Source: Cisco Systems
This chapter describes how to configure OSPF. For a complete description of the OSPF commands in this chapter, refer to the "OSPF Commands" chapter of the Network Protocols Command Reference, Part 1. To locate documentation of other commands that appear in this chapter, use the command reference master index or search online.
Open shortest path first (OSPF) is an IGP developed by the OSPF working group of the Internet Engineering Task Force (IETF). Designed expressly for IP networks, OSPF supports IP subnetting and tagging of externally derived routing information. OSPF also allows packet authentication and uses IP multicast when sending/receiving packets.
We support RFC 1253, Open Shortest Path First (OSPF) MIB, August 1991. The OSPF MIB defines an IP routing protocol that provides management information related to OSPF and is supported by Cisco routers.
For protocol-independent features that include OSPF, see the chapter "Configuring IP Routing Protocol-Independent Features" in this document.
Cisco's implementation conforms to the OSPF Version 2 specifications detailed in the Internet RFC 1583. The list that follows outlines key features supported in Cisco's OSPF implementation:
OSPF typically requires coordination among many internal routers, area border routers (routers connected to multiple areas), and autonomous system boundary routers. At a minimum, OSPF-based routers or access servers can be configured with all default parameter values, no authentication, and interfaces assigned to areas. If you intend to customize your environment, you must ensure coordinated configurations of all routers.
To configure OSPF, complete the tasks in the following sections. Enabling OSPF is mandatory; the other tasks are optional, but might be required for your application.
In addition, you can specify route redistribution; see the task "Redistribute Routing Information" in the chapter "Configuring IP Routing Protocol-Independent Features" for information on how to configure route redistribution.
As with other routing protocols, enabling OSPF requires that you create an OSPF routing process, specify the range of IP addresses to be associated with the routing process, and assign area IDs to be associated with that range of IP addresses. Use the following commands, starting in global configuration mode:
| Step | Command | Purpose |
|---|---|---|
| 1. |
Enable OSPF routing, which places you in router configuration mode. | |
| 2. |
Define an interface on which OSPF runs and define the area ID for that interface. |
Our OSPF implementation allows you to alter certain interface-specific OSPF parameters, as needed. You are not required to alter any of these parameters, but some interface parameters must be consistent across all routers in an attached network. Those parameters are controlled by the ip ospf hello-interval, ip ospf dead-interval, and ip ospf authentication-key commands. Therefore, be sure that if you do configure any of these parameters, the configurations for all routers on your network have compatible values.
In interface configuration mode, use any of the following commands to specify interface parameters as needed for your network:
OSPF classifies different media into the following three types of networks by default:
You can configure your network as either a broadcast or a nonbroadcast multiaccess network.
X.25 and Frame Relay provide an optional broadcast capability that can be configured in the map to allow OSPF to run as a broadcast network. See the x25 map and frame-relay map command descriptions in the Wide-Area Networking Command Reference for more detail.
You have the choice of configuring your OSPF network type as either broadcast or nonbroadcast multiaccess, regardless of the default media type. Using this feature, you can configure broadcast networks as nonbroadcast multiaccess networks when, for example, you have routers in your network that do not support multicast addressing. You also can configure nonbroadcast multiaccess networks (such as X.25, Frame Relay, and SMDS) as broadcast networks. This feature saves you from having to configure neighbors, as described in the section "Configure OSPF for Nonbroadcast Networks."
Configuring nonbroadcast, multiaccess networks as either broadcast or nonbroadcast assumes that there are virtual circuits from every router to every router or fully meshed network. This is not true for some cases, for example, because of cost constraints, or when you have only a partially meshed network. In these cases, you can configure the OSPF network type as a point-to-multipoint network. Routing between two routers not directly connected will go through the router that has virtual circuits to both routers. Note that it is not necessary to configure neighbors when using this feature.
An OSPF point-to-multipoint interface is defined as a numbered point-to-point interface having one or more neighbors. It creates multiple host routes. An OSPF point-to-multipoint network has the following benefits compared to nonbroadcast multiaccess and point-to-point networks:
To configure your OSPF network type, use the following command in interface configuration mode:
| Command | Purpose |
|---|---|
|
ip ospf network {broadcast | non-broadcast | {point-to-multipoint [non-broadcast] }} |
See the "OSPF Point-to-Multipoint Example" section at the end of this chapter for an example of an OSPF point-to-multipoint network.
