OSPF Limit on Number of Redistributed Routes

Open Shortest Path First (OSPF) supports a user-defined maximum number of prefixes (routes) that are allowed to be redistributed into OSPF from other protocols or other OSPF processes. Such a limit could help prevent the router from being flooded by too many redistributed routes.

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Prerequisites for OSPF Limit on Number of Redistributed Routes

It is presumed that you have OSPF configured in your network, along with another protocol or another OSPF process you are redistributing.

Information About OSPF Limit on Number of Redistributed Routes

If large number of IP routes are sent into OSPF by redistributing Border Gateway Protocol (BGP) into OSPF, the network can be severely flooded. Limiting the number of redistributed routes prevents this potential problem.

OSPF can receive and accept packets from non-routable addresses (for example, 0.0.0.0/7) also.

How to Configure OSPF Limit the Number of OSPF Redistributed Routes

This section contains the following procedures, which are mutually exclusive. That is, you cannot both limit redistributed prefixes and also choose to be warned.

Limiting the Number of OSPF Redistributed Routes

This task describes how to limit the number of OSPF redistributed routes. If the number of redistributed routes reaches the maximum value configured, no more routes will be redistributed.

The redistribution limit applies to all IP redistributed prefixes, including summarized ones. The redistribution limit does not apply to default routes or prefixes that are generated as a result of Type-7 to Type-5 translation.

SUMMARY STEPS

  1. enable
  2. configure terminal
  3. router ospf process-id
  4. redistribute protocol [process-id ] [as-number ] [metric metric-value ] [metric-type type-value ] [match {internal | external 1 | external 2 }] [tag tag-value ] [route-map map-tag ] [subnets ]
  5. redistribute maximum-prefix maximum [threshold ]
  6. end
  7. show ip ospf [process-id ]

DETAILED STEPS

  Command or Action Purpose
Step 1

enable

Example:


Router> enable

Enables privileged EXEC mode.

  • Enter your password if prompted.

Step 2

configure terminal

Example:


Router# configure terminal

Enters global configuration mode.

Step 3

router ospf process-id

Example:


Router(config)# router ospf 1

Configures an OSPF routing process.

Step 4

redistribute protocol [process-id ] [as-number ] [metric metric-value ] [metric-type type-value ] [match {internal | external 1 | external 2 }] [tag tag-value ] [route-map map-tag ] [subnets ]

Example:


Router(config-router)# redistribute eigrp 10

Redistributes routes from one routing domain into another routing domain.

Step 5

redistribute maximum-prefix maximum [threshold ]

Example:


Router(config-router)# redistribute maximum-prefix 100 80

Sets a maximum number of IP prefixes that are allowed to be redistributed into OSPF.

  • There is no default value for the maximum argument.

  • The threshold value defaults to 75 percent.

Note 

If the warning-only keyword had been configured in this command, no limit would be enforced; a warning message is simply logged.

Step 6

end

Example:


Router(config-router)# end

Exits router configuration mode.

Step 7

show ip ospf [process-id ]

Example:


Router# show ip ospf 1

(Optional) Displays general information about OSPF routing processes.

  • If a redistribution limit was configured, the output will include the maximum limit of redistributed prefixes and the threshold for warning messages.

Requesting a Warning About the Number of Routes Redistributed into OSPF

SUMMARY STEPS

  1. enable
  2. configure terminal
  3. router ospf process-id
  4. redistribute protocol [process-id ] [as-number ] [metric metric-value ] [metric-type type-value ] [match {internal | external 1 | external 2 }] [tag tag-value ] [route-map map-tag ] [subnets ]
  5. redistribute maximum-prefix maximum [threshold ] warning-only
  6. end

DETAILED STEPS

  Command or Action Purpose
Step 1

enable

Example:


Router> enable

Enables privileged EXEC mode.

  • Enter your password if prompted.

Step 2

configure terminal

Example:


Router# configure terminal

Enters global configuration mode.

Step 3

router ospf process-id

Example:


Router(config)# router ospf 1

Configures an OSPF routing process.

