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  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can easily run BGP now (so long as they don't get the full Internet table!), so the idea of a "Synchronization Rule" is outdated.

    --Andrew

  3. mark1 says:

    Hello,

    I think the sentence above "Lesson learned: You can’t advertise what you don’t have. Create a static route and point it to the null0 interface to create a loopback interface that has a prefix that falls within the summary address range" should read:

    "Lesson learned: You can’t advertise what you don’t have. Create a static route and point it to the null0 interface OR create a loopback interface that has a prefix that falls within the summary address range".

    Or am I still missing something?

    Great lesson as always!

    Warm regards

    Mark

  4. andrew says:

    Thanks Mark and Rohit. I will ask Rene to get that corrected.

  5. 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?

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