5.5 Alternate solution

Hi all,

I am just wondering if we could get R2 to send a long AS_PATH to R5
by using prepend feature. I guess it's an easier workaround for this
case. R5 hears about the VLAN from two sources R2 and R3 and if we add
something like 300,300,300,300,300,300,300 to our AS_PATH leaving AS
300 towards R5 via FR, R5 would go for R1 leading to the HDLC link.

Your thoughts ?

 

Cheers,

Kami

 

Comments

  • I opened this title just to ask the same question. I guess as-prepend would get us the same solution unless there is some other catch in the wordings.

    Please some one post if as-prepend is a valid solution in this case.

    Thanks

  • This is really interstig!!! a friend of mine  got the same quiestion in a mock lab, took the steps we just suggested above and got zero! The proctor says no-advs stuff must be used!!! so the solution we're suggesting is wrong! WHY ????

     

     

  • your solution contradicts with task 5.4, which specifies exact show output.

  • In task 5.4, R5 sets weight for that route, so prepend does not work...

  • Thanks folks, I apprecaite your clarifications.

  • But even the solution IE provided will be contradicted with Task 5.4. After you put the solution in, you can't meet the requirement of Task 5.4

  • This is true, but recall your BGP best path selection algorithm.  In 5.4, we ended up setting a Weight of 100 on the prefix via R2 in order to match the output. Regardless of whether we send a longer AS-Path via the direct link to R2, or anything else on R2, we have no shot of influencing the route to take that path.  Since Weight wins at the top of our path selection criteria, and is locally significant only, our only choice is to remove the announcement via the unwanted path in order to force it to take the path we want.  In order to do that, we have to do a conditional advertisement.

    It does contradict what we did in 5.4, as we will no longer see the required output once we only advertise that path in 1 direction.  In this case, I'd assume that you ask your proctor for clarification to confirm that we're supposed to break 5.4.

     

  • Jeremy,

    Is this the right thing I should get when the HDLC link between R2 and R3 is up because of the weight I put in 5.4.

    R5#sh ip route 136.1.29.0
    Routing entry for 136.1.29.0/24
      Known via "bgp 200", distance 20, metric 0
      Tag 300, type external
      Last update from 136.1.245.2 00:03:35 ago
      Routing Descriptor Blocks:
      * 136.1.245.2, from 136.1.245.2, 00:03:35 ago
          Route metric is 0, traffic share count is 1
          AS Hops 1
          Route tag 300

    R5#sh ip bgp 136.1.29.0
    BGP routing table entry for 136.1.29.0/24, version 73
    Paths: (1 available, best #1, table Default-IP-Routing-Table)
    Flag: 0x820
      Advertised to update-groups:
         1          2        
      300
        136.1.245.2 from 136.1.245.2 (150.1.2.2)
          Origin IGP, metric 0, localpref 100, weight 100, valid, external, best
    R5#

    --------------------------

    When HDLC link is down there is no other path.

    R5#
    R5#sh ip bgp 136.1.29.0
    % Network not in table
    R5#sh ip route 136.1.29.0
    % Subnet not in table
    R5#

     

     

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