Step 4

redistribute protocol [process-id ] [as-number ] [metric metric-value ] [metric-type type-value ] [match {internal | external 1 | external 2 }] [tag tag-value ] [route-map map-tag ] [subnets ]

Example:


Router(config-router)# redistribute eigrp 10

Redistributes routes from one routing domain into another routing domain.

Note 

The redistribution count applies to external IP prefixes, including summarized routes. Default routes and prefixes that are generated as a result of Type-7 to Type-5 translation are not considered.

Step 5

redistribute maximum-prefix maximum [threshold ] warning-only

Example:


Router(config-router)# redistribute maximum-prefix 1000 80 warning-only

Causes a warning message to be logged when the maximum number of IP prefixes has been redistributed into OSPF.

  • Because the warning-only keyword is included, no limit is imposed on the number of redistributed prefixes into OSPF.

  • There is no default value for the maximum argument.

  • The threshold value defaults to 75 percent.

  • This example causes two warnings: one at 80 percent of 1000 (800 routes redistributed) and another at 1000 routes redistributed.

Step 6

end

Example:


Router(config-router)# end

Exits router configuration mode.

Configuration Examples for OSPF Limit on Number of Redistributed Routes

Example OSPF Limit on Number of Redistributed Routes

This example sets a maximum of 1200 prefixes that can be redistributed into OSPF process 1. Prior to reaching the limit, when the number of prefixes redistributed reaches 80 percent of 1200 (960 prefixes), a warning message is logged. Another warning is logged when the limit is reached and no more routes are redistributed. 


router ospf 1
 router-id 10.0.0.1
 domain-id 5.6.7.8
 log-adjacency-changes
 timers lsa-interval 2
 network 10.0.0.1 0.0.0.0 area 0
 network 10.1.5.1 0.0.0.0 area 0
 network 10.2.2.1 0.0.0.0 area 0
 redistribute static subnets
 redistribute maximum-prefix 1200 80

Example Requesting a Warning About the Number of Redistributed Routes

This example allows two warning messages to be logged, the first if the number of prefixes redistributed reaches 85 percent of 600 (510 prefixes), and the second if the number of redistributed routes reaches 600. However, the number of redistributed routes is not limited. 


router ospf 1
 network 10.0.0.0 0.0.0.255 area 0
 redistribute eigrp 10 subnets
 redistribute maximum-prefix 600 85 warning-only

Additional References

Related Documents

Related Topic

Document Title

OSPF commands

Cisco IOS IP Routing: OSPF Command Reference

Standards

Standards

Title

No new or modified standards are supported by this feature, and support for existing standards has not been modified by this feature.

--

MIBs

MIBs

MIBs Link

None

To locate and download MIBs for selected platforms, Cisco software releases, and feature sets, use Cisco MIB Locator found at the following URL:

http://www.cisco.com/go/mibs

RFCs

RFCs

Title

No new or modified RFCs are supported by this feature, and support for existing RFCs has not been modified by this feature.

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Technical Assistance

Description

Link

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http://www.cisco.com/cisco/web/support/index.html

Feature Information for OSPF Limit on Number of Redistributed Routes

The following table provides release information about the feature or features described in this module. This table lists only the software release that introduced support for a given feature in a given software release train. Unless noted otherwise, subsequent releases of that software release train also support that feature.

Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to www.cisco.com/go/cfn. An account on Cisco.com is not required.
Table 1. Feature Information for OSPF Limit on Number of Redistributed Routes

Feature Name

Releases

Feature Information

OSPF Limit on Number of Redistributed Routes

12.0(25)S 12.3(2)T 12.2(18)S 12.2(27)SBC Cisco IOS XE 3.1.0 SG

OSPF supports a user-defined maximum number of prefixes (routes) that are allowed to be redistributed into OSPF from other protocols or other OSPF processes. Such a limit could help prevent the router from being flooded by too many redistributed routes.

The following commands are introduced or modified in the feature documented in this module:

  • redistribute maximum-prefix

  • show ip ospf

  • show ip ospf database