On point-to-multipoint, broadcast networks, there is no need to specify neighbors. However, you can specify neighbors with the neighbor command, in which case you should specify a cost to that neighbor.
Before this feature, some OSPF point-to-multipoint protocol traffic was treated as multicast traffic. Therefore, the neighbor command was not needed for point-to-multipoint interfaces because multicast took care of the traffic. Hellos, updates and acknowledgments were sent using multicast. In particular, multicast hellos discovered all neighbors dynamically.
On any point-to-multipoint interface (broadcast or not), the Cisco IOS software assumed the cost to each neighbor was equal. The cost was configured with the ip ospf cost command. In reality, the bandwidth to each neighbor is different, so the cost should be different. With this feature, you can configure a separate cost to each neighbor. This feature applies to point-to-multipoint interfaces only.
To treat an interface as point-to-multipoint broadcast and assign a cost to each neighbor, use the following commands beginning in interface configuration mode:
Because there might be many routers attached to an OSPF network, a designated router is selected for the network. It is necessary to use special configuration parameters in the designated router selection if broadcast capability is not configured.
These parameters need only be configured in those devices that are themselves eligible to become the designated router or backup designated router (in other words, routers with a nonzero router priority value).
To configure routers that interconnect to nonbroadcast networks, use the following command in router configuration mode:
| Command | Purpose |
|---|---|
|
neighbor ip-address [priority number] [poll-interval seconds] |
Configure a router interconnecting to nonbroadcast networks. |
You can specify the following neighbor parameters, as required:
On point-to-multipoint, nonbroadcast networks, you now use the neighbor command to identify neighbors. Assigning a cost to a neighbor is optional.
Prior to Release 12.0, some customers were using point-to-multipoint on nonbroadcast media (such as classic IP over ATM), so their routers could not dynamically discover their neighbors. This feature allows the neighbor command to be used on point-to-multipoint interfaces.
On any point-to-multipoint interface (broadcast or not), the Cisco IOS software assumed the cost to each neighbor was equal. The cost was configured with the ip ospf cost command. In reality, the bandwidth to each neighbor is different, so the cost should be different. With this feature, you can configure a separate cost to each neighbor. This feature applies to point-to-multipoint interfaces only.
To treat the interface as point-to-multipoint when the media does not support broadcast, use the following commands beginning in interface configuration mode:
Our OSPF software allows you to configure several area parameters. These area parameters, shown in the following table, include authentication, defining stub areas, and assigning specific costs to the default summary route. Authentication allows password-based protection against unauthorized access to an area.
Stub areas are areas into which information on external routes is not sent. Instead, there is a default external route generated by the area border router, into the stub area for destinations outside the autonomous system. To take advantage of the OSPF stub area support, default routing must be used in the stub area. To further reduce the number of link state advertisements sent into a stub area, you can configure no-summary on the ABR to prevent it from sending summary link advertisement (link state advertisements Type 3) into the stub area.
In router configuration mode, specify any of the following area parameters as needed for your network:
NSSA area is similar to OSPF stub area. NSSA does not flood Type 5 external link state advertisements (LSAs) from the core into the area, but it has the ability of importing AS external routes in a limited fashion within the area.
NSSA allows importing of Type 7 AS external routes within NSSA area by redistribution. These Type 7 LSAs are translated into Type 5 LSAs by NSSA ABR which are flooded throughout the whole routing domain. Summarization and filtering are supported during the translation.
Use NSSA to simplify administration if you are an Internet service provider (ISP), or a network administrator that must connect a central site using OSPF to a remote site that is using a different routing protocol.
Prior to NSSA, the connection between the corporate site border router and the remote router could not be run as OSPF stub area because routes for the remote site cannot be redistributed into stub area. A simple protocol like RIP is usually run and handle the redistribution. This meant maintaining two routing protocols. With NSSA, you can extend OSPF to cover the remote connection by defining the area between the corporate router and the remote router as an NSSA.
In router configuration mode, use the following command to specify area parameters as needed to configure OSPF NSSA:
In router configuration mode on the ABR, use the following command to control summarization and filtering of Type 7 LSA into Type 5 LSA:
Evaluate the following considerations before implementing this feature:
If possible, avoid using explicit redistribution on NSSA ABR because confusion may result over which packets are being translated by which router.
Route summarization is the consolidation of advertised addresses. This feature causes a single summary route to be advertised to other areas by an ABR. In OSPF, an ABR will advertise networks in one area into another area. If the network numbers in an area are assigned in a way such that they are contiguous, you can configure the ABR to advertise a summary route that covers all the individual networks within the area that fall into the specified range.
