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Forum Replies

  1. Hi Hamood,

    GNS3 makes this really simple. You can right-click on any interface and select “capture”. It will automatically start Wireshark and show you the traffic that flows through the link.

    Rene

  2. Hi Dhanu,

    The second update message is empty and is used to indicate that all routes have been sent. You can read more about it in RFC4724:

    An UPDATE message with no reachable Network Layer Reachability Information (NLRI) and empty withdrawn NLRI is specified as the End-of-RIB marker that can be used by a BGP speaker to indicate to its peer the completion of the initial routing update after the session is established.

    Rene

  3. Hello Rene,
    I have couple of questions regarding the timers used in BGP. Correct me if I am wrong. Bgp uses keepalive of 60 seconds and hold down timer of 180 seconds by default. So here the keepalive works like hello messages like in ospf and hold down timer works like dead timer in ospf. Am I correct? Let’s say Router A is peering with Router B by EBGP and router A is using keepalive of 60 seconds and hold down timer of 180 seconds whereas Router B is using keepalive of 100 seconds and hold down timer of 300 seconds. In this case, what would be the negotiated keepalive and hold down timer that both routers will use? My second question. Let’s say Router A is peering with Router B by EBGP and and Route-maps are in place for filtering in both routers. In this scenario, if I make any change in the outbound route-map assigned to Router A and do not run clear ip bgp * soft, how long is it going to take Router B to get the update? How long does it take a router to get the update when any route goes down in the neighboring AS router? Thank you in advance.

  4. The default keepalive and holddown timer are 60 and 180 seconds (3x the keepalive):

    R1#show ip bgp neighbors | include keepalive
      Last read 00:00:20, last write 00:00:50, hold time is 180, keepalive interval is 60 seconds
    

    So what happens when you change these? For example, R1 uses a lower keepalive and holddown timer while 192.168.12.2 (R2) uses the default:

    R1(config)#router bgp 1
    R1(config-router)#neighbor 192.168.12.2 timers 10 30 
    

    The end result will be:

    R1#show ip bgp neighbors | include keepalive
      Last read 00:00:08, last write 00:00:08, hold time is 30, keepalive interval is 10 seconds
      Configured hold time is 30, keepalive interval is 10 seconds
    

    And R2:

    R2#show ip bgp neighbors | include keepalive
      Last read 00:00:05, last write 00:00:06, hold time is 30, keepalive interval is 10 seconds
    

    BGP will use the lowest timer values.

    How long it takes before an update is sent depends if you are running eBGP or iBGP and doesn’t have anything to do with the keepalive and holddown timer, those are only for the neighbor adjacency. For eBGP, an update is sent every 30 seconds which you can see in the show ip bgp neighbors command:

    R1#show ip bgp neighbors | include advertisement
      Default minimum time between advertisement runs is 30 seconds
    

    When you run iBGP, it is instant as you can see here:

    R1#show ip bgp neighbors | include advertisement
      Default minimum time between advertisement runs is 0 seconds
    

    Hope this helps!

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