I have the following test lab:
The configuration of 'King' is:
King#sho ip int bri
Interface IP-Address OK? Method Status Protocol
FastEthernet0/0 192.168.122.1 YES NVRAM up up
Serial1/0 192.168.112.1 YES NVRAM up up
Serial1/1 unassigned YES NVRAM administratively down down
Serial1/2 unassigned YES NVRAM administratively down down
Serial1/3 unassigned YES NVRAM administratively down down
Loopback0 1.1.1.1 YES manual up up
King#
King#
King#sho run | sec bgp
router bgp 100
no synchronization
bgp log-neighbor-changes
neighbor 2.2.2.2 remote-as 100
neighbor 2.2.2.2 update-source Loopback0
no auto-summary
King#
King#sho run | sec eigrp
router eigrp 100
network 0.0.0.0
auto-summary
King#
The configuration of 'Kong' is:
Kong#sho ip int bri
Interface IP-Address OK? Method Status Protocol
FastEthernet0/0 192.168.122.2 YES NVRAM up up
Serial1/0 192.168.112.2 YES NVRAM up up
Serial1/1 unassigned YES NVRAM administratively down down
Serial1/2 unassigned YES NVRAM administratively down down
Serial1/3 unassigned YES NVRAM administratively down down
Loopback0 2.2.2.2 YES manual up up
Kong#
Kong#
Kong#sho run | sec bgp
router bgp 100
no synchronization
bgp log-neighbor-changes
neighbor 1.1.1.1 remote-as 100
no auto-summary
Kong#
Kong#sho run | sec eigrp
router eigrp 100
network 0.0.0.0
auto-summary
Kong#
Question:
From what i've read, i understood the following:
To establish a neighbor relationship, the update source IP from router A must match the neighbor command on router B, and vice versa.
However, in my case the update-source is configured on only 'King', how come the bgp neighbor ship is up?
Please explain in terms of packet flow.
Best Answer
To expand on @cown 's answer, the reason this works is because King initiated the connection, sending an open request to 2.2.2.2. Kong received the request on that interface, so it responded from from 2.2.2.2. If Kong had initiated the connection, it would looked up the best outgoing interface, (serial1/0 or 1/1) and used that as thesource. In that case, King would have received an open request from 192.168.122.2, and the BGP session would not have come up.