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Forum Replies

  1. thank you for your lesson, can you explain why we use command bgp log-neighbor-changes no synchronization

  2. bgp log-neighbor-changes
    This command simply causes a message to displayed to the console or in the event log that a status has changed with one of your established BGP neighbors. This is very useful for an administrator to know.

    no synchronization
    This can be considered a legacy command now, because in modern IOS the “no synchronization” is on by default. The explanation of this is a bit long. Your best bet is to review the synchronization lesson.

    In a nutshell, the purpose for Synchronization rule was back when many internal routers didn’t have the CPU and RAM capable of running BGP. If you have a BGP router advertising a network, but the less powerful routers inside your network don’t know about the route being advertised, if traffic needs to cross your internal network, it would be dropped. The Synchronization rule would only allow BGP to advertise a network if that network was already known by an internal routing protocol (like RIP, OSPF, EIGRP, etc). The idea was that if the route is known by an IGP, you wouldn’t act like a “black hole” for the traffic.

    Most modern routers can easily run BGP now (so long as they don’t get the full Internet table!), so the idea of a “Synchronization Rule” is outdated.


  3. Hi Rene,
    I really appreciate your nice explanation. I love the way you explained the things. I have one confusion in below statement.
    "When two EBGP routers that are directly connected do not form a working BGP neighbor adjacency there could be a number of things that are wrong:

    Layer 2 down preventing us from reaching the other side.
    Layer 3 issue: wrong IP address on one of the routers.
    Access-list blocking TCP port 179 (BGP).
    _Wrong IP address configured for BGP neighbor router."

    What do you mean by layer 2 down preventing us from reaching the other side? What could be the layer 2 issues except interface down?

  4. It could be any issue on layer 2…an interface that goes down but a MAC address filter that prevents two routers from communicating with each other is also a layer 2 issue :slight_smile:

  5. Hello sachin

    You could add the network command on the BGP configuration, however this would only advertise the default route to the other routers. It would not allow R1 to advertise to R2. You need the specific destination and subnet mask in the routing table using the two options that are described in the lesson. It would probably be very beneficial if you tried it out in a lab to see first hand how BGP behaves with these configuration commands. If you do try it, let us know your results!

    I hope this has been helpful!


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