To specify an address range, use the following command in router configuration mode:
When redistributing routes from other protocols into OSPF (as described in the chapter "Configuring IP Routing Protocol-Independent Features"), each route is advertised individually in an external link state advertisement (LSA). However, you can configure the Cisco IOS software to advertise a single route for all the redistributed routes that are covered by a specified network address and mask. Doing so helps decrease the size of the OSPF link state database.
To have the software advertise one summary route for all redistributed routes covered by a network address and mask, use the following command in router configuration mode:
| Command | Purpose |
|---|---|
|
Specify an address and mask that covers redistributed routes, so only one summary route is advertised. |
In OSPF, all areas must be connected to a backbone area. If there is a break in backbone continuity, or the backbone is purposefully partitioned, you can establish a virtual link. The two end points of a virtual link are Area Border Routers. The virtual link must be configured in both routers. The configuration information in each router consists of the other virtual endpoint (the other ABR), and the nonbackbone area that the two routers have in common (called the transit area). Note that virtual links cannot be configured through stub areas.
To establish a virtual link, use the following command in router configuration mode:
To display information about virtual links, use the show ip ospf virtual-links EXEC command. To display the router ID of an OSPF router, use the show ip ospf EXEC command.
You can force an autonomous system boundary router to generate a default route into an OSPF routing domain. Whenever you specifically configure redistribution of routes into an OSPF routing domain, the router automatically becomes an autonomous system boundary router. However, an autonomous system boundary router does not, by default, generate a default route into the OSPF routing domain.
To force the autonomous system boundary router to generate a default route, use the following command in router configuration mode:
See the discussion of redistribution of routes in the "Configuring IP Routing Protocol-Independent Features" chapter.
You can configure OSPF to look up Domain Naming System (DNS) names for use in all OSPF show command displays. This feature makes it easier to identify a router, because it is displayed by name rather than by its router ID or neighbor ID.
To configure DNS name lookup, use the following command in global configuration mode:
OSPF uses the largest IP address configured on the interfaces as its router ID. If the interface associated with this IP address is ever brought down, or if the address is removed, the OSPF process must recalculate a new router ID and resend all its routing information out its interfaces.
If a loopback interface is configured with an IP address, the Cisco IOS software will use this IP address as its router ID, even if other interfaces have larger IP addresses. Since loopback interfaces never go down, greater stability in the routing table is achieved.
OSPF automatically prefers a loopback interface over any other kind, and it chooses the highest IP address among all loopback interfaces. If no loopback interfaces are present, the highest IP address in the router is chosen. You cannot tell OSPF to use any particular interface.
To configure an IP address on a loopback interface, use the following commands, starting in global configuration mode:
| Step | Command | Purpose |
|---|---|---|
| 1. |
Create a loopback interface, which places you in interface configuration mode. | |
| 2. |
In Cisco IOS Release 10.3 and later, by default, OSPF calculates the OSPF metric for an interface according to the bandwidth of the interface. For example, a 64K link gets a metric of 1562, while a T1 link gets a metric of 64.
The OSPF metric is calculated as ref-bw divided by bandwidth, with ref-bw equal to 108 by default, and bandwidth determined by the bandwidth command. The calculation gives FDDI a metric of 1. If you have multiple links with high bandwidth, you might want to specify a larger number to differentiate the cost on those links. To do so, use the following command in router configuration mode:
An administrative distance is a rating of the trustworthiness of a routing information source, such as an individual router or a group of routers. Numerically, an administrative distance is an integer between 0 and 255. In general, the higher the value, the lower the trust rating. An administrative distance of 255 means the routing information source cannot be trusted at all and should be ignored.
OSPF uses three different administrative distances: intra-area, inter-area, and external. Routes within an area are intra-area; routes to another area are inter-area; and routes from another routing domain learned via redistribution are external. The default distance for each type of route is 110.
To change any of the OSPF distance values, use the following command in router configuration mode:
For an example of changing administrative distance, see the section "Changing OSPF Administrative Distance" at the end of this chapter.
Because simplex interfaces between two devices on an Ethernet represent only one network segment, for OSPF you must configure the transmitting interface to be a passive interface. This prevents OSPF from sending hello packets for the transmitting interface. Both devices are able to see each other via the hello packet generated for the receiving interface.
