no-prepend is configure, but the question does not ask for. Is this a best practise how it should be configured, or is this wrong since not asked for?



  • Hi Frank,

    I believe "no-prepend" is needed because you don't want to alter the AS path. Can anyone confirm?

  • The question didn't specifically ask for no-prepend, but it might be implied for full reachability in the real lab.

    No-prepend simply doesn't stamp the configured 'local-as' in the outgoing updates to peers that the route wasn't received from (so anything other than BB3). No-prepend shouldn't do anything with BB3 in this case. Check both routing tables and you'll see. In order to affect bb3, thats the 'replace-as' command.

    If you do not configure no-prepend, you'll see that R6 will lose connectivity to BB3 routes when R6 has his session down to BB1. This is because of the bgp loop prevention. If no-prepend isn't on, R6 will see paths from R2->R1->SW1->BB3 but reject them because AS 100 is in the path.

    In short, no-prepend affects upstream peers. If you're using the same local AS in multiple places like lab2, use it or else you'll run into the above scenario.

    replace-as affects the actual peer you configured the command on. This replaces the as in outbound updates so all the peer(bb3) sees is '100' in the path and not 100 400
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