8-33 neighbor ip-address peer-group-name update-source interface-name

8-33 neighbor { ip-address peer- group - name } update-source interface-name

Syntax Description:

• ip-address Neighbor's IP address.

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

• interface-name Any physical or logical router interface. Usually, a loopback interface is used.

Purpose: An IBGP connection can occur as long as there is a TCP/IP path between the routers. If multiple paths exist between the IBGP routers, using a loopback interface as the neighbor's address can add stability to the network. Using loopback interfaces with EBGP speakers is not necessary, because EBGP neighbors are usually directly connected.

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

Configuration Example: Using a Loopback Interface for Network Stability

In Figure 8-32, we want to establish an IBGP session between Routers A and C. From Router A's point of view, we can use one of two addresses in the neighbor command, 172.17.1.2 or 10.1.2.2. From Router C's perspective, either address 172.17.1.1 or 10.1.1.1 can be used to form a neighbor relationship with Router A. The following configurations show these options:

Figure 8-32. When Multiple Paths Exist Between IBGP Peers, Using a Loopback Interface as the Source Adds Stability to the Design

 Router A  router bgp 1   neighbor 172.17.1.2 remote-as 1   or   neighbor 10.1.2.2 remote-as 1   __________________________________________________________________________  Router C  neighbor 172.17.1.1 remote-as 1   or   neighbor 10.1.1.1 remote-as 1  

These configurations assume that Routers A and C have routes to these addresses through either an IGP or static routes. If the physical interface addresses that are used in the neighbor commands are functional, the IBGP session remains active. If one of the physical interfaces used in the neighbor command goes down, the IBGP session is terminated . If loopback addresses are used and we are running an IGP that advertises the loopback addresses, the IBGP session remains established even if one of the paths goes down. Because we are using OSPF, the Ethernet network is the shortest path between Routers A and C. The IP routing table on Router A contains a route to the loopback on Router C via the Ethernet network. If the Ethernet network goes down, OSPF converges and installs the path through Router B into the routing table. IBGP packets now take this route, and the IBGP session is maintained . The update-source option uses the address of the indicated interface as the source IP address in the IBGP packets. When configured on both neighbors, the session remains established as long as a path between the loopback addresses exists. If only one path exists between the IBGP speakers, using loopback addresses does not provide additional stability.

 Router A  interface Loopback0   ip address 172.16.1.1 255.255.255.255   !   interface Ethernet0   ip address 172.17.1.1 255.255.255.0   !   interface Serial0   ip address 10.1.1.1 255.255.255.252   !   router ospf 1   network 10.0.0.0 0.255.255.255 area 0   network 172.16.0.0 0.0.255.255 area 0   network 172.17.0.0 0.0.255.255 area 0   !   router bgp 1   neighbor 172.16.1.3 remote-as 1   neighbor 172.16.1.3 update-source Loopback0   __________________________________________________________________________  Router B  interface Loopback0   ip address 172.16.1.2 255.255.255.0   !   interface Serial0   ip address 10.1.1.2 255.255.255.252   clockrate 64000   !   interface Serial1   ip address 10.1.2.1 255.255.255.252   clockrate 64000   !   router ospf 1   network 10.0.0.0 0.255.255.255 area 0   network 172.16.0.0 0.0.255.255 area 0   !   __________________________________________________________________________  Router C  interface Loopback0   ip address 172.16.1.3 255.255.255.255   !   interface Ethernet0   ip address 172.17.1.2 255.255.255.0   !   interface Serial0   ip address 10.1.2.2 255.255.255.252   !   router ospf 1   network 10.0.0.0 0.255.255.255 area 0   network 172.16.0.0 0.0.255.255 area 0   network 172.17.0.0 0.0.255.255 area 0   !   router bgp 1   neighbor 172.16.1.1 remote-as 1   neighbor 172.16.1.1 update-source Loopback0  

Verification

Verify that the IBGP session is being established by examining the IBGP neighbors on Router A:

 rtrA# BGP neighbor is 172.16.1.3,  remote AS 1, internal link  Index 1, Offset 0, Mask 0x2 BGP version 4, remote router ID 172.16.1.1  BGP state = Established, table version = 3, up for 00:28:24  Last read 00:00:26, hold time is 180, keepalive interval is 60 seconds   Minimum time between advertisement runs is 5 seconds   Received 31 messages, 0 notifications, 0 in queue   Sent 31 messages, 0 notifications, 0 in queue   Prefix advertised 0, suppressed 0, withdrawn 0   Connections established 1; dropped 0   Last reset 00:28:34, due to RR client config change   0 accepted prefixes consume 0 bytes   0 history paths consume 0 bytes Connection state is ESTAB, I/O status: 1, unread input bytes: 0 Local host: 172.16.1.3, Local port: 11026 Foreign host: 172.16.1.1, Foreign port: 179 

Troubleshooting

1. Ensure that the loopback address for the neighbor is in the IP routing table.

2. Each neighbor should be able to ping the other neighbor's loopback address. If the pings are successful, the routers can form an IBGP connection using the loopback interface.

3. Verify the neighbor's IP address and remote AS number in the BGP configuration.