iBGP update-source question

[benson.low]

Hi CSC,

I am puzzled; According to iBGP operations, i can use routers' loopback interfaces in the neighbor command with worrying about the multihops issue. However, with the below config, iBGP neighbors are not establish

on R1 (lo 1.1.1.1)

router bgp 100

no synchronization

bgp log-neighbor-changes

neighbor 3.3.3.3 remote-as 100

neighbor 5.5.5.5 remote-as 100

neighbor 5.5.5.5 route-reflector-client

neighbor 10.1.2.2 remote-as 100

no auto-summary

The debug ip bgp all shows the following

*Mar  1 00:20:16.219: BGP: 5.5.5.5 open active, local address 10.1.3.1

*Mar  1 00:20:16.263: BGP: 5.5.5.5 open failed: Connection refused by remote host, open active delayed 25482ms (35000ms max, 28% jitter)

iBGP neighbors would only formed if neighbor 5.5.5.5 update-source lo0 is used.

Please note that all physical and loopback interfaces are known via the IGP.

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

O       2.2.2.2 [110/2] via 10.1.2.2, 00:20:38, FastEthernet0/1

     3.0.0.0/32 is subnetted, 1 subnets

O       3.3.3.3 [110/2] via 10.1.3.2, 00:20:38, FastEthernet0/0

     4.0.0.0/32 is subnetted, 1 subnets

O       4.4.4.4 [110/3] via 10.1.2.2, 00:20:38, FastEthernet0/1

     5.0.0.0/32 is subnetted, 1 subnets

O       5.5.5.5 [110/3] via 10.1.3.2, 00:20:38, FastEthernet0/0

     10.0.0.0/30 is subnetted, 6 subnets

C       10.1.3.0 is directly connected, FastEthernet0/0

O       10.5.6.0 [110/66] via 10.1.3.2, 00:20:38, FastEthernet0/0

C       10.1.2.0 is directly connected, FastEthernet0/1

O       10.4.5.0 [110/3] via 10.1.3.2, 00:20:38, FastEthernet0/0

                 [110/3] via 10.1.2.2, 00:20:38, FastEthernet0/1

O       10.3.5.0 [110/2] via 10.1.3.2, 00:20:38, FastEthernet0/0

O       10.2.4.0 [110/2] via 10.1.2.2, 00:20:38, FastEthernet0/1

R1#show ip bgp sum

BGP router identifier 1.1.1.1, local AS number 100

BGP table version is 1, main routing table version 1

Neighbor        V    AS MsgRcvd MsgSent   TblVer  InQ OutQ Up/Down  State/PfxRcd

3.3.3.3         4   100       0       0        0    0    0 never    Active

5.5.5.5         4   100       0       0        0    0    0 never    Active

10.1.2.2        4   100      12      12        1    0    0 00:08:45        0

Please note that the above configuration is done using GNS3. Anyone please help to clarify my doubts

1) Why do I need to use update-source loopback in the neighbor command in order to form neighborship?

2) Why if i use physical interface, then neighbors would be formed?

[Jon Marshall]

Your'e right in that you don't need to worry about ebgp multihop but if you use a non-directly connected interface you still need to use the "update-source ..." command.

The update-source command is used to tell the router which interface to use for it's updates. If you are not using the directly connected interface then you have to have a way of telling the router which interface you want to use. That is what this command does. Without this command how could you tell the router that for IBGP updates you want the source address to be the loopback interface.

If you used the physical interface then yes the neighborship would still be formed.

Jon