OSPF Stub Router Advertisement
Last Updated: May 4, 2009
The OSPF Stub Router Advertisement feature allows you to bring a new router into a network without immediately routing traffic through the new router and allows you to gracefully shut down or reload a router without dropping packets that are destined for other networks.
Finding Feature Information
For the latest feature information and caveats, see the release notes for your platform and software release. To find information about the features documented in this module, and to see a list of the releases in which each feature is supported, see the "Feature Information for OSPF Stub Router Advertisement" section.
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Contents
•Information About OSPF Stub Router Advertisement
•How to Configure OSPF Stub Router Advertisement
•Configuration Examples of OSPF Stub Router Advertisement
•Additional References
•Feature Information for OSPF Stub Router Advertisement
Information About OSPF Stub Router Advertisement
Before you configure the OSPF Stub Router Advertisement feature, you should understand the following concepts:
•OSPF Stub Router Advertisement Functionality
•Maximum Metric Allows Routing Tables to Converge
•Maximum Metric Allows Graceful Shutdown of a Router
•Benefits of OSPF Stub Router Advertisement
OSPF Stub Router Advertisement Functionality
The OSPF Stub Router Advertisement feature allows you to bring a new router into a network without immediately routing traffic through the new router and allows you to gracefully shut down or reload a router without dropping packets that are destined for other networks. This feature introduces three configuration options that allow you to configure a router that is running the Open Shortest Path First (OSPF) protocol to advertise a maximum or infinite metric to all neighbors.
When any of these three configuration options are enabled on a router, the router will originate link-state advertisements (LSAs) with a maximum metric (LSInfinity: 0xFFFF) through all nonstub links. The advertisement of a maximum metric causes other routers to assign a cost to the new router that is higher than the cost of using an alternate path. Because of the high cost assigned to paths that pass through the new router, other routers will not use a path through the new router as a transit path to forward traffic that is destined for other networks, which allows switching and routing functions to be up and running and routing tables to converge before transit traffic is routed through this router.
Note Directly connected links in a stub network are not affected by the configuration of a maximum or infinite metric because the cost of a stub link is always set to the output interface cost.
Maximum Metric Allows Routing Tables to Converge
Two configuration options introduced by the OSPF Stub Router Advertisement feature allow you to bring a new router into a network without immediately routing traffic through the new router. These configuration options are useful because Interior Gateway Protocols (IGPs) converge very quickly upon a router during startup or after a reload, often before Border Gateway Protocol (BGP) routing tables have completely converged. If neighbor routers forward traffic through a router while that router is building BGP routing tables, packets that have been received for other destinations may be dropped. Advertising a maximum metric during startup will allow routing tables to converge before traffic that is destined for other networks is sent through the router.
The following two configuration options enable a router to advertise a maximum metric at startup:
•You can configure a timer to advertise a maximum metric when the router is started or reloaded. When this option is configured, the router will advertise a maximum metric, which forces neighbor routers to select alternate paths until the timer expires. When the timer expires, the router will advertise accurate (normal) metrics, and other routers will send traffic to this router depending on the cost. The configurable range of the timer is from 5 to 86,400 seconds.
•You can configure a router to advertise a maximum metric at startup until BGP routing tables converge or until the default timer expires (600 seconds). Once BGP routing tables converge or the default timer expires, the router will advertise accurate (normal) metrics and other routers will send traffic to this router, depending on the cost.
Maximum Metric Allows Graceful Shutdown of a Router
The third configuration option introduced by the OSPF Stub Router Advertisement feature allows you to gracefully remove a router from the network by advertising a maximum metric through all links, which allows other routers to select alternate paths for transit traffic to follow before the router is shut down. There are many situations where you may need to remove a router from the network. If a router is removed from a network and neighbor routers cannot detect that the physical interface is down, neighbors will need to wait for dead timers to expire before the neighbors will remove the adjacency and routing tables will reconverge. This situation may occur when there is a switch between other routers and the router that is shut down. Packets may be dropped while the neighbor routing tables reconverge.
When this third option is configured, the router advertises a maximum metric, which allows neighbor routers to select alternate paths before the router is shut down. This configuration option could also be used to remove a router that is in a critical condition from the network without affecting traffic that is destined for other networks.
Note You should not save the running configuration of a router when it is configured for a graceful shutdown because the router will continue to advertise a maximum metric after it is reloaded.
Benefits of OSPF Stub Router Advertisement
Improved Stability and Availability
Advertising a maximum metric through all links at startup or during a reload will prevent neighbor routers from using a path through the router as a transit path, thereby reducing the number of packets that are dropped and improving the stability and availability of the network.
Graceful Removal from the Network
Advertising a maximum metric before shutdown allows other routers to select alternate paths before the transit path through a router becomes inaccessible.
How to Configure OSPF Stub Router Advertisement
The following tasks configure OSPF to advertise a maximum metric. This feature has three different configuration options. All tasks are optional and should be individually configured.
