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Forum Replies

  1. Hi Alfredo,

    When you send something to the null interface then those packets are discarded. It can be useful if you want to advertise something that you don’t have.

    When you create a static route you need a valid next hop or it can’t be installed in the routing table. Because of this, using the null interface as next hop will help.

    It’s also useful when you create summary routes. With a summary, it’s possible that you receive traffic for networks that you don’t have. In this case the packets can be discarded to the null interface.


  2. Can you shed any light on the default information originate and it’s use in BGP.

  3. Hi Rouzbeh,

    1. If no mask is specified, default mask is used, that is /8 in your example.
    2. When Rene said “exact mask has to be added in network command” meant that if we have a prefix in routing table (say, and we want this prefix to be injected in bgp with network command we should use
    router bgp 1

    in our config.
    If we are going to use:

    router bgp 1

    , router will assume an implicit mask (
    So prefix will not enter bgp table because we do not have in routing table. We have but not
    Briefly, if we have in r. table, we should use EXACT prefix (of course) and mask (!) in network statement
    Hope it helps.

  4. Hello Mohammad

    BGP does use triggered updates when it learns of a change on its internal domain. However, there are a few things that affect the operation of this triggering that will in turn affect the behaviour of BGP.

    Lets say that there is a network on the internal domain that is learned via OSPF, and this network continually goes up and down. BGP updates will be affected by the following:

    1. The detection of the changes - how fast does the router detect that the OSPF route is down? This depends primarily on the BGP scanner process. This process walks the BGP table and confirms reachability of the next hops. The most important issue here is that BGP will do this once a minute.

    2. Propagation of the changes - How fast and often can BGP advertise the changes? By default, BGP waits for the Advertisement Interval to expire before sending any changes. For eBGP, the default is 30 seconds and for iBGP the default is 5 seconds. Even if a change is detected in an OSPF route for example, BGP will not send out any advertisements until this timer expires. The benefit is that updates can be sent more efficiently, solving the problem of stability to a certain degree, but this comes at a cost of convergence time.

    If however you have a flapping route on an internal network, BGP will not issue updates at the same frequency thus giving you some stability until the problem can be solved.

    I hope this has been helpful!


  5. Hi @ReneMolenaar

    Let assume that R1 have three eBGP neighbors ( R2 , R3 , and R4 ) and I need to advertise the network in interface loopback of R1 to only R2.

    Is that possible by using network statement with route-map ?? or there is something else to achieve this ??

    It’s very helpful for me if you can do that by using network statement with route-map.

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