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OSPF Distribute List

Hi,

I have a question regarding the new addition of gateway option in distribute list which is there in IOS 12.2(T). The Cli capture in 12000 series is as shown below:

C12000(config-router)#distribute-list ?

<1-199> IP access list number

<1300-2699> IP expanded access list number

gateway Filtering incoming updates based on gateway

prefix Filter prefixes in routing updates

If anyone has used this in their networks , I would appreciate if you can explain the scenario in which it is used.

Thanks in advance

4 REPLIES
Cisco Employee

Re: OSPF Distribute List

Hi Sudha,

This is a generic options for all the routing protocols not just OSPF. Gateway is the ip address of the neighbor whom you receive a routing update from. This term make more sense in RIP and IGRP. Incase of OSPF its the router ID of the neighbor.

Lets say you want to block full or partial routing update from a neighbor on a broadcast segment like ethernet. If you do passive interface in case of OSPF then it will affect all the neighbors on that segment so one option there is to use gateway with distribute-list.

Note, this option is only valid for inbound distribute-list. Outbound distribute-list will not work and it does not make sense, thats why its not supported.

Hope it helps,

Faraz

Re: OSPF Distribute List

Hi Faraz,

Thanks a lot!!!!! It was really good information. Do you know some information about database-filter. My understanding is that if it filters all outgoing lsa's then there will be inconsistancy in the ospf database, how is this taken care of.

Sudha

Cisco Employee

Re: OSPF Distribute List

Hi Sudha,

Database-filer is used for flood supression on redundant link. It will not cause inconsistency because you will be learning the database via other redundant links. You actually exchange the empty DBD packets across the link with database-filter command enabled. There is a nice explaination under CSCdk03136 release notes.

Hope it helps,

Faraz

Re: OSPF Distribute List

Hi,

In the options given below :

c7202(config-router)#distribute-list gateway aaa ?

in Filter incoming routing updates

out Filter outgoing routing updates

c7202(config-router)#distribute-list gateway aaa in ?

Async Async interface

BVI Bridge-Group Virtual Interface

CTunnel CTunnel interface

Dialer Dialer interface

FastEthernet FastEthernet IEEE 802.3

Lex Lex interface

Loopback Loopback interface

Multilink Multilink-group interface

Null Null interface

POS Packet over Sonet

Port-channel Ethernet Channel of interfaces

Tunnel Tunnel interface

Vif PGM Multicast Host interface

Virtual-Template Virtual Template interface

Virtual-TokenRing Virtual TokenRing

XTagATM Extended Tag ATM interface

Can anyone tell me, how to specify that a particular neighbour is the gateway is there any other configurations for that . Also there is an option to configure prefix list both after and before gateway , what will be the affect of this in case I configure under ospf.

Thanks in anticipation

Sudha

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