•Configuring Advertisement on Startup (optional)
•Configuring Advertisement Until Routing Tables Converge (optional)
•Configuring Advertisement for a Graceful Shutdown (optional)
•Verifying the Advertisement of a Maximum Metric (optional)
•Monitoring and Maintaining OSPF Stub Router Advertisement (optional)
Configuring Advertisement on Startup
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Step 1 |
Router(config)# router ospf process-id |
Places the router in router configuration mode and enables an OSPF routing process. |
Step 2 |
Router(config-router)# max-metric router-lsa on-startup announce-time |
Configures OSPF to advertise a maximum metric during startup for a configured period of time. The announce-time argument is a configurable timer that must follow the on-startup keyword to be configured. There is no default timer value. The configurable time range is from 5 to 86,400 seconds. |
Configuring Advertisement Until Routing Tables Converge
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Step 1 |
Router(config)# router ospf process-id |
Places the router in router configuration mode and enables an OSPF routing process. |
Step 2 |
Router(config-router)# max-metric router-lsa on-startup wait-for-bgp |
Configures OSPF to advertise a maximum metric until BGP routing tables have converged or until the default timer has expired. The wait-for-bgp keyword must follow the on-startup keyword to be configured. The default timer value is 600 seconds. |
Configuring Advertisement for a Graceful Shutdown
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Step 1 |
Router(config)# router ospf process-id |
Places the router in router configuration mode and enables an OSPF routing process. |
Step 2 |
Router(config-router)# max-metric router-lsa |
Configures OSPF to advertise a maximum metric until the router is shut down. |
Step 3 |
Router(config-router)# end |
Ends configuration mode and places the router in privileged EXEC mode. |
Step 4 |
Router# show ip ospf |
Displays general information about OSPF routing processes. •Use the show ip ospf command to verify that the max-metric router-lsa command has been enabled before the router is shut down or reloaded. |
Note Do not save the running configuration of a router when it is configured for a graceful shutdown because the router will continue to advertise a maximum metric after it is reloaded.
Verifying the Advertisement of a Maximum Metric
To verify that the advertisement of a maximum metric has been configured correctly, use the show ip ospf or show ip ospf database command.
The output of the show ip ospf command will display the condition, state, and remaining time delay of the advertisement of a maximum metric, depending on which options were configured with the max-metric router-lsa command.
The following sample output is similar to the output that will be displayed when the on-startup keyword and announce-time argument are configured with the max-metric router-lsa command:
Routing Process "ospf 1998" with ID 10.18.134.155
Supports only single TOS(TOS0) routes
It is an area border and autonomous system boundary router
Redistributing External Routes from,
static, includes subnets in redistribution
Originating router-LSAs with maximum metric, Time remaining: 00:01:18
Condition: on startup for 300 seconds, State: active
SPF schedule delay 5 secs, Hold time between two SPFs 10 secs
Minimum LSA interval 5 secs. Minimum LSA arrival 1 secs
Number of external LSA 7. Checksum Sum 0x47261
Number of opaque AS LSA 0. Checksum Sum 0x0
Number of DCbitless external and opaque AS LSA 0
Number of DoNotAge external and opaque AS LSA 0
Number of areas in this router is 2. 1 normal 0 stub 1 nssa
External flood list length 0
Number of interfaces in this area is 1
Area has no authentication
SPF algorithm executed 3 times
Number of LSA 8. Checksum Sum 0x474AE
Number of opaque link LSA 0. Checksum Sum 0x0
The following sample output is similar to the output that will be displayed when the on-startup and wait-for-bgp keywords are configured with the max-metric router-lsa command:
Routing Process "ospf 1998" with ID 10.18.134.155
Supports only single TOS(TOS0) routes
It is an area border and autonomous system boundary router
Redistributing External Routes from,
static, includes subnets in redistribution
Originating router-LSAs with maximum metric, Time remaining: 00:01:18
Condition: on startup while BGP is converging, State: active
SPF schedule delay 5 secs, Hold time between two SPFs 10 secs
Minimum LSA interval 5 secs. Minimum LSA arrival 1 secs
Number of external LSA 7. Checksum Sum 0x47261
Number of opaque AS LSA 0. Checksum Sum 0x0
Number of DCbitless external and opaque AS LSA 0
Number of DoNotAge external and opaque AS LSA 0
Number of areas in this router is 2. 1 normal 0 stub 1 nssa
External flood list length 0
Number of interfaces in this area is 1
Area has no authentication
SPF algorithm executed 3 times
Number of LSA 8. Checksum Sum 0x474AE
Number of opaque link LSA 0. Checksum Sum 0x0
The following sample output is similar to the output that will be displayed when the max-metric router-lsa command is configured without any keywords or arguments:
Routing Process "ospf 1998" with ID 10.18.134.155
Supports only single TOS(TOS0) routes
It is an area border and autonomous system boundary router
Redistributing External Routes from,
static, includes subnets in redistribution
Originating router-LSAs with maximum metric
Condition: always, State: active
SPF schedule delay 5 secs, Hold time between two SPFs 10 secs
