Source: Cisco Systems
This chapter describes how to configure Border Gateway Protocol (BGP). For a complete description of the BGP commands in this chapter, refer to the "BGP Commands" chapter of the Cisco IOS IP and IP Routing Command Reference. To locate documentation of other commands that appear in this chapter, use the command reference master index or search online. For multiprotocol BGP configuration information and examples, refer to the "Configuring Multiprotocol BGP Extensions for IP Multicast" chapter of the Cisco IOS IP and IP Routing Configuration Guide. For multiprotocol BGP command descriptions, refer to the "Multiprotocol BGP Extensions for IP Multicast Commands" chapter of the Cisco IOS IP and IP Routing Command Reference.
The Border Gateway Protocol, as defined in RFCs 1163 and 1267, is an Exterior Gateway Protocol (EGP). It allows you to set up an interdomain routing system that automatically guarantees the loop-free exchange of routing information between autonomous systems.
For protocol-independent features, see the chapter "Configuring IP Routing Protocol-Independent Features" in this document.
In BGP, each route consists of a network number, a list of autonomous systems that information has passed through (called the autonomous system path), and a list of other path attributes. We support BGP Versions 2, 3, and 4, as defined in RFCs 1163, 1267, and 1771, respectively.
The primary function of a BGP system is to exchange network reachability information with other BGP systems, including information about the list of autonomous system paths. This information can be used to construct a graph of autonomous system connectivity from which routing loops can be pruned and with which autonomous system-level policy decisions can be enforced.
You can configure the value for the Multi Exit Discriminator (MED) metric attribute using route maps. (The name of this metric for BGP Versions 2 and 3 is INTER_AS_METRIC.) When an update is sent to an IBGP peer, the MED is passed along without any change. This action enables all the peers in the same autonomous system to make a consistent path selection.
A next-hop router address is used in the NEXT_HOP attribute, regardless of the autonomous system of that router. The Cisco IOS software automatically calculates the value for this attribute.
Transitive, optional path attributes are passed along to other BGP-speaking routers.
BGP Version 4 supports classless interdomain routing (CIDR), which lets you reduce the size of your routing tables by creating aggregate routes, resulting in supernets. CIDR eliminates the concept of network classes within BGP and supports the advertising of IP prefixes. CIDR routes can be carried by OSPF, Enhanced IGRP, and ISIS-IP, and RIP.
See the "BGP Route Map Examples" section at the end of this chapter for examples of how to use route maps to redistribute BGP Version 4 routes.
A router running Cisco IOS Release 12.0 or later does not select or use an IBGP route unless both of the following are true:
BGP bases its decision process on the attribute values. When faced with multiple routes to the same destination, BGP chooses the best route for routing traffic toward the destination. The following process summarizes how BGP chooses the best route.
1. If the next hop is inaccessible, do not consider it.
This is why it is important to have an IGP route to the next hop.
2. If the path is internal, synchronization is enabled, and the route is not in the IGP, do not consider the route.
3. Prefer the path with the largest weight (weight is a Cisco proprietary parameter).
4. If the routes have the same weight, prefer the route with the largest local preference.
5. If the routes have the same local preference, prefer the route that was originated by the local router.
For example, a route might be originated by the local router using the network bgp command, or through redistribution from an IGP.
6. If the local preference is the same, or if no route was originated by the local router, prefer the route with the shortest autonomous system path.
7. If the autonomous system path length is the same, prefer the route with the lowest origin code (IGP < EGP < INCOMPLETE).
8. If the origin codes are the same, prefer the route with the lowest Multi Exit Discriminator (MED) metric attribute.
This comparison is only done if the neighboring autonomous system is the same for all routes considered, unless bgp always-compare-med is enabled.
9. Prefer the external (EBGP) path over the internal (IBGP) path.
All confederation paths are considered internal paths.
10. Prefer the route that can be reached through the closest IGP neighbor (the lowest IGP metric).
This means the router will prefer the shortest internal path within the autonomous system to reach the destination (the shortest path to the BGP next-hop).
11. If the following conditions are all true, insert the route for this path into the IP routing table:
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Note EBGP load sharing can occur at this point, which means that multiple paths can be installed in the forwarding table. |
12. If multipath is not enabled, prefer the route with the lowest IP address value for the BGP router ID.
The router ID is usually the highest IP address on the router or the loopback (virtual) address, but might be implementation-specific.
When a BGP speaker learns two identical EBGP paths for a prefix from a neighboring AS, it will choose the path with the lowest route-id as the best path. This best path is installed in the IP routing table. If BGP multipath support is enabled and the EBGP paths are learned from the same neighboring AS, instead of picking one best path, multiple paths are installed in the IP routing table.
During packet switching, depending on the switching mode, either per-packet or per-destination load balancing is performed among the multiple paths. A maximum of six paths is supported. The maximum-paths router configuration command controls the number of paths allowed. By default, BGP will install only one path to the IP routing table.
The BGP configuration tasks are divided into basic and advanced tasks. The first two basic tasks are required to configure BGP; the remaining basic and advanced tasks are optional.
Basic BGP configuration tasks are discussed in the following sections:
Advanced, optional BGP configuration tasks are discussed in the following sections:
For information on configuring features that apply to multiple IP routing protocols (such as redistributing routing information), see the chapter "Configuring IP Routing Protocol-Independent Features."
The tasks in this section are for configuring basic BGP features.
To enable BGP routing, establish a BGP routing process by using the following commands beginning in global configuration mode:
| Command | Purpose | |
|---|---|---|
| Step 1 |
Enables a BGP routing process, which places you in router configuration mode. | |
| Step 2 |
Flags a network as local to this autonomous system and enter it to the BGP table. |
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Note For exterior protocols, a reference to an IP network from the network router configuration command controls only which networks are advertised. This is in contrast to Interior Gateway Protocols (IGP), such as IGRP, which also use the network command to determine where to send updates. |
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Note The network command is used to inject IGP routes into the BGP table. The network-mask portion of the command allows supernetting and subnetting. The router's resources, such as configured NVRAM or RAM, determine the upper limit of the number of network commands you can use. Alternatively, you could use the redistribute command to achieve the same result. |
Like other Exterior Gateway Protocols (EGPs), BGP must completely understand the relationships it has with its neighbors. Therefore, this task is required.
BGP supports two kinds of neighbors: internal and external. Internal neighbors are in the same autonomous system; external neighbors are in different autonomous systems. Normally, external neighbors are adjacent to each other and share a subnet, while internal neighbors may be anywhere in the same autonomous system.
To configure BGP neighbors, use the following command, beginning in router configuration mode:
| Command | Purpose |
|---|---|
See the "BGP Neighbor Configuration Examples" section at the end of this chapter for an example of configuring BGP neighbors.
Routing policies for a peer include all the configurations such as route-map, distribute-list, prefix-list, and filter-list that may impact inbound or outbound routing table updates. Whenever there is a change in the routing policy, the BGP session must be soft cleared, or soft reset, for the new policy to take effect. Performing inbound reset enables the new inbound policy to take effect. Performing outbound reset causes the new local outbound policy take effect without resetting the BGP session. As a new set of updates is sent during outbound policy reset, a new inbound policy of the neighbor can also take effect.
There are two types of reset, hard reset and soft reset. Table 7 lists the advantages and disadvantages hard reset and soft reset.
Table 7 Advantages and Disadvantages of Hard and Soft Resets
| Type of Reset | Advantages | Disadvantages |
|---|---|---|
|
The prefixes in the BGP, IP, and FIB tables provided by the neighbor are lost. Not recommended. | ||
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No configuration, no storing of routing table updates. The procedure for an outbound reset is described in the section "Configuring BGP Soft Reset Using Stored Routing Policy Information." |
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Does not clear the BGP session and cache. Does not require storing of routing table updates, and has no memory overhead. |
Both BGP routers must support the route refresh capability (in Cisco IOS Release 12.1 and later releases). | |
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configured inbound soft reset (uses the neighbor soft-reconfiguration command) |
Can be used when both BGP routers do not support the automatic route refresh capability. |
Stores all received (inbound) routing policy updates without modification; is memory-intensive. Recommended only when absolutely necessary, such as when both BGP routers do not support the automatic route refresh capability. |
Once you have defined two routers to be BGP neighbors, they will form a BGP connection and exchange routing information. If you subsequently change a BGP filter, weight, distance, version, or timer, or make a similar configuration change, you must reset BGP connections for the configuration change to take effect.
A soft reset updates the routing table for inbound and outbound routing updates. Cisco IOS software releases 12.1 and later support soft reset without any prior configuration. This soft reset allows the dynamic exchange of route refresh requests and routing information between BGP routers, and the subsequent re-advertisement of the respective outbound routing table.
To use soft reset without preconfiguration, both BGP peers must support the soft route refresh capability, which is advertised in the OPEN message sent when the peers establish a TCP session. Routers running Cisco IOS software releases prior to release 12.1 do not support the route refresh capability and must clear the BGP session using the neighbor soft-reconfiguration command, described in "Configuring BGP Soft Reset Using Stored Routing Policy Information." This will have a negative impact upon network operations and should only be used as a last resort.
If both the local BGP router and the neighbor router support the route refresh capability, you can perform a dynamic soft inbound reset. This type of reset has the following advantages over a soft inbound reset using stored routing update information:
To determine whether a router supports the route refresh capability, use the show ip bgp neighbors command, beginning in EXEC mode:
| Command | Purpose |
|---|---|
|
Shows whether a neighbor supports the route refresh capability. If the specified router supports the route refresh capability, the following message is displayed: |
If all the BGP routers support the route refresh capability, you can use the dynamic soft reset method for resetting the inbound routing table. To perform a dynamic soft reset of the inbound routing table, use the clear ip bgp command beginning in EXEC mode:
See the "BGP Soft Reset Examples" section at the end of this chapter for examples of both types of BGP soft resets.
Outbound soft resets do not require any preconfiguration. Using the keyword soft specifies that a soft reset be performed. To perform an outbound soft reset, use the clear ip bgp command, beginning in EXEC mode:
If all of the BGP routers in the connection do not support the route refresh capability, use the soft reset method that generates a new set of inbound routing table updates from information previously stored. To initiate storage of inbound routing table updates, you must first preconfigure the router using the neighbor soft-reconfiguration command. The clear ip bgp command initiates the soft reset, which generates a new set of inbound routing table updates using the stored information.
Keep in mind that the memory requirements for storing the inbound update information can become quite large.To configure BGP soft reset using stored routing policy information, use the following commands, beginning in router configuration mode:
See the "BGP Path Filtering by Neighbor Example" section at the end of this chapter for an example of BGP path filtering by neighbor.
You can verify whether a soft reset is successful by checking information about the routing table and about BGP neighbors.
Step 2 Enter the show ip bgp neighbors command to display information about the BGP and TCP connections to neighbors.
If your autonomous system will be passing traffic through it from another autonomous system to a third autonomous system, it is very important that your autonomous system be consistent about the routes that it advertises. For example, if your BGP were to advertise a route before all routers in your network had learned about the route through your IGP, your autonomous system could receive traffic that some routers cannot yet route. To prevent this from happening, BGP must wait until the IGP has propagated routing information across your autonomous system. This causes BGP to be synchronized with the IGP. Synchronization is enabled by default.
In some cases, you do not need synchronization. If you will not be passing traffic from a different autonomous system through your autonomous system, or if all routers in your autonomous system will be running BGP, you can disable synchronization. Disabling this feature can allow you to carry fewer routes in your IGP and allow BGP to converge more quickly. To disable synchronization, use the no synchronization command, beginning in router configuration mode:
| Command | Purpose |
|---|---|
See the "BGP Path Filtering by Neighbor Example" section at the end of this chapter for an example of BGP synchronization.
In general, you will not want to redistribute most BGP routes into your IGP. A common design is to redistribute one or two routes and to make them exterior routes in IGRP, or have your BGP speaker generate a default route for your autonomous system. When redistributing from BGP into IGP, only the routes learned using EBGP get redistributed.
In most circumstances, you also will not want to redistribute your IGP into BGP. Just list the networks in your autonomous system with network router configuration commands and your networks will be advertised. Networks that are listed this way are referred to as local networks and have a BGP origin attribute of "IGP." They must appear in the main IP routing table and can have any source; for example, they can be directly connected or learned via an IGP. The BGP routing process periodically scans the main IP routing table to detect the presence or absence of local networks, updating the BGP routing table as appropriate.
If you do perform redistribution into BGP, you must be very careful about the routes that can be in your IGP, especially if the routes were redistributed from BGP into the IGP elsewhere. This creates a situation where BGP is potentially injecting information into the IGP and then sending such information back into BGP, and vice versa. Incorrectly redistributing routes into BGP can result in the loss of critical information, such as the AS-path, that is required for BGP to function properly.
Networks that are redistributed into BGP from the EGP protocol will be given the BGP origin attribute "EGP." Other networks that are redistributed into BGP will have the BGP origin attribute of "incomplete." The origin attribute in our implementation is only used in the path selection process.
A weight is a number that you can assign to a path so that you can control the path selection process. The administrative weight is local to the router. A weight can be a number from 0 to 65535. Any path that a Cisco router originates will have a default weight of 32768; other paths have weight 0. If you have particular neighbors that you want to prefer for most of your traffic, you can assign a higher weight to all routes learned from that neighbor.
Weights can be assigned based on autonomous system path access lists. A given weight becomes the weight of the route if the autonomous system path is accepted by the access list. Any number of weight filters are allowed. Weights can only be assigned via route maps.
RFC 1771, the IETF document defining BGP, does not include as-path as part of the "tie-breaker" decision algorithm. By default, Cisco IOS software considers the as-path as a part of the decision algorithm. This enhancement makes it possible to modify the decision algorithm, bringing the router's behavior in selecting a path more in line with the IETF specification.
To prevent the router from considering the as-path length when selecting a route, use the bgp bestpath as-path ignore command, beginning in router configuration mode:
You can filter BGP advertisements in two ways:
Filtering using prefix lists is described in the "Configuring BGP Filtering Using Prefix Lists" section.
If you want to restrict the routing information that the Cisco IOS software learns or advertises, you can filter BGP routing updates to and from particular neighbors. To do this, you can either define an access list or a prefix list and apply it to the updates.
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Note Distribute-list filters are applied to network numbers and not autonomous system paths. |
To filter BGP routing updates, use the neighbor distribute-list command, beginning in router configuration mode:
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Note Although the neighbor prefix-list router configuration command can be used as an alternative to the neighbor distribute-list command, do not use attempt to apply both the neighbor prefix-list and neighbor distribute-list command filtering to the same neighbor in any given direction. These two commands are mutually exclusive, and only one command (neighbor prefix-list or neighbor distribute-list) can be applied for each inbound or outbound direction. |
Prefix lists can be used as an alternative to access lists in many BGP route filtering commands. "How the System Filters Traffic by Prefix List" describes the way prefix list filtering works. The advantages of using prefix lists are:
Filtering using extended access lists does not support incremental updates.
The command-line interface for using access lists to filter BGP updates is difficult to understand and use, since it uses the packet filtering format.
Before using a prefix list in a command, you must set up a prefix list, and you may want to assign sequence numbers to the entries in the prefix list.
Filtering by prefix list involves matching the prefixes of routes with those listed in the prefix list. When there is a match, the route is used. More specifically, whether a prefix is permitted or denied is based upon the following rules:
The router begins the search at the top of the prefix list, with the sequence number 1. Once a match or deny occurs, the router does not need to go through the rest of the prefix list. For efficiency, you may want to put the most common matches or denies near the top of the list, using the argument seq in the ip prefix-list command. The show commands always include the sequence numbers in their output.
Sequence numbers are generated automatically unless you disable this automatic generation. If you disable the automatic generation of sequence numbers, you must specify the sequence number for each entry using the seq-value argument of the ip prefix-list command.
Regardless of whether the default sequence numbers are used in configuring a prefix list, a sequence number does not need to be specified when removing a configuration entry.
Show commands include the sequence numbers in their output.
To create a prefix list, use the ip prefix-list command, beginning in router configuration mode:
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Note To create a prefix list you must enter at least one permit or deny clause. |
To remove a prefix list and all of its entries, use the no ip prefix-list command, beginning in router configuration mode:
You can add entries to a prefix list individually. To configure an entry in a prefix list, use the ip prefix-list command, beginning in router configuration mode:
The optional keywords ge and le can be used to specify the range of the prefix length to be matched for prefixes that are more specific than network/len. An exact match is assumed when neither ge nor le is specified. The range is assumed to be from ge-value to 32 if only the ge attribute is specified, and from len to le-value if only the le attribute is specified.
A specified ge-value and/or le-value must satisfy the following condition:
For example, to deny all prefixes matching /24 in 128.0.0.0/8, you would use:
By default, the sequence numbers are automatically generated when you create a prefix list entry. Sequence numbers can be suppressed with the command no ip prefix-list sequence-number. Sequence values are generated in increments of 5. The first sequence value generated in a prefix list would be 5, then 10, then 15, and so on. If you specify a value for an entry and then do not specify values for subsequent entries, the assigned (generated) sequence values are incremented in units of five. For example, if you specify that the first entry in the prefix list have a sequence value of 3, and then do not specify sequence values for the other entries, the automatically generated numbers will be 8, 13, 18, and so on.
To disable the automatic generation of sequence numbers, use the no ip prefix-list sequence-number command, beginning in router configuration mode:
To reenable automatic generation of the sequence numbers of prefix list entries, use the ip prefix-list sequence number command, beginning in router configuration mode:
| Command | Purpose |
|---|---|
|
Enables the automatic generation of the sequence numbers of prefix list entries. The default is enable. |
If you disable automatic generation of sequence numbers in a prefix list, you must specify the sequence number for each entry using the seq-value argument of the ip prefix-list command.
Regardless of whether the default sequence numbers are used in configuring a prefix list, a sequence number does not need to be specified when de-configuring an entry. Show commands include the sequence numbers in their output.
To delete a prefix list, use the no ip prefix-list command, beginning in router configuration mode:
You can delete entries from a prefix list individually. To delete an entry in a prefix list, use the no ip prefix-list seq command, beginning in router configuration mode:
| Command | Purpose |
|---|---|
|
Note The sequence number of an entry does not need to be specified when you delete the entry. |
To display information about prefix tables, prefix table entries, the policy associated with a node, or specific information about an entry, use the following commands, beginning in EXEC mode:
To clear the hit count table of prefix list entries, use the clear ip prefix-list command, beginning in EXEC mode:
| Command | Purpose |
In addition to filtering routing updates based on network numbers, you can specify an access list filter on both incoming and outbound updates based on the BGP autonomous system paths. Each filter is an access list based on regular expressions. To do this, define an autonomous system path access list and apply it to updates to and from particular neighbors. See the "Regular Expressions" appendix in the Cisco IOS Dial Services Command Reference for more information on forming regular expressions.
To configure BGP path filtering, use the following commands beginning in global configuration mode:
| Command | Purpose | |
|---|---|---|
| Step 1 | ||
| Step 2 | ||
| Step 3 |
See the "BGP Path Filtering by Neighbor Example" section at the end of this chapter for an example of BGP path filtering by neighbor.
You can configure the Cisco IOS software to disable next-hop processing for BGP updates to a neighbor. This might be useful in nonmeshed networks such as Frame Relay or X.25, where BGP neighbors might not have direct access to all other neighbors on the same IP subnet. There are two ways to disable next-hop processing:
To disable next-hop processing and provide a specific address to be used instead of the next-hop address, use the neighbor next-hop-self command, beginning in router configuration mode:
| Command | Purpose |
|---|---|
Configuring this command causes the current router to advertise its peering address as the next hop for the specified neighbor. Therefore, other BGP neighbors will forward to it packets for that address. This is useful in a nonmeshed environment, since you know that a path exists from the present router to that address. In a fully meshed environment, this is not useful, since it will result in unnecessary extra hops and because there might be a direct access through the fully meshed cloud with fewer hops.
To override the inbound next hop setting for BGP routes and specify that the next hop of the matching routes is to be the IP address of the remote peer, or to set the peering address of the local router to be the next hop of the matching routes, use the neighbor next-hop-self command.
To configure the neighbor peering address to be used for the next hop address, use the set ip next-hop command, beginning in route map configuration mode:
By default, BGP sessions begin using BGP Version 4 and negotiating downward to earlier versions if necessary. To prevent negotiation and force the BGP version used to communicate with a neighbor, use the neighbor version command, beginning in router configuration mode:
BGP uses the Multi Exit Discriminator (MED) metric as a hint to external neighbors about preferred paths. (The name of this metric for BGP Versions 2 and 3 is INTER_AS_METRIC.) You can set the MED of the redistributed routes by performing the following task. All the routes without a MED will also be set to this value. Use the default-metric command, beginning in router configuration mode:
Alternatively, you can set the MED using the route-map command. See the "BGP Route Map Examples" section at the end of this chapter for examples of using BGP route maps.
The tasks in this section are for configuring advanced BGP features.
You can use a route map on a per-neighbor basis to filter updates and modify various attributes. A route map can be applied to either inbound or outbound updates. Only the routes that pass the route map are sent or accepted in updates.
On both the inbound and the outbound updates, we support matching based on autonomous system path, community, and network numbers. Autonomous system path matching requires the as-path access-list command, community based matching requires the community-list command and network-based matching requires the ip access-list command. Use the neighbor route-map command, beginning in router configuration mode:
| Command | Purpose |
|---|---|
See the "BGP Route Map Examples" section at the end of this chapter for BGP route-map examples.
Normally, when a link between external neighbors goes down, the BGP session will not be reset immediately. If you want the EBGP session to be reset as soon as an interface goes down, use the bgp fast-external-fallover command, beginning in router configuration mode:
| Command | Purpose |
Classless interdomain routing (CIDR) enables you to create aggregate routes (or supernets) to minimize the size of routing tables. You can configure aggregate routes in BGP either by redistributing an aggregate route into BGP or by using the conditional aggregation feature described in the following task table. An aggregate address will be added to the BGP table if there is at least one more specific entry in the BGP table.
To create an aggregate address in the routing table, use one or more of the following commands, beginning in router configuration mode:
| Command | Purpose |
|---|---|
|
Generates an aggregate based on conditions specified by the route map. | |
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Generates an aggregate with attributes specified in the route map. |
See the "BGP Aggregate Route Examples" section at the end of this chapter for examples of using BGP aggregate routes.
In BGP Version 3, when a subnet is redistributed from an IGP into BGP, only the network route is injected into the BGP table. By default, this automatic summarization is enabled. To disable automatic network number summarization, use the no auto-summary command, beginning in router configuration mode:
BGP supports transit policies via controlled distribution of routing information. The distribution of routing information is based on one of the following three values:
The COMMUNITIES attribute is a way to group destinations into communities and apply routing decisions based on the communities. This method simplifies a BGP speaker's configuration that controls distribution of routing information.
A community is a group of destinations that share some common attribute. Each destination can belong to multiple communities. Autonomous system administrators can define to which communities a destination belongs. By default, all destinations belong to the general Internet community. The community is carried as the COMMUNITIES attribute.
The COMMUNITIES attribute is an optional, transitive, global attribute in the numerical range from 1 to 4,294,967,200. Along with Internet community, there are a few predefined, well-known communities, as follows:
Based on the community, you can control which routing information to accept, prefer, or distribute to other neighbors. A BGP speaker can set, append, or modify the community of a route when you learn, advertise, or redistribute routes. When routes are aggregated, the resulting aggregate has a COMMUNITIES attribute that contains all communities from all the initial routes.
You can use community lists to create groups of communities to use in a match clause of a route map. Just like an access list, a series of community lists can be created. Statements are checked until a match is found. As soon as one statement is satisfied, the test is concluded.
To create a community list, use the ip community-list command, beginning in global configuration mode:
| Command | Purpose |
|---|---|
To set the COMMUNITIES attribute and match clauses based on communities, see the match community-list and set community commands in the "Redistribute Routing Information" section in the "Configuring IP Routing Protocol-Independent Features" chapter.
By default, no COMMUNITIES attribute is sent to a neighbor. You can specify that the COMMUNITIES attribute be sent to the neighbor at an IP address by using the neighbor send-community command, beginning in router configuration mode:
To remove communities from the community attribute of an inbound or outbound update using a route map to filter and determine the communities to be deleted, use the set comm-list delete command, beginning in router configuration mode:
| Command | Purpose |
|---|---|
|
Removes communities in a community attribute that match a standard or extended community list. |
A BGP community is displayed in a two-part format two bytes long in the show ip bgp community command output, as well as wherever communities are displayed in the router configuration, such as router maps and community lists. In the most recent version of the RFC for BGP, a community is of the form AA:NN, where the first part is the AS number and the second part is a 2 byte number. The Cisco default community format is in the format NNAA.
To display BGP communities in the new format, use the ip bgp-community new-format command, beginning in global configuration mode:
| Command | Purpose |
|---|---|
The BGP advertises routes from its routing table to external peers (peers in different autonomous systems) by default. The BGP Conditional Advertisement feature provides additional control of route advertisement depending on the existence of other prefixes in the BGP table. Normally, routes are propagated regardless of the existence of a different path. The BGP Conditional Advertisement feature uses the non-exist-map and the advertise-map to track routes by the route prefix. If a route prefix is not present in the non-exist-map, the route specified by the advertise-map is announced. The announced route is installed to the BGP routing table as a locally originated route and will behave as a locally originated route. The announced route will be originated by BGP only if the corresponding IGP route exists. After the prefix is locally originated by BGP, BGP will advertise the prefix to internal and external peers. If the route prefix is present, the route in the advertise-map is not announced.
This feature can be useful in a multihomed network, in which some prefixes are to be advertised to one of the providers, only if information from the other provider is missing. This condition would indicate a failure in the peering session, or partial reachability.
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Note The conditional BGP announcements are sent in addition to the normal announcements that a BGP router sends to its peers. |
If the same information is advertised to all providers in a multihomed environment, the information is duplicated in the global BGP table. When the BGP Conditional Advertisement feature is used, only partial routes are advertised to each provider, and the size of the global BGP table is not increased with redundant information.
The user can guarantee the path that inbound traffic will follow because only specific paths are advertised to providers.
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Note Autonomous system path list information cannot be used for conditional advertisement because the IP routing table does not contain autonomous system path information. |
See the following section for configuration tasks for the BGP Conditional Advertisement feature. Each task in the list indicates if the task is optional or required.
To conditionally advertise a set of routes, use the following commands begining in router configuration mode:
See the "BGP Conditional Advertisement Configuration Examples" section at the end of this chapter for an example configuration of BGP conditional advertisement.
To verify that the BGP Condition Advertisement feature has been configured, use the show ip bgp neighbor command. The show ip bgp neighbor EXEC command will show the status of the BGP Conditional Advertisement feature as initialized or uninitialized. The following example shows output from the show ip bgp neighbor EXEC command:
This section provides troubleshooting information for the BGP conditional advertisement feature.
The BGP Conditional Advertisement feature is based on the nonexistence of a prefix and the advertisement of another. Normally, only two problems can occur:
One way to reduce the IBGP mesh is to divide an autonomous system into multiple sub-autonomous systems and group them into a single confederation. To the outside world, the confederation looks like a single autonomous system. Each autonomous system is fully meshed within itself, and has a few connections to other autonomous systems in the same confederation. Even though the peers in different autonomous systems have EBGP sessions, they exchange routing information as if they were IBGP peers. Specifically, the next-hop, MED, and local preference information is preserved. This enables to us to retain a single Interior Gateway Protocol (IGP) for all of the autonomous systems.
To configure a BGP confederation, you must specify a confederation identifier. To the outside world, the group of autonomous systems will look like a single autonomous system with the confederation identifier as the autonomous system number.
To configure a BGP confederation identifier, use the bgp confederation identifier command, beginning in router configuration mode:
| Command | Purpose |
|---|---|
In order to treat the neighbors from other autonomous systems within the confederation as special EBGP peers, use the bgp confederation peers command, beginning in router configuration mode:
See the "BGP Community with Route Maps Examples" section at the end of this chapter for an example configuration of several peers in a confederation.
For an alternative way to reduce the IBGP mesh, see the next section, "Configuring a Route Reflector."
BGP requires that all of the IBGP speakers be fully meshed. However, this requirement does not scale when there are many IBGP speakers. Instead of configuring a confederation, another way to reduce the IBGP mesh is to configure a route reflector.
Figure 39 illustrates a simple IBGP configuration with three IBGP speakers (Routers A, B, and C). Without route reflectors, when Router A receives a route from an external neighbor, it must advertise it to both Routers B and C. Routers B and C do not readvertise the IBGP learned route to other IBGP speakers because the routers do not pass routes learned from internal neighbors on to other internal neighbors, thus preventing a routing information loop.
With route reflectors, all IBGP speakers need not be fully meshed because there is a method to pass learned routes to neighbors. In this model, an internal BGP peer is configured to be a route reflector responsible for passing IBGP learned routes to a set of IBGP neighbors. In Figure 40, Router B is configured as a route reflector. When the route reflector receives routes advertised from Router A, it advertises them to Router C, and vice versa. This scheme eliminates the need for the IBGP session between Routers A and C.
The internal peers of the route reflector are divided into two groups: client peers and all the other routers in the autonomous system (nonclient peers). A route reflector reflects routes between these two groups. The route reflector and its client peers form a cluster. The nonclient peers must be fully meshed with each other, but the client peers need not be fully meshed. The clients in the cluster do not communicate with IBGP speakers outside their cluster.
Figure 41 illustrates a more complex route reflector scheme. Router A is the route reflector in a cluster with Routers B, C, and D. Routers E, F, and G are fully meshed, nonclient routers.
When the route reflector receives an advertised route, depending on the neighbor, it does the following:
To configure a route reflector and its clients, use the neighbor route-reflector-client command, beginning in router configuration mode:
| Command | Purpose |
|---|---|
|
Configures the local router as a BGP route reflector and the specified neighbor as a client. |
Along with route reflector-aware BGP speakers, it is possible to have BGP speakers that do not understand the concept of route reflectors. They can be members of either client or nonclient groups. This allows easy, gradual migration from the old BGP model to the route reflector model. Initially, you could create a single cluster with a route reflector and a few clients. All the other IBGP speakers could be nonclient peers to the route reflector and then more clusters could be created gradually.
An autonomous system can have multiple route reflectors. A route reflector treats other route reflectors just like other IBGP speakers. A route reflector can be configured to have other route reflectors in a client group or nonclient group. In a simple configuration, the backbone could be divided into many clusters. Each route reflector would be configured with other route reflectors as nonclient peers (thus, all the route reflectors will be fully meshed). The clients are configured to maintain IBGP sessions with only the route reflector in their cluster.
Usually a cluster of clients will have a single route reflector. In that case, the cluster is identified by the router ID of the route reflector. To increase redundancy and avoid a single point of failure, a cluster might have more than one route reflector. In this case, all route reflectors in the cluster must be configured with the 4-byte cluster ID so that a route reflector can recognize updates from route reflectors in the same cluster. All the route reflectors serving a cluster should be fully meshed and all of them should have identical sets of client and nonclient peers.
If the cluster has more than one route reflector, configure the cluster ID by using the bgp cluster-id command, beginning in router configuration mode:
Use the show ip bgp command to display the originator ID and the cluster-list attributes.
By default, the clients of a route reflector are not required to be fully meshed and the routes from a client are reflected to other clients. However, if the clients are fully meshed, the route reflector does not need to reflect routes to clients. To disable client-to-client route reflection, use the no bgp client-to-client reflection command, beginning in router configuration mode:
| Command | Purpose |
|---|---|
As the IBGP learned routes are reflected, it is possible for routing information to loop. The route reflector model has the following mechanisms to avoid routing loops:
Often, in a BGP speaker, many neighbors are configured with the same update policies (that is, the same outbound route maps, distribute lists, filter lists, update source, and so on). Neighbors with the same update policies can be grouped into peer groups to simplify configuration and, more importantly, to make updating more efficient. When you have many peers, this approach is highly recommended.
The three steps to configure a BGP peer group, described in the following sections, are as follows:
2. Assigning Options to the Peer Group
3. Making Neighbors Members of the Peer Group
You can disable a BGP peer or peer group without removing all the configuration information using the neighbor shutdown command.
To create a BGP peer group, use the neighbor peer-group command, beginning in router configuration mode:
| Command | Purpose |
|---|---|
After you create a peer group, you configure the peer group with neighbor commands. By default, members of the peer group inherit all the configuration options of the peer group. Members can also be configured to override the options that do not affect outbound updates.
Peer group members will always inherit the following: remote-as (if configured), version, update-source, out-route-map, out-filter-list, out-dist-list, minimum-advertisement-interval, and next-hop-self. All the peer group members will inherit changes made to the peer group.
To assign configuration options to an individual neighbor, specify any of the following commands using the IP address. To assign the options to a peer group, specify any of the commands using the peer group name. Use any of these commands, beginning in router configuration mode.
If a peer group is not configured with a remote-as, the members can be configured with the neighbor remote-as command. This allows you to create peer groups containing EBGP neighbors.
You can customize inbound policies for peer group members, (using, for example, a distribute list, route map, or filter list) because one identical copy of an update is sent to every member of a group. Therefore, neighbor options related to outgoing updates cannot be customized for peer group members.
External BGP peers normally must reside on a directly connected network. Sometimes it is useful to relax this restriction in order to test BGP; do so by specifying the neighbor ebgp-multihop command.
|
Note To avoid the accidental creation of loops through oscillating routes, the multihop session will not be established if the only route to the multihop peer's address is the default route (0.0.0.0). |
Members of a peer group can pass routes from one member of the peer group to another. For example, if router B is peering with routers A and C, router B can pass router A's routes to router C.
For internal BGP, you might want to allow your BGP connections to stay up regardless of which interface is used to reach a neighbor. To do this, you first configure a loopback interface and assign it an IP address. Next, configure the BGP update source to be the loopback interface. Finally, configure your neighbor to use the address on the loopback interface. Now the IBGP session will be up as long as there is a route, regardless of any interface.
You can set the minimum interval of time between BGP routing updates.
You can invoke MD5 authentication between two BGP peers, meaning that each segment sent on the TCP connection between them is verified. This feature must be configured with the same password on both BGP peers; otherwise, the connection between them will not be made. The authentication feature uses the MD5 algorithm. Invoking authentication causes the Cisco IOS software to generate and check the MD5 digest of every segment sent on the TCP connection. If authentication is invoked and a segment fails authentication, then a message appears on the console.
See the "BGP Peer Group Examples" at the end of this chapter for an example of enabling MD5 authentication.
Finally, to configure a BGP neighbor to be a member of that BGP peer group, use the neighbor peer-group command, beginning in router configuration mode, using the same peer group name:
| Command | Purpose |
|---|---|
See the "BGP Peer Group Examples" section at the end of this chapter for examples of IBGP and EBGP peer groups.
To disable an existing BGP neighbor or neighbor peer group, use the neighbor shutdown command, beginning in router configuration mode:
| Command | Purpose |
|---|---|
To enable a previously existing neighbor or neighbor peer group that had been disabled using the neighbor shutdown command, use the no neighbor shutdown command, beginning in router configuration mode:
| Command | Purpose |
|---|---|
You can indicate which networks are reachable by using a backdoor route that the border router should use. A backdoor network is treated as a local network, except that it is not advertised. To configure backdoor routes, use the network backdoor command, beginning in router configuration mode:
| Command | Purpose |
|---|---|
By default, when a BGP route is put into the IP routing table, the MED is converted to an IP route metric, the BGP next hop is used as the next hop for the IP route, and the tag is not set. However, you can use a route map to perform mapping. To modify metric and tag information when the IP routing table is updated with BGP learned routes, use the table-map command, beginning in router configuration mode:
Administrative distance is a measure of the preference of different routing protocols. BGP uses three different administrative distances: external, internal, and local. Routes learned through external BGP are given the external distance, routes learned with internal BGP are given the internal distance, and routes that are part of this autonomous system are given the local distance. To assign a BGP administrative distance, use the distance bgp command beginning in router configuration mode:
| Command | Purpose |
|---|---|
Changing the administrative distance of BGP routes is considered dangerous and generally is not recommended. The external distance should be lower than any other dynamic routing protocol, and the internal and local distances should be higher than any other dynamic routing protocol.
BGP uses certain timers to control periodic activities such as the sending of keepalive messages, and the interval after not receiving a keepalive message after which the Cisco IOS software declares a peer dead. By default, the keepalive timer is 60 seconds, and the holdtime timer is 180 seconds.You can adjust these timers. When a connection is started, BGP will negotiate the hold time with the neighbor. The smaller of the two hold times will be chosen. The keepalive timer is then set based on the negotiated hold time and the configured keepalive time. To adjust BGP timers for all neighbors, use the timers bgp command, beginning in router configuration mode:
| Command | Purpose |
|---|---|
To adjust BTP keepalive and holdtime timers for a specific neighbor, use the neighbor timers command, beginning in router configuration mode:
| Command | Purpose |
|---|---|
|
Sets the keepalive and holdtime timers (in seconds) for the specified peer or peer group. |
|
Note The timers configured for a specific neighbor or peer group override the timers configured for all BGP neighbors using the timers bgp command. |
To clear the timers for a BGP neighbor or peer group, use the no form of the neighbor timers command.
You can define a particular path as more preferable or less preferable than other paths by changing the default local preference value of 100. To assign a different default local preference value, use the bgp default local-preference command, beginning in router configuration mode:
| Command | Purpose |
|---|---|
You can use route maps to change the default local preference of specific paths. See the "BGP Route Map Examples" section at the end of this chapter for examples when used with BGP route maps.
By default, you are not allowed to redistribute network 0.0.0.0. To permit the redistribution of network 0.0.0.0, use the default-information originate command, beginning in router configuration mode:
| Command | Purpose |
|---|---|
To configure the router to consider a path with a missing MED attribute as the worst path, use the bgp bestpath med missing-as-worst command, beginning in router configuration mode:
| Command | Purpose |
|---|---|
|
Configures the router to consider a missing MED as having a value of infinity, making the path without a MED value the least desirable path. |
The MED is one of the parameters that is considered when selecting the best path among many alternative paths. The path with a lower MED is preferred over a path with a higher MED.
By default, during the best-path selection process, MED comparison is done only among paths from the same autonomous system. You can allow comparison of MEDs among paths regardless of the autonomous system from which the paths are received. To do so, use the bgp always-compare-med command, beginning in router configuration mode:
| Command | Purpose |
|---|---|
|
Allows the comparison of MEDs for paths from neighbors in different autonomous systems. |
To configure the router to consider the MED value in choosing a path, use the bgp bestpath med confed command, beginning in router configuration mode:
| Command | Purpose |
|---|---|
|
Configures the router to consider the MED in choosing a path from among those advertised by different sub-ASs within a confederation. |
The comparison between MEDs is only made if there are no external ASes in the path (an external AS is an AS that is not within the confederation). If there is an external AS in the path, then the external MED is passed transparently through the confederation, and the comparison is not made.
For e and we are comparing route A with these paths:
In this case, path 1 would be chosen if bgp bestpath med confed is enabled. The fourth path has a lower MED, but it is not involved in the MED comparison because there is an external AS in this path.
To configure the router to use the MED to select the best path from among paths advertised by a single sub-AS within a confederation, use the bgp deterministic med command, beginning in router configuration mode:
| Command | Purpose |
|---|---|
|
Configures the router to compare the MED variable when choosing among routes advertised by different peers in the same autonomous system. |
|
Note If bgp always-compare-med is enabled, all paths are fully comparable, including those from other ASes in the confederation, even if bgp deterministic med is also enabled. |
Route dampening is a BGP feature designed to minimize the propagation of flapping routes across an internetwork. A route is considered to be flapping when it is repeatedly available, then unavailable, then available, then unavailable, and so on.
For example, consider a network with three BGP autonomous systems: AS 1, AS 2, and AS 3. Suppose the route to network A in AS 1 flaps (it becomes unavailable). Under circumstances without route dampening, AS 1's EBGP neighbor to AS 2 sends a Withdraw message to AS 2. The border router in AS 2, in turn, propagates the Withdraw to AS 3. When the route to network A reappears, AS 1 sends an Advertisement to AS 2, which sends it to AS 3. If the route to network A repeatedly becomes unavailable, then available, many Withdrawals and Advertisements are sent. This is a problem in an internetwork connected to the Internet because a route flap in the Internet backbone usually involves many routes.
|
Note No penalty is applied to a BGP peer reset when route dampening is enabled. Although the reset withdraws the route, there is no penalty applied in this instance, even if route flap dampening is enabled. |
The route dampening feature minimizes the flapping problem as follows. Suppose again that the route to network A flaps. The router in AS 2 (where route dampening is enabled) assigns network A a penalty of 1000 and moves it to "history" state. The router in AS 2 continues to advertise the status of the route to neighbors. The penalties are cumulative. When the route flaps so often that the penalty exceeds a configurable suppress limit, the router stops advertising the route to network A, regardless of how many times it flaps. Thus, the route is dampened.
The penalty placed on network A is decayed until the reuse limit is reached, upon which the route is once again advertised. At half of the reuse limit, the dampening information for the route to network A is removed.
The following terms are used when describing route dampening:
The routes external to an AS learned via IBGP are not dampened. This policy prevent the IBGP peers from having a higher penalty for routes external to the AS.
To enable BGP route dampening, use the bgp dampening command, beginning in global configuration mode:
To change the default values of various dampening factors, use the bgp dampening command, beginning in global configuration mode:
| Command | Purpose |
|---|---|
You can monitor the flaps of all the paths that are flapping. The statistics will be deleted once the route is not suppressed and is stable for at least one half-life. To display flap statistics, use the following commands, beginning in EXEC mode:
| Command | Purpose |
|---|---|
|
Displays BGP flap statistics for all paths that match the regular expression. | |
|
Displays BGP flap statistics for all paths that pass the filter. | |
To clear BGP flap statistics (thus making it less likely that the route will be dampened), use the following commands, beginning in EXEC mode:
| Command | Purpose |
|---|---|
|
Clears BGP flap statistics for all paths that match the regular expression. | |
|
Clears BGP flap statistics for all paths that pass the filter. | |
|
Note The flap statistics for a route are also cleared when a BGP peer is reset. Although the reset withdraws the route, there is no penalty applied in this instance, even if route flap dampening is enabled. |
Once a route is dampened, you can display BGP route dampening information, including the time remaining before the dampened routes will be unsuppressed. To display the information, use the show ip bgp dampened-paths command, beginning in EXEC mode:
| Command | Purpose |
|---|---|
|
Displays the dampened routes, including the time remaining before they will be unsuppressed. |
You can clear BGP route dampening information and unsuppress any suppressed routes by using the clear ip bgp dampening command, beginning in EXEC mode:
You can remove all contents of a particular cache, table, or database. You also can display specific statistics. The following sections describe each of these tasks.
You can remove all contents of a particular cache, table, or database. Clearing a cache, table, or database can become necessary when the contents of the particular structure have become, or are suspected to be, invalid.
The following table lists the tasks associated with clearing caches, tables, and databases for BGP. Use these commands, beginning in EXEC mode:
| Command | Purpose |
|---|---|
You can display specific statistics such as the contents of BGP routing tables, caches, and databases. Information provided can be used to determine resource utilization and solve network problems. You can also display information about node reachability and discover the routing path your device's packets are taking through the network.
To display various routing statistics, use the following commands, beginning in EXEC mode:
To enable the logging of messages generated when a BGP neighbor resets, comes up, or goes down, use the bgp log-neighbor changes command, beginning in router configuration mode:
The following sections provide BGP configuration examples:
The following example shows how you can use route maps to modify incoming data from a neighbor. Any route received from 140.222.1.1 that matches the filter parameters set in autonomous system access list 200 will have its weight set to 200 and its local preference set to 250, and it will be accepted.
In the following example, route map freddy marks all paths originating from autonomous system 690 with a Multi Exit Discriminator (MED) metric attribute of 127. The second permit clause is required so that routes not matching autonomous system path list 1 will still be sent to neighbor 1.1.1.1.
The following example shows how you can use route maps to modify redistributed information from the IP forwarding table:
It is proper behavior to not accept any autonomous system path not matching the match clause of the route map. This means that you will not set the metric and the Cisco IOS software will not accept the route. However, you can configure the software to accept autonomous system paths not matched in the match clause of the route map command by using multiple maps of the same name, some without accompanying set commands.
The following example shows how you can use route maps in a reverse operation to set the route tag (as defined by the BGP/OSPF interaction document, RFC 1403) when exporting routes from BGP into the main IP routing table:
The following example shows how the route map called set-as-path is applied to outbound updates to the neighbor 200.69.232.70. The route map will prepend the autonomous system path "100 100" to routes that pass access list 1. The second part of the route map is to permit the advertisement of other routes.
Inbound route-maps could do prefix-based matching and set various parameters of the update. Inbound prefix matching is available in addition to as-path and community-list matching. The following example shows how the set local preference command sets the local preference of the inbound prefix 140.10.0.0/16 to 120.
The following examples show how to ensure that traffic from one router on a shared LAN will always be passed through a second router, rather than being sent directly to a third router on the same LAN.
Routers A, B, and C connect to the same LAN. Router A peers with B, and B peers with C. B sends traffic over the routes of router A to router C, but wants to make sure that all traffic from C to A goes through B, rather than directly from C to A over the shared LAN. This configuration can be useful for traffic accounting purposes or to satisfy the peering agreement between C and B. You can achieve this configuration by using the set ip next-hop command as shown in the following two examples.
Example one applies an inbound route map on the BGP session of router C with router B.
Example two applies an outbound route map on the BGP session of router B with router C.
The following example shows how BGP neighbors on an autonomous system are configured to share information. In the example, a BGP router is assigned to autonomous system 109, and two networks are listed as originating in the autonomous system. Then the addresses of three remote routers (and their autonomous systems) are listed. The router being configured will share information about networks 131.108.0.0 and 192.31.7.0 with the neighbor routers. The first router listed is in a different autonomous system; the second neighbor command specifies an internal neighbor (with the same autonomous system number) at address 131.108.234.2; and the third neighbor command specifies a neighbor on a different autonomous system.
In Figure 42, Router A is being configured. The internal BGP neighbor is not directly linked to Router A. External neighbors (in autonomous system 167 and autonomous system 99) must be linked directly to Router A.
The following examples show route filtering using a single prefix list and a group of prefixes, and how to add or delete an individual entry from a prefix list.
The following example shows how a prefix list denies the default route 0.0.0.0/0:
The following example shows how a prefix list permits a route that matches the prefix 35.0.0.0/8:
The following example shows how to configure the BGP process so that it only accept prefixes with a prefix length of /8 to /24:
The following example configuration shows how to conditionally originate a default route(0.0.0.0/0) in RIP when there exists a prefix 10.1.1.0/24 in the routing table:
The following example shows how to configure BGP to accept routing updates from 192.1.1.1 only, besides filtering on the prefix length:
The following example shows how to direct the BGP process to filter incoming updates to the prefix using name1, and match the gateway (next-hop) of the prefix being updated to the prefix list name2, on the interface ethernet 0.
The following example shows how to configure BGP to permit routes with prefix length up to 24 in network 192/8:
The following example shows how to configure BGP to deny routes with prefix length greater than in 25 in 192/8:
The following example shows how to configure BGP to permit routes with prefix length greater than 8 and less than 24 in all address space:
The following example shows how to configure BGP to deny routes with prefix length greater than 25 in all address space:
The following example shows how to configure BGP to deny all routes in 10/8, since any route in the Class A network 10.0.0.0/8 is denied if its mask is less than or equal to 32 bits:
The following example shows how to configure BGP to deny routes with a mask greater than 25 in 204.70.1/24:
The following example shows how to configure BGP to permit all routes:
You can add or delete individual entries in a prefix list. For example, if a prefix list has the following initial configuration:
The following example shows how to delete an entry from the prefix list so that 192.168.0.0 is not permitted, and add a a new entry that permits 10.0.0.0/8:
The following examples show two ways to reset the connection for BGP peer 131.108.1.1.
The following examples shows the clear ip bgp 131.108.1.1 soft in command used to initiate a dynamic soft reconfiguration in the BGP peer 131.108.1.1. This command requires that the peer supports the route refresh capability.
The following example shows how to enable inbound soft reconfiguration for the neighbor 131.108.1.1. All the updates received from this neighbor will be stored unmodified, regardless of the inbound policy. When inbound soft reconfiguration is performed later, the stored information will be used to generate a new set of inbound updates.
The following example clears the session with the neighbor 131.108.1.1.
