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Showing smaller routes that are part of bigger BGP routes.

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  • 1.  Showing smaller routes that are part of bigger BGP routes.

    Posted 06-06-2018 06:42

    Hi all,

     

    When I do a show route on an IP address, our routing table shows it's part of a /24 learned from a peering exchange session with a Route Server. However, this /24 is part of a bigger /21 that we learn from bgp sessions with our transit tier pone providers.

     

    So...when I show route to see what my router has, should I expect to see this /21 in the list too as it is announced out to the world by this ISP under their ASN too and the /24 is part of it? I guess I'd only see it if this /24 route was pulled via BGP.

     

    I'm just used to seeing lots of routes returned with different local preferences on BGP when looking at an IP address, and this has thrown me.

     

    Thanks,

    Gavin.



  • 2.  RE: Showing smaller routes that are part of bigger BGP routes.

     
    Posted 06-07-2018 02:54

    Is the route table you are checking peered to both the exchange and inet peer directly or does it learn the full table routes from a separate bgp neighbor?

    And if so, is that iBGP or eBGP between these peers and the topology?

     

    Basically, I would follow the path for the /21 route in the internal network by peer.

    Confirm this enters from the expected peer

    show route receive-protocol bgp 10.1.1.1 | match 10.10.10.0/21

    Confirm added to the active table

    show route protocol bgp 10.10.10.0/21

    confirm it is then advertised to the next peer internally

    show route advertising-protocol bgp 10.1.1.2 | match 10.10.10.0/21

     

    and then on the peer 10.1.1.2 for the check on receive to see if it accepts this prefix

    and so on until arriving on the peer in question

     

     



  • 3.  RE: Showing smaller routes that are part of bigger BGP routes.

    Posted 06-07-2018 06:13

    The /24 is learned from one peer and the /21 is learned from our Tier 1 transit. Both eBGP.

     

    Should I expect to see the returned routes for show route IP_address to list them both as it's the same AS that is announcing them out to the world.



  • 4.  RE: Showing smaller routes that are part of bigger BGP routes.
    Best Answer

     
    Posted 06-07-2018 16:04

    Use the /21 mask in your show route requests

     

    example routes:

    10.0.0.0/24

    10.0.0.0/21

     

    If you use a single ip address the result will be only the longest match route.

     

    show route protocol bgp 10.0.0.0

    gets

    10.0.0.0/24

     

    Using the longer mask will return only the longer route

    show route protocol bgp 10.0.0.0/24

    gets just

    10.0.0.0/24

     

    Using the shorter mask will show both

    show route protocol bgp 10.0.0.0/21

    gets

    10.0.0.0/21

    10.0.0.0/24