Minimum LSA interval 5 secs. Minimum LSA arrival 1 secs
Number of external LSA 7. Checksum Sum 0x47261
Number of opaque AS LSA 0. Checksum Sum 0x0
Number of DCbitless external and opaque AS LSA 0
Number of DoNotAge external and opaque AS LSA 0
Number of areas in this router is 2. 1 normal 0 stub 1 nssa
External flood list length 0
Number of interfaces in this area is 1
Area has no authentication
SPF algorithm executed 3 times
Number of LSA 8. Checksum Sum 0x474AE
Number of opaque link LSA 0. Checksum Sum 0x0
The output of the show ip ospf database command will display information about OSPF LSAs and indicate if the router is announcing maximum cost links. The following sample output is similar to the output that will be displayed when any form of the max-metric router-lsa command is configured:
Router# show ip ospf database
Exception Flag: Announcing maximum link costs
Options: (No TOS-capability, DC)
Link State ID: 172.18.134.155
Advertising Router: 172.18.134.155
Link connected to: a Transit Network
(Link ID) Designated Router address: 192.168.1.11
(Link Data) Router Interface address: 192.168.1.14
TOS 0 Metrics: 65535 (metric used for local calculation: 10)
Link connected to: a Transit Network
(Link ID) Designated Router address: 10.1.145.11
(Link Data) Router Interface address: 10.1.145.14
TOS 0 Metrics: 65535 (metric used for local calculation: 10)
Link connected to: a Stub Network
(Link ID) Network/subnet number: 10.11.12.0
(Link Data) Network Mask: 255.255.255.0
Monitoring and Maintaining OSPF Stub Router Advertisement
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Router# show ip ospf |
Displays general information about OSPF routing processes and provides information about the configuration settings and status of the OSPF Stub Router Advertisement feature. |
Router# show ip ospf database router |
Displays information about router LSAs, and indicates if a router is announcing maximum link costs. |
Configuration Examples of OSPF Stub Router Advertisement
This section provides the following configuration examples:
•Example: Advertisement on Startup
•Example: Advertisement Until Routing Tables Converge
•Example: Graceful Shutdown
Example: Advertisement on Startup
In the following example, a router that is running OSPF is configured to advertise a maximum metric at startup for 300 seconds:
Router(config)# router ospf 100
Router(config-router)# max-metric router-lsa on-startup 300
Example: Advertisement Until Routing Tables Converge
In the following example, a router that is running OSPF is configured to advertise a maximum metric until BGP routing tables converge or until the default timer expires (600 seconds):
Router(config)# router ospf 100
Router(config-router)# max-metric router-lsa on-startup wait-for-bgp
Example: Graceful Shutdown
In the following example, a router that is running OSPF is configured to advertise a maximum metric until the router is shut down:
Router(config)# router ospf 100
Router(config-router)# max-metric router-lsa
Router(config-router)# end
Additional References
The following sections provide references related to OSPF Stub Router Advertisement.
Related Documents
Standards
MIBs
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No new or modified MIBs are supported by this feature, and support for existing MIBs has not been modified by this feature. |
To locate and download MIBs for selected platforms, Cisco IOS XE software releases , and feature sets, use Cisco MIB Locator found at the following URL: http://www.cisco.com/go/mibs |
RFCs
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RFC 3137 |
OSPF Stub Router Advertisement |
Technical Assistance
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The Cisco Support and Documentation website provides online resources to download documentation, software, and tools. Use these resources to install and configure the software and to troubleshoot and resolve technical issues with Cisco products and technologies. Access to most tools on the Cisco Support and Documentation website requires a Cisco.com user ID and password. |
http://www.cisco.com/cisco/web/support/index.html |
Feature Information for OSPF Stub Router Advertisement
Table 1 lists the release history for this feature.
Use Cisco Feature Navigator to find information about platform support and software image support. Cisco Feature Navigator enables you to determine which Cisco IOS XE software images support a specific software release, feature set, or platform. To access Cisco Feature Navigator, go to http://www.cisco.com/go/cfn. An account on Cisco.com is not required.
Note Table 1 lists only the Cisco IOS XE software release that introduced support for a given feature in a given Cisco IOS XE software release train. Unless noted otherwise, subsequent releases of that Cisco IOS XE software release train also support that feature.
Table 1 Feature Information for OSPF Stub Router Advertisement
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OSPF Stub Router Advertisement |
Cisco IOS XE Release 2.1 |
The OSPF Stub Router Advertisement feature allows you to bring a new router into a network without immediately routing traffic through the new router and allows you to gracefully shut down or reload a router without dropping packets that are destined for other networks. The following commands are introduced or modified in the feature documented in this module: •max-metric router-lsa •show ip ospf |
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