To configure OSPF on simplex Ethernet interfaces, use the following command in router configuration mode:
You can configure the delay time between when OSPF receives a topology change and when it starts a shortest path first (SPF) calculation. You can also configure the hold time between two consecutive SPF calculations. To do this, use the following command in router configuration mode:
The OSPF on demand circuit is an enhancement to the OSPF protocol that allows efficient operation over on demand circuits like ISDN, X.25 SVCs and dial-up lines. This feature supports RFC 1793, Extending OSPF to Support Demand Circuits.
Prior to this feature, OSPF periodic hello and link state advertisements (LSAs) updates would be exchanged between routers that connected the on demand link, even when no changes occurred in the hello or LSA information.
With this feature, periodic hellos are suppressed and the periodic refreshes of LSAs are not flooded over the demand circuit. These packets bring up the link only when they are exchanged for the first time, or when a change occurs in the information they contain. This operation allows the underlying datalink layer to be closed when the network topology is stable.
This feature is useful when you want to connect telecommuters or branch offices to an OSPF backbone at a central site. In this case, OSPF for on demand circuits allows the benefits of OSPF over the entire domain, without excess connection costs. Periodic refreshes of hello updates, LSA updates, and other protocol overhead are prevented from enabling the on demand circuit when there is no "real" data to transmit.
Overhead protocols such as hellos and LSAs are transferred over the on demand circuit only upon initial setup and when they reflect a change in the topology. This means that critical changes to the topology that require new SPF calculations are transmitted in order to maintain network topology integrity. Periodic refreshes that do not include changes, however, are not transmitted across the link.
To configure OSPF for on demand circuits, use the following commands, beginning in global configuration mode:
If the router is part of a point-to-point topology, then only one end of the demand circuit must be configured with this command. However, all routers must have this feature loaded.
If the router is part of a point-to-multipoint topology, only the multipoint end must be configured with this command.
For an example of OSPF over an on-demand circuit, see the section "OSPF over On-Demand Routing Example" at the end of this chapter.
Evaluate the following considerations before implementing this feature:
http://www.cisco.com/warp/public/104/dcprob.html#reason5
To configure the router to send a syslog message when an OSPF neighbor goes up or down, use the following command in router configuration mode:
| Command | Purpose |
|---|---|
Configure this command if you want to know about OSPF neighbors going up or down without turning on the debugging command debug ip ospf adjacency. The ospf log-adjacency-changes command provides a higher level view of such changes with less output.
The OSPF LSA group pacing feature allows the router to group together OSPF link state advertisements (LSAs) and pace the refreshing, checksumming, and aging functions. The group pacing results in more efficient use of the router.
The router groups together OSPF LSAs and paces the refreshing, checksumming, and aging functions so that sudden hits on CPU usage and network resources are avoided. This feature is most beneficial to large OSPF networks.
OSPF LSA group pacing is enabled by default. For typical customers, the default group pacing interval for refreshing, checksumming, and aging is appropriate and you need not configure this feature.
Each OSPF LSA has an age, which indicates whether the LSA is still valid. Once the LSA reaches the maximum age (one hour), it is discarded. During the aging process, the originating router sends a refresh packet every 30 minutes to refresh the LSA. Refresh packets are sent to keep the LSA from expiring, whether there has been a change in the network topology or not. Checksumming is performed on all LSAs every 10 minutes. The router keeps track of LSAs it generates and LSAs it receives from other routers. The router refreshes LSAs it generated; it ages the LSAs it received from other routers.
Prior to the LSA group pacing feature, the Cisco IOS software would perform refreshing on a single timer, and checksumming and aging on another timer. In the case of refreshing, for example, the software would scan the whole database every 30 minutes, refreshing every LSA the router generated, no matter how old it was. Figure 21 illustrates all the LSAs being refreshed at once. This process wasted CPU resources because only a small portion of the database needed to be refreshed. A large OSPF database (several thousand LSAs) could have thousands of LSAs with different ages. Refreshing on a single timer resulted in the age of all LSAs becoming synchronized, which resulted in much CPU processing at once. Furthermore, a huge number of LSAs could cause a sudden increase of network traffic, consuming a large amount of network resources in a short period of time.
This problem is solved by each LSA having its own timer. Again using the example of refreshing, each LSA gets refreshed when it is 30 minutes old, independent of other LSAs. So CPU is used only when necessary. However, LSAs being refreshed at frequent, random intervals would require many packets for the few refreshed LSAs the router must send out. That would be inefficient use of bandwidth.
