The use of the update-source command

I am a little confused as to the purpose of the the update-source command in BGP. In the example from the Internal BGP lesson, Rene says that the update-source command is used to specify that we will use the loopback interfaces as the source for the IBGP session.

R4(config-router)# neighbor update-source loopback 0

Does this mean that R4 will be sourcing from R2’s loopback, meaning the neighbor’s loop back? or does it refer to R4’s loopback? I am not clear on this. Can someone please explain? Thanks!


let’s start from fundamentals; by default when BGP is establishing the adjacency with another neighbor, that relationship will be formed using the IP address of the outgoing/closet interface to your neighbor. However this design might cause issues in some design.

Take a look at this topology : R1 fa0/1-2=========== fa0/1-2 R2 , R1 is connected to R2 via two interfaces for redundancy. R1 will use one interface to form the neighborship, let’s say fa0/1 in this case. If Fa0/1 goes down for some reason, our BGP relationship will go down immediately then another session will start using the 2nd interface. On a production network, this will definitely introduce an outage while another neighborship is forming.

To overcome this outage in the future, we can create a Loopback interface on our local router, then use it as the source of our TCP session while establishing neighborship by using neighbor x.x.x.x update-source Loopback x command.

Now I go back to your question:
R4(config-router)# neighbor update-source loopback 0

Can you now try to answer the question? if not, let me know then @lagapides or @ReneMolenaar might explain better


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thank you so much for this explanation. It makes perfect sense.

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