OSPF Inbound Filtering Using Route Maps with a Distribute List

The OSPF Inbound Filtering Using Route Maps with a Distribute List feature allows users to define a route map to prevent Open Shortest Path First (OSPF) routes from being added to the routing table. In the route map, the user can match on any attribute of the OSPF route.

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Prerequisites for OSPF Inbound Filtering Using Route Maps with a Distribute List

It is presumed that you have OSPF configured in your network.

Information About OSPF Inbound Filtering Using Route Maps with a Distribute List

Users can define a route map to prevent OSPF routes from being added to the routing table. This filtering happens at the moment when OSPF is installing the route in the routing table. This feature has no effect on link-state advertisement (LSA) flooding. In the route map, the user can match on any attribute of the OSPF route. That is, the route map could be based on the following match options:

  • match interface

  • match ip address

  • match ip next-hop

  • match ip route-source

  • match metric

  • match route-type

  • match tag

This feature can be useful during redistribution if the user tags prefixes when they get redistributed on Autonomous System Boundary Routers (ASBRs) and later uses the tag to filter the prefixes from being installed in the routing table on other routers.

Filtering Based on Route Tag

Users can assign tags to external routes when they are redistributed to OSPF. Then the user can deny or permit those routes in the OSPF domain by identifying that tag in the route-map and distribute-list in commands.

Filtering Based on Route Type

In OSPF, the external routes could be Type 1 or Type 2. Users can create route maps to match either Type 1 or Type 2 and then use the distribute-list in command to filter certain prefixes. Also, route maps can identify internal routes (interarea and intra-area) and then those routes can be filtered.

Filtering Based on Route Source

When a match is done on the route source, the route source represents the OSPF Router ID of the LSA originator of the LSA in which the prefix is advertised.

Filtering Based on Interface

When a match is done on the interface, the interface represents the outgoing interface for the route that OSPF is trying to install in the routing table.

Filtering Based on Next Hop

When a match is done on the next hop, the next hop represents the next hop for the route that OSPF is trying to install in the routing table.


Note

The distribute-list in command can be configured to prevent routes from being installed in the global Routing Information Base (RIB). Prior to the implementation of OSPF local RIB (for feature information on OSPF local RIB, see OSPFv2 Local RIB), OSPF would attempt to install a less preferred route (e.g. an inter-area route when the intra-area path is filtered). With OSPF local RIB, only the best route is considered (because this is the only route the local RIB maintains). There is no concept of a "second-best" OSPF route. For more information on the routing algorithm used by Cisco OSPF routers, please refer to RFC 2328.

How to Configure OSPF Inbound Filtering Using Route Maps

Configuring OSPF Route Map-Based Filtering

SUMMARY STEPS

    1.    enable

    2.    configure terminal

    3.    route-map map-tag [permit | deny] [sequence-number]

    4.    match tag tag-name

    5.    Repeat Steps 3 and 4 with other route-map and match commands.

    6.    exit

    7.    router ospf process-id

    8.    distribute-list route-map map-tag in

    9.    end


DETAILED STEPS
     Command or ActionPurpose
    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 route-map map-tag [permit | deny] [sequence-number]


    Example:
    Router(config)# route-map tag-filter deny 10
     

    Defines a route map to control filtering.

     
    Step 4 match tag tag-name


    Example:
    Router(config-router)# match tag 777
     

    Matches routes with a specified name, to be used as the route map is referenced.

    • At least one match command is required, but it need not be this matchcommand. This is just an example.

    • The list of match commands available to be used in this type of route map appears on the distribute-list in command reference page.

    • This type of route map will have no set commands.

     
    Step 5 Repeat Steps 3 and 4 with other route-map and match commands. 

    Optional.

     
    Step 6 exit


    Example:
    Router(config-router)# exit
     

    Exits router configuration mode.

     
    Step 7 router ospf process-id


    Example:
    Router(config)# router ospf 1
     

    Configures an OSPF routing process.

     
    Step 8 distribute-list route-map map-tag in


    Example:
    Router(config-router)# distribute-list route-map tag-filter in
     

    Enables filtering based on an OSPF route map.

     
    Step 9 end


    Example:
    Router(config-router)# end
     

    Exits router configuration mode.

     

    Configuration Examples for OSPF Inbound Filtering Using Route Maps with a Distribute List

    Example OSPF Route Map-Based Filtering

    In this example, OSPF external LSAs have a tag. The value of the tag is examined before the prefix is installed in the routing table. All OSPF external prefixes that have the tag value of 777 are filtered (prevented from being installed in the routing table). The permit statement with sequence number 20 has no match conditions, and there are no other route-map statements after sequence number 20, so all other conditions are permitted.

    route-map tag-filter deny 10
 match tag 777
route-map tag-filter permit 20
!
router ospf 1
 router-id 10.0.0.2
 log-adjacency-changes
 network 172.16.2.1 0.0.0.255 area 0
 distribute-list route-map tag-filter in

    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.

    --

    Technical Assistance

    Description

    Link

    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 Inbound Filtering Using Route Maps with a Distribute List

    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 . An account on Cisco.com is not required.
    Table 1 Feature Information for OSPF Inbound Filtering Using Route Maps with a Distribute List

    Feature Name

    Releases

    Feature Information

    OSPF Inbound Filterng Using Route Maps with a Distribute List

    12.0(24)S 12.2(15)T 12.2(18)S 12.2(27)SBC Cisco IOS XE 3.1.0 SG

    The OSPF Inbound Filtering Using Route Maps with a Distribute List feature allows users to define a route map to prevent Open Shortest Path First (OSPF) routes from being added to the routing table. In the route map, the user can match on any attribute of the OSPF route..

    The following command was introduced or modified: distribute-list in (IP).