The basic BGP config on each router looks like this:
R1#sh run | s router router bgp 1 no synchronization bgp log-neighbor-changes neighbor 2.2.2.2 remote-as 23 neighbor 2.2.2.2 ebgp-multihop 2 neighbor 2.2.2.2 update-source Loopback0 neighbor 3.3.3.3 remote-as 23 neighbor 3.3.3.3 ebgp-multihop 2 neighbor 3.3.3.3 update-source Loopback0 no auto-summary R2#sh run | s router router bgp 23 no synchronization bgp log-neighbor-changes neighbor 1.1.1.1 remote-as 1 neighbor 1.1.1.1 ebgp-multihop 2 neighbor 1.1.1.1 update-source Loopback0 neighbor 3.3.3.3 remote-as 23 neighbor 3.3.3.3 ebgp-multihop 2 neighbor 3.3.3.3 update-source Loopback0 no auto-summary R3#sh run | s router router bgp 23 no synchronization bgp log-neighbor-changes neighbor 1.1.1.1 remote-as 1 neighbor 1.1.1.1 ebgp-multihop 2 neighbor 1.1.1.1 update-source Loopback0 neighbor 2.2.2.2 remote-as 23 neighbor 2.2.2.2 ebgp-multihop 2 neighbor 2.2.2.2 update-source Loopback0 no auto-summary
We’ve now got peering between all three routers.
R1#sh ip bgp neighbors | i BGP BGP neighbor is 2.2.2.2, remote AS 23 external link BGP version 4, remote router ID 2.2.2.2 BGP state = Established, up for 05:17:02 BGP table version 1, neighbor version 1/0 External BGP neighbor may be up to 2 hops away. BGP neighbor is 3.3.3.3, remote AS 23 external link BGP version 4, remote router ID 3.3.3.3 BGP state = Established, up for 05:16:57 BGP table version 1, neighbor version 1/0 External BGP neighbor may be up to 2 hops away. R1#sh ip route {output omitted} Gateway of last resort is not set 1.0.0.0/32 is subnetted, 1 subnets C 1.1.1.1 is directly connected, Loopback0 2.0.0.0/32 is subnetted, 1 subnets S 2.2.2.2 [1/0] via 10.0.12.2 3.0.0.0/32 is subnetted, 1 subnets S 3.3.3.3 [1/0] via 10.0.13.3 10.0.0.0/29 is subnetted, 2 subnets C 10.0.12.0 is directly connected, FastEthernet0/0 C 10.0.13.0 is directly connected, FastEthernet1/0
There are two main methods of advertising routes into BGP – network statements and redistribution.
Network Statements
On R2 we will use a network statement to advertise the 192.168.2.0 /24 network into BGP (note that bgp network statements use subnet masks rather than wildcard masks):
R2(config)#router bgp 23 R2(config-router)#network 192.168.2.0 mask 255.255.255.0
If we check R1′s BGP table we see that it has learned of 192.168.2.0/24 route, also that the best route is via R2, and that the path is through AS23:
R1#sh ip bgp
BGP table version is 36, local router ID is 1.1.1.1
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete
Network Next Hop Metric LocPrf Weight Path
* 192.168.2.0 3.3.3.3 0 23 i
*> 2.2.2.2 0 0 23 i
The best route is now also in R1′s routing table (note the administrative distance of 20 for external BGP):
R1#sh ip route bgp
B 192.168.2.0/24 [20/0] via 2.2.2.2, 00:25:55
Redistribution
On R3 we will redistribute connected networks so that we advertise the 192.168.3.0 /24 network into BGP:
R3(config)#router bgp 23 R3(config-router)#redistribute connected
This works but is a little clumsy. We can now see the 192.168.3.0 /24 network in R1′s routing table, but we also see the 10.0.23.0 /29 transit network between R2 and R3:
R1#show ip route bgp
10.0.0.0/29 is subnetted, 3 subnets
B 10.0.23.0 [20/0] via 3.3.3.3, 00:01:22
B 192.168.2.0/24 [20/0] via 2.2.2.2, 00:20:27
B 192.168.3.0/24 [20/0] via 3.3.3.3, 00:01:22
Plus we also get some RIB failure messages relating to R3′s other connected networks:
R1#sh ip bgp
BGP table version is 8, local router ID is 1.1.1.1
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete
Network Next Hop Metric LocPrf Weight Path
r 3.3.3.3/32 2.2.2.2 0 23 ?
r> 3.3.3.3 0 0 23 ?
r 10.0.13.0/29 2.2.2.2 0 23 ?
r> 3.3.3.3 0 0 23 ?
* 10.0.23.0/29 2.2.2.2 0 23 ?
*> 3.3.3.3 0 0 23 ?
* 192.168.2.0 3.3.3.3 0 23 i
*> 2.2.2.2 0 0 23 i
* 192.168.3.0 2.2.2.2 0 23 ?
*> 3.3.3.3 0 0 23 ?
This is because R1 already has a static route to 3.3.3.3/32 and is directly connected to 10.0.13.0/29.
To tidy this up we’ll filter routes using a route map that references an access list to match only the 192.168.3.0/24 network:
R3(config)#route-map R3RouteMap permit 10 R3(config-route-map)#match ip address 10 R3(config)#access-list 10 permit 192.168.3.0 0.0.0.255 R3(config)#router bgp 23 R3(config-router)#redistribute connected route-map R3RouteMap
Now R1 only learns the route to 192.168.3.0/24 from R3, as the route map is preventing the other networks from being advertised:
R1#sh ip bgp
BGP table version is 17, local router ID is 1.1.1.1
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete
Network Next Hop Metric LocPrf Weight Path
* 192.168.2.0 3.3.3.3 0 23 i
*> 2.2.2.2 0 0 23 i
* 192.168.3.0 2.2.2.2 0 23 ?
*> 3.3.3.3 0 0 23 ?
R1#sh ip route bgp
B 192.168.2.0/24 [20/0] via 2.2.2.2, 02:20:54
B 192.168.3.0/24 [20/0] via 3.3.3.3, 02:16:40
