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Forum Replies

  1. hi

    will I need to advertise the internal routing network into BGP
    for example internal EIGRP is using subnet 126.89.120.64/26

  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 192.168.1.0/25), and we want this prefix to be injected in bgp with network command we should use
    router bgp 1
    network 192.168.1.0 255.255.255.128
    

    in our config.
    If we are going to use:

    router bgp 1
    network 192.168.1.0
    

    , router will assume an implicit mask (255.255.255.0).
    So 192.168.1.0/24 prefix will not enter bgp table because we do not have 192.168.1.0/24 in routing table. We have 192.168.1.0/25 but not 192.168.1.0/24.
    Briefly, if we have 192.168.1.0/25 in r. table, we should use EXACT prefix (of course) and mask (!) in network statement
    network 192.168.1.0 255.255.255.128
    Hope it helps.
    A.

  4. HI Rene,
    SInce BGP only advertises network which matches in the routing table, why doesn’t it advertise prefixes learnt from other neighbor as its own network to other neighbors .
    the show ip route command does show route is being self-originated but the last update still shows that is prefix is being learnt from neighbor
    Please advise

  5. 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!

    Laz

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