Therefore, the router delays the LSA refresh function for an interval of time instead of performing it when the individual timers are reached. The accumulated LSAs constitute a group, which is then refreshed and sent out in one packet or more. Thus, the refresh packets are paced, as are the checksumming and aging. The pacing interval is configurable; it defaults to 4 minutes, which is randomized to further avoid synchronization.
Figure 22 illustrates the case of refresh packets. The first timeline illustrates individual LSA timers; the second timeline illustrates individual LSA timers with group pacing.
The group pacing interval is inversely proportional to the number of LSAs the router is refreshing, checksumming, and aging. For example, if you have approximately 10,000 LSAs, decreasing the pacing interval would benefit you. If you have a very small database (40 to 100 LSAs), increasing the pacing interval to 10 to 20 minutes might benefit you slightly.
The default value of pacing between LSA groups is 240 seconds (4 minutes). The range is 10 seconds to 1800 seconds (half an hour). To change the LSA group pacing interval, use the following command in router configuration mode:
For an example, see the section "LSA Group Pacing Example" at the end of this chapter.
By default, OSPF floods new LSAs over all interfaces in the same area, except the interface on which the LSA arrives. Some redundancy is desirable, because it ensures robust flooding. However, too much redundancy can waste bandwidth and might destabilize the network due to excessive link and CPU usage in certain topologies. An example would be a fully meshed topology.
You can block OSPF flooding of LSAs two ways, depending on the type of networks:
On broadcast, nonbroadcast, and point-to-point networks, to prevent flooding of OSPF LSAs, use the following command in interface configuration mode:
| Command | Purpose |
|---|---|
On point-to-multipoint networks, to prevent flooding of OSPF LSAs, use the following command in router configuration mode:
For an example of blocking LSA flooding, see the section "Block LSA Flooding Example" at the end of this chapter.
Cisco routers do not support LSA Type 6 (MOSPF), and they generate syslog messages if they receive such packets. If the router is receiving many MOSPF packets, you might want to configure the router to ignore the packets and thus prevent a large number of syslog messages. To do so, use the following command in router configuration mode:
| Command | Purpose |
|---|---|
|
Prevent the router from generating syslog messages when it receives MOSPF LSA packets. |
For an example of suppressing MOSPF LSA packets, see the section "Ignore MOSPF LSA Packets Example" at the end of this chapter.
You can display specific statistics such as the contents of IP 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 in EXEC mode:
The following sections provide OSPF configuration examples:
In Figure 23, Mollie uses DLCI 201 to communicate with Neon, DLCI 202 to Jelly, and DLCI 203 to Platty. Neon uses DLCI 101 to communicate with Mollie and DLCI 102 to communicate with Platty. Platty communicates with Neon (DLCI 401) and Mollie (DLCI 402). Jelly communicates with Mollie (DLCI 301).
The following example illustrates a point-to-multipoint network with broadcast:
The following example shows the configuration of the neighbor at 10.0.1.3:
The output shown for neighbors in the first configuration above looks like this:
The route information in the first configuration above looks like this:
The following example illustrates a point-to-multipoint network with nonbroadcast:
The following example is the configuration for the router on the other side:
The output shown for neighbors in the first configuration above looks like this:
OSPF, static routes, and IS-IS support variable-length subnet masks (VLSMs). With VLSMs, you can use different masks for the same network number on different interfaces, which allows you to conserve IP addresses and more efficiently use available address space.
In the following example, a 30-bit subnet mask is used, leaving two bits of address space reserved for serial line host addresses. There is sufficient host address space for two host endpoints on a point-to-point serial link.
OSPF typically requires coordination among many internal routers, area border routers, and autonomous system boundary routers. At a minimum, OSPF-based routers can be configured with all default parameter values, with no authentication, and with interfaces assigned to areas.
The following example illustrates a simple OSPF configuration that enables OSPF routing process 9000, attaches Ethernet 0 to area 0.0.0.0, and redistributes RIP into OSPF, and OSPF into RIP:
The following example illustrates the assignment of four area IDs to four IP address ranges. In the example, OSPF routing process 109 is initialized, and four OSPF areas are defined: 10.9.50.0, 2, 3, and 0. Areas 10.9.50.0, 2, and 3 mask specific address ranges, while Area 0 enables OSPF for all other networks.
Each network area router configuration command is evaluated sequentially, so the order of these commands in the configuration is important. The Cisco IOS software sequentially evaluates the address/wildcard-mask pair for each interface. See the "OSPF Commands" chapter of the Network Protocols Command Reference, Part 1 for more information.
