5.3


Hi,

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?

Frank

Comments

  • Hi Frank,

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

    Thanks,
    Dennis
  • 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
Sign In or Register to comment.