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  1. Quick question Rene on this topic. The lab works fine as I followed along with my own topology. But I am just wondering why R2 should send the community only to R3? For example, you mentioned: "R2 sets the community so make sure that it advertises it to R3".... why should it not send the community to R4 as well. I know R4 is in another AS, but R4 would have filtered it out as well, wouldn't it?

  2. Hi Mario,

    On R2 I configured the route-map inbound (facing R1) so that R2 sets the local AS community. Because of this, R2 will no longer advertise prefixes to R4 since it's another sub-AS. R3 will still receive it since it's in the same sub-AS as R2.

    To make sure R5 (and R4) learn anything through R3, we need to advertise the local AS community to R3.

    Rene

  3. Hi,

    I am confused about BGP selecting path.Here topology

    R5 is in sub-AS 45.And advertise prefix 9.9.9.9/32.

    R5#sh ip bgp             
    BGP table version is 14, local router ID is 5.5.5.5
    Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
                  r RIB-failure, S Stale
    Origin codes: i - IGP, e - EGP, ? - incomplete
    
       Network          Next Hop            Metric LocPrf Weight Path
    *> 9.9.9.9/32       0.0.0.0                  0         32768 i
    *> 192.168.12.0     2.2.2.2                  0    100      0 (23) i
    * i                 2.2.2.2                  0    100      0 (23) i

    R4 is in sub-AS 45 same R5 and it learn prefix 9.9.9.9/32 from R5.

    R4#sh ip bg              
    BGP table version is 15, local router ID is 4.4.4.4
    Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
                  r RIB-failure, S Stale
    Origin codes: i - IGP, e - EGP, ? - incomplete
    
       Network          Next Hop            Metric LocPrf Weight Path
    *>i9.9.9.9/32       5.5.5.5                  0    100      0 i
    * i192.168.12.0     2.2.2.2                  0    100      0 (23) i
    *>                  2.2.2.2                  0    100      0 (23) i
    R4#sh ip bgp 9.9.9.9/32
    BGP routing table entry for 9.9.9.9/32, version 10
    Paths: (1 available, best #1, table Default-IP-Routing-Table)
      Advertised to update-groups:
            2
      Local
        5.5.5.5 (metric 11) from 5.5.5.5 (5.5.5.5)
          Origin IGP, metric 0, localpref 100, valid, confed-internal, best

    R2 is in sub-AS 23 and it learn prefix 9.9.9.9/32 from R4.

    R2#sh ip bgp
    BGP table version is 4, local router ID is 2.2.2.2
    Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
                  r RIB-failure, S Stale
    Origin codes: i - IGP, e - EGP, ? - incomplete
    
       Network          Next Hop            Metric LocPrf Weight Path
    *> 9.9.9.9/32       5.5.5.5                  0    100      0 (45) i
    *> 192.168.12.0     0.0.0.0                  0         32768 i
    R2#
    R2#sh ip bgp 9.9.9.9/32
    BGP routing table entry for 9.9.9.9/32, version 4
    Paths: (1 available, best #1, table Default-IP-Routing-Table)
      Advertised to update-groups:
            1    3
      (45)
        5.5.5.5 (metric 21) from 4.4.4.4 (4.4.4.4)
          Origin IGP, metric 0, localpref 100, valid, confed-external, best

    R3 is in sub-AS 23 and it learn prefix 9.9.9.9/32 from both R2 and R5.

    R3#sh ip bgp 
    BGP table version is 3, local router ID is 3.3.3.3
    Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
                  r RIB-failure, S Stale
    Origin codes: i - IGP, e - EGP, ? - incomplete
    
       Network          Next Hop            Metric LocPrf Weight Path
    *  9.9.9.9/32       5.5.5.5                  0    100      0 (45) i
    *>i                 5.5.5.5                  0    100      0 (45) i
    *>i192.168.12.0     2.2.2.2                  0    100      0 i
    R3#
    R3#
    R3#sh ip bgp 9.9.9.9/32
    BGP routing table entry for 9.9.9.9/32, version 2
    Paths: (2 available, best #2, table Default-IP-Routing-Table)
      Advertised to update-groups:
            2
      (45)
        5.5.5.5 (metric 11) from 5.5.5.5 (5.5.5.5)
          Origin IGP, metric 0, localpref 100, valid, confed-external
      (45)
        5.5.5.5 (metric 11) from 2.2.2.2 (2.2.2.2)
          Origin IGP, metric 0, localpref 100, valid, confed-internal, best

    The question is, Why R3 select confed-internal(2.2.2.2) instead confed-external(5.5.5.5)?

    Thank you in advance.

  4. Hi Ler Sak,

    BGP doesn't differentiate between confed-internal or confed-external. When the two paths are the same, it's up to the router ID to decide which one will be selected.

    Here's an example from the topology I used in this lesson:

    R4#show ip bgp 55.55.55.55/32
    BGP routing table entry for 55.55.55.55/32, version 5
    Paths: (2 available, best #1, table default)
    Flag: 0x800
      Not advertised to any peer
      Refresh Epoch 1
      (35)
        5.5.5.5 (metric 2) from 2.2.2.2 (2.2.2.2)
          Origin IGP, metric 0, localpref 100, valid, confed-internal, best
          rx pathid: 0, tx pathid: 0x0
      Refresh Epoch 1
      (35)
        5.5.5.5 (metric 2) from 5.5.5.5 (55.55.55.55)
          Origin IGP, metric 0, localpref 100, valid, confed-external
          rx pathid: 0, tx pathid: 0

    Above you can see the path from 2.2.2.2 has been selected. Let's change the router ID on R2:

    R2(config)#router bgp 24
    R2(config-router)#bgp router-id 222.222.222.222

    Now it prefers R5:

    R4#show ip bgp 55.55.55.55/32
    BGP routing table entry for 55.55.55.55/32, version 13
    Paths: (2 available, best #2, table default)
    Flag: 0x800
      Advertised to update-groups: (Pending Update Generation)
         2          4         
      Refresh Epoch 1
      (35)
        5.5.5.5 (metric 2) from 2.2.2.2 (222.222.222.222)
          Origin IGP, metric 0, localpref 100, valid, confed-internal
          rx pathid: 0, tx pathid: 0
      Refresh Epoch 1
      (35)
        5.5.5.5 (metric 2) from 5.5.5.5 (55.55.55.55)
          Origin IGP, metric 0, localpref 100, valid, confed-external, best
          rx pathid: 0, tx pathid: 0x0

    Hope this helps!

    Rene

  5. Hi Rene,

    Thank you so much.

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