Consider the first network area command. Area ID 10.9.50.0 is configured for the interface on which subnet 131.108.20.0 is located. Assume that a match is determined for interface Ethernet 0. Interface Ethernet 0 is attached to Area 10.9.50.0 only.
The second network area command is evaluated next. For Area 2, the same process is then applied to all interfaces (except interface Ethernet 0). Assume that a match is determined for interface Ethernet 1. OSPF is then enabled for that interface and Ethernet 1 is attached to Area 2.
This process of attaching interfaces to OSPF areas continues for all network area commands. Note that the last network area command in this example is a special case. With this command, all available interfaces (not explicitly attached to another area) are attached to Area 0.
The following example outlines a configuration for several routers within a single OSPF autonomous system. Figure 24 provides a general network map that illustrates this example configuration.
In this configuration, five routers are configured with OSPF:
Note It is not necessary to include definitions of all areas in an OSPF autonomous system in the configuration of all routers in the autonomous system. You must only define the directly connected areas. In the example that follows, routes in Area 0 are learned by the routers in Area 1 (Router A and Router B) when the area border router (Router C) injects summary link state advertisements (LSAs) into Area 1.
The OSPF domain in BGP autonomous system 109 is connected to the outside world via the BGP link to the external peer at IP address 11.0.0.6.
The following example configuration accomplishes several tasks in setting up an ABR. These tasks can be split into two general categories:
The specific tasks outlined in this configuration are detailed briefly in the following descriptions. Figure 25 illustrates the network address ranges and area assignments for the interfaces.
The basic configuration tasks in this example are as follows:
Configuration tasks associated with redistribution are as follows:
The following is an example OSPF configuration:
OSPF is on network 131.119.0.0:
IGRP autonomous system 200 is on 131.119.0.0:
The examples in this section illustrate the use of redistribution, with and without route maps. Examples from both the IP and CLNS routing protocols are given.
The following example redistributes all OSPF routes into IGRP:
The following example redistributes RIP routes with a hop count equal to 1 into OSPF. These routes will be redistributed into OSPF as external link state advertisements with a metric of 5, metric type of Type 1, and a tag equal to 1.
The following example redistributes OSPF learned routes with tag 7 as a RIP metric of 15:
The following example redistributes OSPF intra-area and interarea routes with next-hop routers on serial interface 0 into BGP with an INTER_AS metric of 5:
The following example redistributes two types of routes into the integrated IS-IS routing table (supporting both IP and CLNS). The first are OSPF external IP routes with tag 5; these are inserted into Level 2 IS-IS LSPs with a metric of 5. The second are ISO-IGRP derived CLNS prefix routes that match CLNS access list 2000. These will be redistributed into IS-IS as Level 2 LSPs with a metric of 30.
With the following configuration, OSPF external routes with tags 1, 2, 3, and 5 are redistributed into RIP with metrics of 1, 1, 5, and 5, respectively. The OSPF routes with a tag of 4 are not redistributed.
The following configuration sets the condition that if there is an OSPF route to network 140.222.0.0, generate the default network 0.0.0.0 into RIP with a metric of 1:
In the following configuration, a RIP learned route for network 160.89.0.0 and an ISO-IGRP learned route with prefix 49.0001.0002 will be redistributed into an IS-IS Level 2 LSP with a metric of 5:
The following configuration example illustrates how a route map is referenced by the default-information router configuration command. This is called conditional default origination. OSPF will originate the default route (network 0.0.0.0) with a Type 2 metric of 5 if 140.222.0.0, with network 0.0.0.0 in the routing table. Extended access-lists cannot be used in a route map for conditional default origination.
The following example changes the external distance to 200, making it less trustworthy. Figure 26 illustrates the example.
The following configuration allows OSPF over an on-demand circuit, as shown in Figure 27. Note that the on-demand circuit is defined on one side only (BRI 0 on RouterA). It is not required to be configured on both sides.
The following example changes the OSPF pacing between LSA groups to 60 seconds:
The following example prevents flooding of OSPF LSAs to broadcast, nonbroadcast, or point-to-point networks reachable through Ethernet interface 0:
The following example prevents flooding of OSPF LSAs to point-to-multipoint networks to the neighbor at IP address 1.2.3.4:
The following example configures the router to suppress the sending of syslog messages when it receives MOSPF packets: