BGP Path selection question

Hi all,

From the following output why is the second path best?

R5#sh ip bgp 114.0.0.0
BGP routing table entry for 114.0.0.0/8, version 6
Paths: (2 available, best #2, table default)
  Advertised to update-groups:
     6          7          9        
  54
    155.1.108.10 (metric 2560002816) from 155.1.0.3 (150.1.3.3)
      Origin IGP, metric 0, localpref 100, valid, internal
      Originator: 150.1.8.8, Cluster list: 150.1.3.3
  54, (Received from a RR-client)
    155.1.108.10 (metric 2560002816) from 155.1.58.8 (150.1.8.8)
      Origin IGP, metric 0, localpref 100, valid, internal, best

Both are valid, same weight, LP, AS_Path length, origin, metric, internal, igp metric to NH.

Next criteria should be oldest eBGP path, but does not apply because they are internal.

Next, Lowest router id should be best. But in case of reflection, originator id is compared so they are the same (150.1.8.8).

Last one should be lowest neighbor IP which is the first entry, not the second.

Am i missing something?

Comments

  • Hi Cosmin,

    It's number 8 in the BGP Best Path Selection algorithm : lowest IGP metric to the BGP next hop.

    We are comparing the metric to 155.1.0.3 to the metric to 155.1.58.8. Since the underlying IGP is EIGRP, the metric over the DMVPN tunnel is higher (low bandwidth and high delay) as compared to the metric over a Gig interface.

     

  • I don't think you are right.

    Its the metric to the NH like you said, not the metric to the router ID (also like you said). Even so, which one is biger 2560002816 or 2560002816 :) ?

  • I may be wrong ... but ... isn't 2560002816 the metric to 155.1.108.10 ? as opposed the metric to IPs I mentioned ? 

    Are you practicing a specific scenario from the workbook ? If that's the case, let me know which one specifically is and I'll take a look.

  • You are right, thats the metric to NH. The IP that you mentioned are not the NH for that prefix, are RIDs. The algorithm compares the igp metric to the next-hop, not to the router ID - those values that appear inside () when received from iBGP are compared.

    Anyway, still not sure why the second entry is chosen. Even if i make the second path newer it is still chosen as best.

     

    I was playing with BGP best-path scenarios, the ones that have Basic BGP routing preconfigured.

  • Hi,

    Increase the EIGRP composite metric to 155.1.58.8 to be higher than metric to 155.1.0.3 and let me know if the result of BGP  best path selection changes.

  • Hi

    If you change igp metric than step 8 will be used

    Now we have the same metric to igp next-hop and bgp will go to next step

    I think it will be step 12, from rr we have cluster list = 1 from second neighbor = 0

    but I 'm not shure that this step will be used when we have updates from rr clients and normal ibgp client [non-client]

    tips [for shure enter 'clear ip bgp *'] and check again

     

    12 If the originator or router ID is the same for multiple paths, prefer the path with the minimum cluster list length.

    This is only present in BGP RR environments. It allows clients to peer with RRs or clients in other clusters. In this scenario, the client must be aware of the RR-specific BGP attribute.

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