The example shown in Figure 43 shows how to use the no synchronization command. In the figure, synchronization is on, and Router B does not advertise network 198.92.68.0 to Router A until an IGRP route for network 198.92.68.0 exists. If you specify the no synchronization router configuration command, Router B advertises network 198.92.68.0 as soon as possible. However, because routing information still must be sent to interior peers, you must configure a full internal BGP mesh.
The following example shows BGP path filtering by neighbor. Only the routes that pass as-path access list 2 will be sent to 193.1.12.10. Similarly, only routes passing access list 3 will be accepted from 193.1.12.10.
The following examples show how you can use aggregate routes in BGP either by redistributing an aggregate route into BGP or by using the conditional aggregate routing feature.
In the following example, the redistribute static command is used to redistribute aggregate route 193.*.*.*:
The following configuration shows how to create an aggregate entry in the BGP routing table when there is at least one specific route that falls into the specified range. The aggregate route will be advertised as coming from your autonomous system and has the atomic aggregate attribute set to show that information might be missing. (By default, atomic aggregate is set unless you use the as-set keyword in the aggregate-address command.)
The following example shows how to create an aggregate entry using the same rules as in the previous example, but the path advertised for this route will be an AS_SET consisting of all elements contained in all paths that are being summarized:
The following example shows how to create the aggregate route for 193.*.*.*, and also suppress advertisements of more specific routes to all neighbors:
This section contains three examples of the use of BGP communities with route maps, and two examples that also contain confederation configurations. For an example of how to configure a BGP confederation, see "BGP Confederation Example" in this chapter.
The first example shows how the route map set-community is applied to the outbound updates to the neighbor 171.69.232.50. The routes that pass access list 1 have the special community attribute value "no-export." The remaining routes are advertised normally. This special community value automatically prevents the advertisement of those routes by the BGP speakers in autonomous system 200.
The second example shows how the route map set-community is applied to the outbound updates to neighbor 171.69.232.90. All the routes that originate from AS 70 have the community values 200 200 added to their already existing values. All other routes are advertised as normal.
The third example shows how community-based matching is used to selectively set MED and local-preference for routes from neighbor 171.69.232.55. All the routes that match community list 1 get the MED set to 8000. This includes any routes that have the communities "100 200 300" or "900 901." These routes could have other community values also.
All the routes that pass community list 2 get the local preference set to 500. This includes the routes that have community values 88 or 90. If they belong to any other community, they will not be matched by community list 2.
All the routes that match community list 3 get the local-preference set to 50. Community list 3 will match all the routes because all the routes are members of the Internet community. Thus, all the remaining routes from neighbor 171.69.232.55 get a local preference 50.
The next two examples show how BGP community attributes are used with BGP confederation configurations to filter routes.
The next example shows how the route map set-community is applied to the outbound updates to the neighbor 171.69.232.50 and the "local-as" community attribute is used to filter the routes. The routes that pass access list 1 have the special community attribute value "local-as." The remaining routes are advertised normally. This special community value automatically prevents the advertisement of those routes by the BGP speakers outside autonomous system 200.
The following example shows how to use the "local-as" community attribute to filter the routes. Confederation 100 contains three autonomous systems: 100, 200, and 300. For network 1.0.0.0, the route map set-local-as specifies that the advertised routes have the community attribute "local-as." These routes are not advertised to any EBGP peer outside the local autonomous system. For network 2.0.0.0, the route map set-no-export specifies that the routes advertised have the community attribute "no-export."
A route between router 6500 and router 65001 does not cross the boundary between autonomous systems within the confederation. A route between sub-autonomous systems for which router 65000 is the controlling router does not cross the boundary between the confederation and an external autonomous system, and also does not cross the boundary between sub-autonomous systems within the local autonomous system. A route to from router 65000 to router 65001 would not be acceptable for network 1.0.0.0 because it crosses the boundary between sub-autonomous systems within the confederation.
This section provides a configuration example of the BGP Conditional Advertisement feature. In the following example, the ip-address argument refers to the IP address of the neighbor, and the map1-name arguments and map2-name, refer to the names of the route maps:
neighbor{ip-address} advertise-map {map1-name} non-exist-map {map2-name}
no neighbor{ip-address} advertise-map {map1-name} non-exist-map {map2-name}
The route map associated with the non-exist-map specifies the prefix that the BGP speaker tracks. The route map associated with the advertise map specifies the prefix that is advertised when the prefix in the non-exist-map no longer exists. The prefix tracked by the BGP speaker must be present in the IP routing table for the conditional advertisement not to take place. In the following example, the router advertises 172.16.0.0/16 to its neighbor only if 192.168.7.0/24 is not present in the IP routing table.
To conditionally advertise a set of routes, use the following commands in router configuration mode:
The following is a sample configuration that shows several peers in a confederation. The confederation consists of three internal autonomous systems with autonomous system numbers 6001, 6002, and 6003. To the BGP speakers outside the confederation, the confederation looks like a normal autonomous system with autonomous system number 666 (specified via the bgp confederation identifier command).
In a BGP speaker in autonomous system 6001, the bgp confederation peers command marks the peers from autonomous systems 6002 and 6003 as special EBGP peers. Hence peers 171.69.232.55 and 171.69.232.56 will get the local-preference, next-hop and MED unmodified in the updates. The router at 160.69.69.1 is a normal EBGP speaker and the updates received by it from this peer will be just like a normal EBGP update from a peer in autonomous system 666.
In a BGP speaker in autonomous system 6002, the peers from autonomous systems 6001 and 6003 are configured as special EBGP peers. 170.70.70.1 is a normal IBGP peer and 199.99.99.2 is a normal EBGP peer from autonomous system 700.
In a BGP speaker in autonomous system 6003, the peers from autonomous systems 6001 and 6002 are configured as special EBGP peers. 200.200.200.200.200 is a normal EBGP peer from autonomous system 701.
The following is a part of the configuration from the BGP speaker 200.200.200.205 from autonomous system 701 in the same example. Neighbor 171.69.232.56 is configured as a normal EBGP speaker from autonomous system 666. The internal division of the autonomous system into multiple autonomous systems is not known to the peers external to the confederation.
For examples of how the BGP set-community command can be used with a confederation configuration, see the last two examples in "BGP Community with Route Maps Examples" in this chapter.
This section contains an IBGP peer group example and an EBGP peer group example.
The following example shows how the peer group named internal configures the members of the peer group to be IBGP neighbors. By definition, this is an IBGP peer group because the router bgp command and the neighbor remote-as command indicate the same autonomous system (in this case, AS 100). All the peer group members use loopback 0 as the update source and use set-med as the outbound route-map. The example also shows that, except for the neighbor at address 171.69.232.55, all the neighbors have filter-list 2 as the inbound filter list.
The following example shows how the peer group external-peers is defined without the neighbor remote-as command. This is what makes it an EBGP peer group. Each member of the peer group is configured with its respective autonomous system number separately. Thus, the peer group consists of members from autonomous systems 200, 300, and 400. All the peer group members have set-metric route map as an outbound route map and filter-list 99 as an outbound filter list. Except for neighbor 171.69.232.110, all have 101 as the inbound filter list.
The following example specifies that the router and its BGP peer at 145.2.2.2 invoke MD5 authentication on the TCP connection between them:
