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Forum Replies

  1. awesome lessons! didnt know you can use extended access-list like this!

    correct me if im wrong, the difference i see here is that, its working like a prefix-list but you can only use “greater than or equal to”?

    on your examples, it only shows like:

    -/24 to /32

    -/25 to /32

    -/26 to /32

    is it possible to have /24 to /30 only? /26 to /29? what will be your subnet and subnet wild card?



  2. Hi John,

    Good question, it can’t be done…let’s look at an example:

    00000000 /24
    10000000 /25
    11000000 /26
    11100000 /27
    11110000 /28
    11111000 /29
    11111100 /30
    11111110 /31
    11111111 /32

    Let’s say you want to match /26 up to /29, the problem is that they don’t have a lot of bits in common…only the first two bits are the same:

    11000000 /26
    11100000 /27
    11110000 /28
    11111000 /29

    Now if you would use wildcard 00111111 (63 in decimal) then it matches /26, /27, /28, /29 but also /30, /31 and /32.

    It can’t be done in one statement but of course you can use multiple statements…just create one for /26, /27, /28 and /29 and you are done.


  3. Hi Shady,

    If you want to enforce one path for outgoing traffic from AS 1 to AS 2 then it’s best to influence the attributes. Don’t let the router ID decide it. If you want to do this for the entire AS, it’s best to configure local preference inbound on R1 and/or R3.


  4. Hi, Rene.

    I’m replicating this same scenario in GNS3 and even applying the filters to some specific routes, I lose all routes from the routing table and in the BGP table. Why is this so?

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