BGP route selction

Can please let me know on what criteria route to 6.6.6.6 is selcted via  155.1.37.7 ?

Rack1R3#sh ip bgp 6.6.6.6
BGP routing table entry for 6.6.6.6/32, version 46
Paths: (2 available, best #2, table Default-IP-Routing-Table)
Flag: 0x820
  Advertised to update-groups:
        1    3    4
  100
    155.1.13.1 from 155.1.13.1 (150.1.1.1)
      Origin IGP, localpref 100, valid, external
  300 100
    155.1.37.7 from 155.1.37.7 (150.1.77.77)
      Origin IGP, localpref 100, valid, external, best <<<<<<<<=====


If shut the neighbour  155.1.13.1 and unshut it agin... and clear the BGP....then as BEST to reach 6.6.6.6 is 155.1.13.1

Rack1R3#conf t
Rack1R3(config)#router bg 200
Rack1R3(config-router)# neighbor 155.1.13.1 shutdown
Rack1R3(config-router)# no neighbor 155.1.13.1 shutdown

Rack1R3(config-router)#^Z

Rack1R3#clear ip bg *

Rack1R3#sh ip bgp 6.6.6.6
BGP routing table entry for 6.6.6.6/32, version 7
Paths: (2 available, best #1, table Default-IP-Routing-Table)
Flag: 0x820
  Advertised to update-groups:
        1    2    3
  100
    155.1.13.1 from 155.1.13.1 (150.1.1.1)
      Origin IGP, localpref 100, valid, external, best <<<<<<<============
  300 100
    155.1.37.7 from 155.1.37.7 (150.1.77.77)
      Origin IGP, localpref 100, valid, external


I have configured "AS-PATH ignore" in the BGP of R3.Below is the configuration on R3.

router bgp 200
 no synchronization
 bgp always-compare-med
 bgp log-neighbor-changes
 bgp bestpath as-path ignore
 network 3.3.3.3 mask 255.255.255.255
 network 155.1.13.0 mask 255.255.255.0
 aggregate-address 10.0.0.0 255.255.252.0 summary-only
 neighbor 155.1.0.5 send-community both
 neighbor 155.1.0.5 soft-reconfiguration inbound
 neighbor 155.1.0.5 route-map NEXTHOP in
 neighbor 155.1.13.1 remote-as 100
 neighbor 155.1.13.1 send-community both
 neighbor 155.1.13.1 route-map TOAS_60 in
 neighbor 155.1.37.7 remote-as 300
 neighbor 155.1.37.7 send-community both
 neighbor 155.1.37.7 advertise-map AS254 non-exist-map LINKS1/2
 no auto-summary

 


is there any command to check on what criteria BGP best path is selected ?

/Ganpat

 

 

Comments

  • What is the IGP metric to both next-hop?

    This is the order of path selection:

    highest WEIGHT- you have same
    highest LOCAL_PREF - you have same
    shortest AS_PATH but you have as-path ignore, skips this step
    lowest origin type, IGP origin type is prefer than EGP you have all IGP origin
    Lowest MED : you have same
    Prefer eBGP over iBGP : you have both eBGP peering
    Prefer Lowest IGP metric of BGP next-hop: This could be the selection method

    Please check IGP metric of BGP next-hop.

     

  •  

    Metric details to next hop as


    Rack1R3#sh ip route 155.1.13.1
    Routing entry for 155.1.13.0/24
      Known via "connected", distance 0, metric 0 (connected, via interface) ====>Metric 0
      Redistributing via eigrp 100, rip
      Advertised by rip
                    bgp 200
      Routing Descriptor Blocks:
      * directly connected, via Serial1/2
          Route metric is 0, traffic share count is 1

    Rack1R3#sh ip route 155.1.37.7
    Routing entry for 155.1.37.0/24
      Known via "connected", distance 0, metric 0 (connected, via interface)====>Metric 0
      Redistributing via eigrp 100, rip
      Advertised by rip
      Routing Descriptor Blocks:
      * directly connected, via FastEthernet0/0
          Route metric is 0, traffic share count is 1

     

    Looking like metric is zero..please suggest.

  • My guess is it is picking the oldest route. That is why the selection changes when you clear BGP on one peer.

  • its also good to remeber steps 11, 12 and 13 in the bgp path selection ..

    11.Prefer the route that comes from the BGP router with the lowest router ID.

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

    13, Prefer the path that comes from the lowest neighbor address.

    what i think (IMHO) is that is happening is this

    First time you runb the command sh ip bgp , its the oldest , as daniel has said.

    when you reset the peering then its step 13 , lowest neibhour.

    you have seen two seperate route calculations with two seperate outcomes.

    HTH

  • <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">I also guess Its because of the oldest route only

    Sent on my BlackBerry®

    From: daniel.dib <[email protected]>
    Date: 23 Feb 2012 00:48:39 -0800
    Subject: Re: [CCIE R&S] BGP route selction

    My guess is it is picking the oldest route. That is why the selection changes when you clear BGP on one peer.




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  • My guess is it is picking the oldest route. That is why the selection changes when you clear BGP on one peer.

    Thats also my guess. When you use "maximum path 1" in OSPF for example, there also the oldest route should win.

    Regards!

     

  • My vote is on oldest route :)

  • Rack1R3#sh ip route 155.1.13.1
    Routing entry for 155.1.13.0/24
      Known via "connected", distance 0, metric 0 (connected, via interface) ====>Metric 0
      Redistributing via eigrp 100, rip
      Advertised by rip
                    bgp 200
      Routing Descriptor Blocks:
      * directly connected, via Serial1/2
          Route metric is 0, traffic share count is 1

    Rack1R3#sh ip route 155.1.37.7
    Routing entry for 155.1.37.0/24
      Known via "connected", distance 0, metric 0 (connected, via interface)====>Metric 0
      Redistributing via eigrp 100, rip
      Advertised by rip
      Routing Descriptor Blocks:
      * directly connected, via FastEthernet0/0
          Route metric is 0, traffic share count is 1

    Here two same routes are learned via ebgp and upto oldest bgp routes in best path selection are same. So oldest route prefered as other are also mentioned in this post. We can overcome this behaviour by using "bgp bestpath compare-routerid" this command will allow us to prefer routes based on the router-id of the receiving routes. Lower router-id is prefered.

  • Rack1R3#sh ip route 155.1.13.1
    Routing entry for 155.1.13.0/24
      Known via "connected", distance 0, metric 0 (connected, via interface) ====>Metric 0
      Redistributing via eigrp 100, rip
      Advertised by rip
                    bgp 200
      Routing Descriptor Blocks:
      * directly connected, via Serial1/2
          Route metric is 0, traffic share count is 1

    Rack1R3#sh ip route 155.1.37.7
    Routing entry for 155.1.37.0/24
      Known via "connected", distance 0, metric 0 (connected, via interface)====>Metric 0
      Redistributing via eigrp 100, rip
      Advertised by rip
      Routing Descriptor Blocks:
      * directly connected, via FastEthernet0/0
          Route metric is 0, traffic share count is 1

    Here two same routes are learned via ebgp and upto oldest bgp routes in best path selection are same. So oldest route prefered as other are also mentioned in this post. We can overcome this behaviour by using "bgp bestpath compare-routerid" this command will allow us to prefer routes based on the router-id of the receiving routes. Lower router-id is prefered.

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