8-9 neighbor ip-address peer-group-name ebgp-multihop maximum-hop-count

8-9 neighbor { ip-address peer- group - name } ebgp- multihop maximum- hop-count

Syntax Description:

• ip-address Neighbor's IP address.

• peer-group-name Name of the peer group. See section 8-19.

• maximum-hop-count Optional parameter with a value of 1 to 255. The default hop count is 255.

Purpose: EBGP neighbors are typically directly connected. In situations in which EBGP neighbors are not directly connected, the ebgp-multihop option must be used in order to form a neighbor relationship.

Cisco IOS Software Release: 10.0. Peer group support was added in Release 11.0.

Configuration Example: Nonconnected EBGP Neighbors

Figure 8-10 shows a situation in which we are trying to establish an EBGP connection between two routers that are not directly connected. BGP uses TCP, and Routers A and C must have routes to each other. The static routes on Routers A and C are necessary in order for BGP to establish the connection. On Router A, the default hop count of 255 is used. On Router C, a maximum hop count of 2 is used. A maximum hop count of 1 is interpreted as directly connected and therefore has no effect.

Figure 8-10. ebgp-multihop Is Used to Form an EBGP Connection Between Peers That Are Not Directly Connected

 Router A  router bgp 1   neighbor 10.1.2.2 remote-as 2    neighbor 10.1.2.2 ebgp-multihop 255    !   ip route 10.1.2.0 255.255.255.252 serial0   ___________________________________________________________________________  Router C  router bgp 2   neighbor 10.1.1.1 remote-as 1    neighbor 10.1.1.1 ebgp-multihop 2    !   ip route 10.1.1.0 255.255.255.252 serial 0  

Verification

Verification is the same as with directly connected EBGP neighbors. Use the show ip bgp neighbors command to verify that the relationship has been established:

 rtrA#  show ip bgp neighbors  BGP neighbor is 10.1.2.2,  remote AS 2, external link  Index 1, Offset 0, Mask 0x2   BGP version 4, remote router ID 172.17.1.2   BGP state = Established, table version = 1, up for 00:07:52   Last read 00:00:52, hold time is 180, keepalive interval is 60 seconds   Minimum time between advertisement runs is 30 seconds   Received 10 messages, 0 notifications, 0 in queue   Sent 10 messages, 0 notifications, 0 in queue   Prefix advertised 0, suppressed 0, withdrawn 0   Connections established 1; dropped 0   Last reset never   0 accepted prefixes consume 0 bytes   0 history paths consume 0 bytes  External BGP neighbor may be up to 255 hops away.  Connection state is ESTAB, I/O status: 1, unread input bytes: 0 Local host: 10.1.1.1, Local port: 179 Foreign host: 10.1.2.2, Foreign port: 11048  ___________________________________________________________________________  rtrC#  show ip bgp neighbors  BGP neighbor is 10.1.1.1,  remote AS 1, external link  Index 1, Offset 0, Mask 0x2   BGP version 4, remote router ID 172.17.1.1   BGP state = Established, table version = 1, up for 00:02:01   Last read 00:00:01, hold time is 180, keepalive interval is 60 seconds   Minimum time between advertisement runs is 30 seconds   Received 20 messages, 0 notifications, 0 in queue   Sent 20 messages, 0 notifications, 0 in queue   Prefix advertised 0, suppressed 0, withdrawn 0   Connections established 2; dropped 1   Last reset 00:03:37, due to User reset   0 accepted prefixes consume 0 bytes   0 history paths consume 0 bytes  External BGP neighbor may be up to 2 hops away.  Connection state is ESTAB, I/O status: 1, unread input bytes: 0 Local host: 10.1.2.2, Local port: 179 Foreign host: 10.1.1.1, Foreign port: 11011 

Troubleshooting

1. If the neighbors are not in the Established state, make sure each neighbor can ping the address used in the neighbor command.

2. If the ping is successful, ensure that you are using the correct neighbor IP address, remote AS number, and hop count in the respective BGP configurations.

3. If the ping is unsuccessful , trace the route to the EBGP neighbor to determine where your IP routing breaks down.