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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 Rene,

    In your comment you mentioned "To make sure R5 (and R4) learn anything through R3, we need to advertise the local AS community to R3" . How can R3 advertise to R4 and R5 when they in different Sub AS.

    Thanks in advance.

  4. 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.

  5. 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

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