Networking with Cisco

I was wondering about something to write about, and I hope this is an interesting subject.
If there is anything you want me to write about, or something you wonder about or think I am mistaking about – please don’t be shy.. Just use the comment box!

Quick overlook:

Router1
ASN: 1
Prefix from OSPF: 192.168.0.0/24
IP for BGP: 172.16.1.1/24

Router2
ASN: 2
IP for BGP: 172.16.1.200/24

Verify OSPF route
Router1#sh ip route | include ^O
O E2 192.168.0.0/24 [110/20] via 10.0.10.2, 00:02:10, FastEthernet0/0

Redistribute OSPF route to BGP table with a community
I created a prefix-list to match the prefixes in the route-map:
ip prefix-list ourPrefixes seq 5 permit 192.168.0.0/24

Then I went on and created the route-map that matches this prefix-list and set the community 1:150 (65686)
route-map ospfTag permit 10
match ip address prefix-list ourPrefixes
set community 65686

Then I did redistribution of OSPF into BGP with this command (in config-router (bgp configuration)):
redistribute ospf 1 route-map ospfTag

So I go on and verify the prefix is in the BGP table with the right community:
Router1#sh ip bgp 192.168.0.0/24 | i Comm
Community: 65686

Perfect!  Now I went on to create a community list for matching the communities in a route-map
ip community-list 1 permit 1:150

As you can see, the router converted this number to the long format number again for me:
Router1(config)#do sh run | i community-list
ip community-list 1 permit 65686

Current announcement to Router2:
Router2(config)#do sh ip bgp | i \*\>
*> 10.20.30.0/24    172.16.1.1               0             0 1 ?
*> 192.168.0.0      172.16.1.1               0             0 1 ?

So far, so good!   The 10.20.30.0/24 network is added as a twist, and it should disappear when the route-map outbound is working!  It is my test to see if things got applied.
Then I went on to create a route-map to match with this community list:
Router1(config)#route-map communityFilter permit 10
Router1(config-route-map)# match community 1

Then I applied the route map on to the BGP peer
Router1(config-route-map)#router bgp 1
Router1(config-router)#neighbor 172.16.1.200 route-map communityFilter out

Okay, after clearing the peer, do we have one less address in BGP then?
Router2#sh ip bgp | i \*\>
*> 192.168.0.0      172.16.1.1              20             0 1 ?
Router2#

Voila!  Please use the comment box if you spot errors, this tutorial was written kind of in a jiffy!

Tags: bgp, Cisco configuration, community, ospf, redistribution, route filtering

This entry was posted on Tuesday, September 9th, 2008 at 10:31 pm and is filed under Cisco configuration. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.