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New Member

BGP Path Selection

With reference to cisco's document on BGP Best Path Selection Algorithm (http://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/13753-25.html).

 

Out of given 9 paths why 6th has been selected even though AS_PATH for 8th route is better.

 

Can anyone explains here, as this document has not considered the AS-PATH during path selection and used lowest ROUTER ID only.

 

Thanks in advance and expect technical explanation here.

1 REPLY
New Member

Hey BuddyThe AS_PATH for both

Hey Buddy

The AS_PATH for both is only 1, don't get confused by (AS_SET) which only counts as 1 no matter how many AS are in the set.  Refer to section "How the Best Path Algorithm Works"


4.Prefer the path with the shortest AS_PATH.

Note: Be aware of these items:

◦An AS_SET counts as 1, no matter how many ASs are in the set.


So bearing the above in mind

Example: BGP Best Path Selection

Path6
  (64955 65003) 65089
--- this equals 1
    172.16.254.226 (metric 20645) from 10.57.255.11 (10.57.255.11)
      Origin IGP, metric 0, localpref 100, valid, confed-external, best
      Extended Community: RT:1100:1001
      mpls labels in/out nolabel/362

!--- BGP selects this as the Best Path on comparing
!--- with all the other routes and selected based on lower router ID.

Path8
  (65003) 65089
--- this equals 1
    172.16.254.226 (metric 20645) from 172.16.254.234 (172.16.254.234)
      Origin IGP, metric 0, localpref 100, valid, confed-external
      Extended Community: RT:1100:1001
      mpls labels in/out nolabel/362


Comparing path 6 with path 8:
 Both paths have reachable next hops
 Both paths have a WEIGHT of 0
 Both paths have a LOCAL_PREF of 100
 Both paths are learned
 Both paths have AS_PATH length 1
--- because the (AS_SET) always equals 1
 Both paths are of origin IGP
 Both paths have the same neighbor AS, 65089, so comparing MED.
 Both paths have a MED of 0
 Both paths are confed-external
 Both paths have an IGP metric to the NEXT_HOP of 20645
Path 6 is better than path 8 because it has a lower Router-ID.

 

Hope it helps (:

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