Prefix-based outbound route filtering is used so a local router can tell it's peer what routes it should send/filter. This prevents unnecessary resources from being used. There is no sense in a router sending a bunch of route updates, if they are only going to get filtered anyway.
In this example we have EBGP peers R4 and R3:
[R4]---[R3]
R3 is receiving a bunch of routes from R4:
R3#show ip bgp Network Next Hop Metric LocPrf Weight Path
*> 1.0.0.0/24 172.12.34.4 0 400 65000 i
*> 1.0.1.0/24 172.12.34.4 0 400 65000 i
*> 1.0.2.0/24 172.12.34.4 0 400 65000 i
*> 1.0.3.0/24 172.12.34.4 0 400 65000 i
*> 2.0.0.0/24 172.12.34.4 0 400 65000 i
*> 2.0.1.0/24 172.12.34.4 0 400 65000 i
*> 2.0.2.0/24 172.12.34.4 0 400 65000 i
*> 2.0.3.0/24 172.12.34.4 0 400 65000 i
*> 3.3.3.0/24 0.0.0.0 0 32768 i
*> 4.0.0.0/24 172.12.34.4 0 0 400 i
*> 4.0.1.0/24 172.12.34.4 0 0 400 i
*> 4.0.2.0/24 172.12.34.4 0 0 400 i
*> 4.0.3.0/24 172.12.34.4 0 0 400 i
R3 only wants to receive 3 routes:
1.0.0.0/24
2.0.0.0/24
4.0.0.0/24
R3 can create a prefix-list allowing these 3 routes only and advertise this to R4. R4 will use this list as a outbound filter. Let's configure it. First you need enable the advertisement of the orf capability. R3 is the one sending the prefix-list so use the send keyword. R4 is receiving the prefix-list.
R3(config)#router bgp 65003
R3(config-router)#neighbor 172.12.34.4 capability orf prefix-list send
R4(config)#router bgp 400
R4(config-router)#neighbor 172.12.34.3 capability orf prefix-list receive
Now configure the prefix-list and apply it to the neighbor:
R3(config)#ip prefix-list ZERO seq 5 permit 1.0.0.0/24
R3(config)#ip prefix-list ZERO seq 10 permit 2.0.0.0/24
R3(config)#ip prefix-list ZERO seq 15 permit 4.0.0.0/24
R3(config)#router bgp 65003
R3(config-router)#neighbor 172.12.34.4 prefix-list ZERO in
R3#clear ip bgp * soft in prefix-filter
Here is the final result:
R3#show ip bgp
Network Next Hop Metric LocPrf Weight Path
*> 1.0.0.0/24 172.12.34.4 0 400 65000 i
*> 2.0.0.0/24 172.12.34.4 0 400 65000 i
*> 3.3.3.0/24 0.0.0.0 0 32768 i
*> 4.0.0.0/24 172.12.34.4 0 0 400 i
Here are some captures I took in dynamips. The first shows the advertisement of the orf capability. The second shows the actually prefix-list R3 is sending. Wireshark shows this as "route-refresh" message. Pretty cool, eh?
Restrictions:
I used the bgp upgrade-cli command to configure these neighbors in AF mode.
Also, prefix-lists must be used, not ACL or distribute lists
In this example we have EBGP peers R4 and R3:
[R4]---[R3]
R3 is receiving a bunch of routes from R4:
R3#show ip bgp Network Next Hop Metric LocPrf Weight Path
*> 1.0.0.0/24 172.12.34.4 0 400 65000 i
*> 1.0.1.0/24 172.12.34.4 0 400 65000 i
*> 1.0.2.0/24 172.12.34.4 0 400 65000 i
*> 1.0.3.0/24 172.12.34.4 0 400 65000 i
*> 2.0.0.0/24 172.12.34.4 0 400 65000 i
*> 2.0.1.0/24 172.12.34.4 0 400 65000 i
*> 2.0.2.0/24 172.12.34.4 0 400 65000 i
*> 2.0.3.0/24 172.12.34.4 0 400 65000 i
*> 3.3.3.0/24 0.0.0.0 0 32768 i
*> 4.0.0.0/24 172.12.34.4 0 0 400 i
*> 4.0.1.0/24 172.12.34.4 0 0 400 i
*> 4.0.2.0/24 172.12.34.4 0 0 400 i
*> 4.0.3.0/24 172.12.34.4 0 0 400 i
R3 only wants to receive 3 routes:
1.0.0.0/24
2.0.0.0/24
4.0.0.0/24
R3 can create a prefix-list allowing these 3 routes only and advertise this to R4. R4 will use this list as a outbound filter. Let's configure it. First you need enable the advertisement of the orf capability. R3 is the one sending the prefix-list so use the send keyword. R4 is receiving the prefix-list.
R3(config)#router bgp 65003
R3(config-router)#neighbor 172.12.34.4 capability orf prefix-list send
R4(config)#router bgp 400
R4(config-router)#neighbor 172.12.34.3 capability orf prefix-list receive
Now configure the prefix-list and apply it to the neighbor:
R3(config)#ip prefix-list ZERO seq 5 permit 1.0.0.0/24
R3(config)#ip prefix-list ZERO seq 10 permit 2.0.0.0/24
R3(config)#ip prefix-list ZERO seq 15 permit 4.0.0.0/24
R3(config)#router bgp 65003
R3(config-router)#neighbor 172.12.34.4 prefix-list ZERO in
R3#clear ip bgp * soft in prefix-filter
Here is the final result:
R3#show ip bgp
Network Next Hop Metric LocPrf Weight Path
*> 1.0.0.0/24 172.12.34.4 0 400 65000 i
*> 2.0.0.0/24 172.12.34.4 0 400 65000 i
*> 3.3.3.0/24 0.0.0.0 0 32768 i
*> 4.0.0.0/24 172.12.34.4 0 0 400 i
Here are some captures I took in dynamips. The first shows the advertisement of the orf capability. The second shows the actually prefix-list R3 is sending. Wireshark shows this as "route-refresh" message. Pretty cool, eh?
Restrictions:
I used the bgp upgrade-cli command to configure these neighbors in AF mode.
Also, prefix-lists must be used, not ACL or distribute lists
