BGP Multi-Homed but traffic likes Sprint better =(

Discussion in 'Cisco' started by mhoppes, Aug 21, 2007.

  1. mhoppes

    mhoppes Guest

    Hi,
    I have two Cisco routes that are running BGP between Level3 and
    Sprint. The BGP configs on both are pretty much the same and look
    something like this:

    router bgp 12345
    no synchronization
    bgp log-neighbor-changes
    neighbor 192.168.1.1 remote-as 26383
    neighbor 192.168.1.1 description iBGP link to hydra
    neighbor 192.168.1.1 password 7 password
    neighbor 192.168.1.1 version 4
    neighbor 192.168.1.1 soft-reconfiguration inbound
    neighbor 160.81.248.77 remote-as 1239
    neighbor 160.81.248.77 description ebgp link to sprint
    neighbor 160.81.248.77 version 4
    neighbor 160.81.248.77 soft-reconfiguration inbound
    no auto-summary

    The other router is configured similarly. HOWEVER, if I do a show ip
    bgp 209.245.19.41 (level3.net) I get:
    BGP routing table entry for 209.244.0.0/14, version 891496
    Paths: (2 available, best #1, table Default-IP-Routing-Table)
    Not advertised to any peer
    1239 3356, (aggregated by 3356 4.69.130.12), (received & used)
    160.81.248.77 (metric 3) from 65.165.94.78 (160.81.248.78)
    Origin IGP, metric 34, localpref 100, valid, internal, atomic-
    aggregate, best
    32327 7911 3356, (aggregated by 3356 4.69.130.10), (received & used)
    209.183.177.125 from 209.183.177.125 (209.183.191.194)
    Origin IGP, localpref 100, valid, external, atomic-aggregate

    Why is Level3 prefering to go out Sprint (160.81.248.77) rather then
    go out Level3 (209.183.177.125)?
    I see there seems to be an extra AS hop there, but why? Shouldn't a
    L3 pipe be a better route to L3?
     
    mhoppes, Aug 21, 2007
    #1
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  2. mhoppes

    mhoppes Guest

    AH! Answered my own question. My L3 pipe is going through another
    carrier (I knew that), who is putting another AS number in there... so
    my question, then, is.. what do I need to do to get the routes to look
    even? Appending another AS number will, I believe, only affect
    inbound traffic.
     
    mhoppes, Aug 21, 2007
    #2
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  3. route map addmyas permit 10
    set as-prepend <your as>
    !
    router bgp <your as>
    neighbour sprint route-map addmyas in
    neighbour sprint route-map addmyas out
     
    Lutz Donnerhacke, Aug 21, 2007
    #3
  4. mhoppes

    mhoppes Guest

    Ok.. that worked as expected:
    Hydra#show ip bgp 206.80.67.28
    BGP routing table entry for 206.80.64.0/21, version 402504
    Paths: (2 available, best #1, table Default-IP-Routing-Table)
    Flag: 0x208
    Not advertised to any peer
    26383 1239 3356 30512, (received & used)
    160.81.248.77 (metric 3) from 65.165.94.78 (160.81.248.78)
    Origin IGP, metric 34, localpref 100, valid, internal, best
    32327 7911 3356 30512, (received-only)
    209.183.177.125 from 209.183.177.125 (209.183.191.194)
    Origin IGP, localpref 100, valid, external



    However, why is Sprint still being preferred? I don't want to send
    ALL traffic down L3.. just traffic that is, well you know, going to
    the L3 network! Even level3.net (the last entry there at 4.68.95.11)
    has fewer AS hops going over L3, yet it still wants to go out Sprint!

    The other odd thing is if I do a show command for L3 (4.68.95.11) on
    my L3 router I get:
    Hydra#show ip bgp 4.68.95.11
    BGP routing table entry for 4.0.0.0/9, version 225677
    Paths: (2 available, best #2, table Default-IP-Routing-Table)
    Not advertised to any peer
    32327 7911 3356, (aggregated by 3356 4.69.130.10), (received-only)
    209.183.177.125 from 209.183.177.125 (209.183.191.194)
    Origin IGP, localpref 100, valid, external, atomic-aggregate
    26383 1239 3356, (aggregated by 3356 4.69.130.12), (received & used)
    160.81.248.77 (metric 3) from 65.165.94.78 (160.81.248.78)
    Origin IGP, metric 34, localpref 100, valid, internal, atomic-
    aggregate, best
    Hey it's working! But if I do it on my Sprint router I get:

    medusa#show ip bgp 4.68.95.11
    BGP routing table entry for 4.0.0.0/9, version 923707
    Paths: (2 available, best #1, table Default-IP-Routing-Table)
    Advertised to non peer-group peers:
    65.165.94.77 65.173.16.33
    26383 1239 3356, (aggregated by 3356 4.69.130.12)
    160.81.248.77 from 160.81.248.77 (144.228.242.72)
    Origin IGP, metric 34, localpref 100, valid, external, atomic-
    aggregate, best
    1239 3356, (aggregated by 3356 4.69.130.12), (received-only)
    160.81.248.77 from 160.81.248.77 (144.228.242.72)
    Origin IGP, metric 34, localpref 100, valid, external, atomic-
    aggregate
    eh?
     
    mhoppes, Aug 21, 2007
    #4
  5. It's called hot potato routing. Every router prefers it's own peers if all
    other paramters are equal. In order to load balance your outgoing traffic,
    you have to load balance the traffic in your own network to the routers.

    OTOH from two routes with equal length, weight and preference the oldest onw
    is prefered (assuming this one is more stable).
     
    Lutz Donnerhacke, Aug 21, 2007
    #5
  6. mhoppes

    mhoppes Guest

    Ok.. I understand this.. and indeed things now look like they should
    on the L3 router.. so my question would then be... what would I need
    to do on the Sprint router to get it to ship traffic over to the L3
    router? Ideally our L3 traffic would go there (a few subnets), but
    best case would be I could get about half our traffic going out there,
    and leave half on Sprint.
     
    mhoppes, Aug 21, 2007
    #6
  7. mhoppes

    Chris Guest

    Match the L3 prefixes coming in via the L3 connection and then set a higher
    local pref to be used within your network to prefer the L3 connection over
    the Sprint connection.

    Chris.
     
    Chris, Aug 21, 2007
    #7
  8. mhoppes

    mhoppes Guest

    Can you, or someone else, provide an example? Although I'm fairly
    fluent in CISO-ios, my BGP knowledge is rather rusty.
     
    mhoppes, Aug 22, 2007
    #8
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