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Latest post 01-02-2017 9:06 AM by peety. 8 replies.
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  • 12-31-2016 11:00 AM

    BGP Best Path Selection vs IP Route

    Happy new Year Experts !

    wonder any one has a better understanding for relationship between the best path result obtained in sh ip bgp vs sh ip route?Geeked

    Refer to below output

    R1#sh ip bgp
    BGP table version is 245925, local router ID is 10.192.7.151
    Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
                  S Stale
    Origin codes: i - IGP, e - EGP, ? - incomplete

       Network          Next Hop            Metric LocPrf  Weight      Path
    *> 10.0.0.0         10.192.5.102                        400          0               65206 i
    *                        10.192.5.101                        400          0               65206 i                   
    * i                      10.192.7.152                 0      400         0                65206 i

    R1#sh ip ro | i 10.0.0.0
         10.0.0.0/8 is variably subnetted, 1298 subnets, 14 masks
    B       10.0.0.0/8 [20/0] via 10.192.5.101, 2w2d

     

    Question : from the sh ip bgp output indicate that the best valid path to reach 10.0.0.0 is via next hop 10.192.5.102

                    likewise from the sh ip ro output , it show that the route to reach 10.0.0.0/8 is via next hop of 10.192.5.101

     

    anyone can shed some light , as i expecting both result of sh ip bgp and sh ip ro should be the same next hop ip address?

    but in reality i getting different ip 10.192.5.102 & 10.192.5.101

    thanks alot !!!

    Mr.BLGeeked

     

    • Post Points: 35
  • 12-31-2016 1:30 PM In reply to

    Re: BGP Best Path Selection vs IP Route

    well, but it is close, close enough does not count?   .102 & .101 Big Smile

    there is 2nd line in bgp with 10.192.5.101, 2 entries but 1st the best , so  what does show ip bgp 10.0.0.0/8 show ?

    • Post Points: 20
  • 12-31-2016 1:48 PM In reply to

    • JoeM
    • Top 10 Contributor
    • Joined on 04-15-2012
    • Guadalajara, Mexico
    • Elite
    • Points 31,465

    Re: BGP Best Path Selection vs IP Route

    I also would like to see the more descriptive output.

    show ip bgp 10.0.0.0

    show ip route 10.0.0.0

     

    Thanks

    • Post Points: 5
  • 12-31-2016 2:12 PM In reply to

    Re: BGP Best Path Selection vs IP Route

    hi Martini, thx for looking into this, ya it does not count as basically 101 and 102 are different bgp router neighbor

    R1#sh ip bgp 10.0.0.0
    BGP routing table entry for 10.0.0.0/8, version 245731
    Paths: (5 available, best #1, table Default-IP-Routing-Table)
    Multipath: eBGP iBGP
      Advertised to update-groups:
         1                            2
      65206
        10.192.5.102 from 10.192.5.102 (10.192.7.102)
          Origin IGP, localpref 400, valid, external, multipath, best
      65206, (received-only)
        10.192.5.102 from 10.192.5.102 (10.192.7.102)
          Origin IGP, metric 1, localpref 100, valid, external
      65206
        10.192.5.101 from 10.192.5.101 (10.192.7.101)
          Origin IGP, localpref 400, valid, external, multipath
      65206, (received-only)
        10.192.5.101 from 10.192.5.101 (10.192.7.101)
          Origin IGP, metric 1, localpref 100, valid, external
      65206, (received & used)
        10.192.7.152 (metric 2) from 10.192.7.152 (10.192.7.152)
          Origin IGP, metric 0, localpref 400, valid, internal

    • Post Points: 20
  • 12-31-2016 2:22 PM In reply to

    Re: BGP Best Path Selection vs IP Route

    well well, Geeked , my piping | include command make me overlook another route in the ip routing table,...revisted below is the result ...my apology

    R1#sh ip route bgp | b 10.0.0.0

     10.0.0.0/8 is variably subnetted, 1296 subnets, 14 masks

    B       10.0.0.0/8 [20/0] via 10.192.5.101, 2w3d
                            [20/0] via 10.192.5.102, 2w3d

    so BGP injected both route into routing table ...now it started to make some sense : )

    • Post Points: 5
  • 12-31-2016 2:27 PM In reply to

    Re: BGP Best Path Selection vs IP Route

    (received & used) versus received-only --- i think is due to bgp multipath feature.

    in sh run | s r bgp do u ahve command    maximum -path x

    • Post Points: 35
  • 12-31-2016 10:07 PM In reply to

    Re: BGP Best Path Selection vs IP Route

    yes , it is multipath with following command line in the router bgp section config:

     maximum-paths 4
     maximum-paths ibgp  4

    • Post Points: 5
  • 12-31-2016 10:43 PM In reply to

    Re: BGP Best Path Selection vs IP Route

    well, another puzzle, if this is a multipath, it should show a *m identifier in the sh ip bgp  : )

     

     

    • Post Points: 5
  • 01-02-2017 9:06 AM In reply to

    • peety
    • Top 25 Contributor
    • Joined on 02-23-2011
    • Redmond, WA, USA
    • Elite
    • Points 19,700

    Re: BGP Best Path Selection vs IP Route

    Martinl:

    (received & used) versus received-only --- i think is due to bgp multipath feature.

    Nope, it's due to soft-reconfiguration inbound being enabled. If enabled for a particular neighbor, the learned routes could have three different possible outcomes:

    (received & used) - means the prefix was not altered by the inbound filtering mechanisms (if any), and is being used in the same form as it was learned.

    (received) - means this is how the prefix was learned, prior to any inbound filtering/manipulation mechanisms being applied.

    [nothing in parenthesis] - means this is how the prefix exists in the BGP RIB, after all inbound filtering/manipulation is applied.

    Note that soft-reconfig in is configured on a per-neighbor basis. A neighbor without this feature enabled would simply show the prefixes after filtering.

    CCIE R&S #34583

    Now based in Redmond, WA

    • Post Points: 5
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