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BGP Best Path Selection with Example

Border Gateway Protocol (BGP) routers typically receive multiple paths to the same destination. The BGP best path algorithm decides which is the best path to install in the IP routing table and to use for traffic forwarding.

Assume that all paths that a router receives for a particular prefix are arranged in a list. The list is similar to the output of the show ip bgp longer-prefixes command. 

In this case, some paths are not considered as candidates for the best path. Such paths typically do not have the valid flag in the output of the show ip bgp longer-prefixes command.

Fig 1.1- Sample Topology BGP

How the BGP Best Path Algorithm Works
BGP assigns the first valid path as the current best path. BGP then compares the best path with the next path in the list, until BGP reaches the end of the list of valid paths. This list provides the rules that are used to determine the best path:
  • Prefer the path with the highest WEIGHT
  • Prefer the path with the highest LOCAL_PREF
  • Prefer the path that was locally originated via a network or aggregate BGP subcommand or through redistribution from an IGP.
  • Prefer the path with the shortest AS_PATH
  • Prefer the path with the lowest origin type.
  • IGP is lower than Exterior Gateway Protocol (EGP), and EGP is lower than INCOMPLETE.
  • Prefer the path with the lowest multi-exit discriminator (MED).
  • Prefer eBGP over iBGP paths.
  • If bestpath is selected, go to Step 9 (multi-path).
  • Prefer the path with the lowest IGP metric to the BGP next hop.
  • Determine if multiple paths require installation in the routing table for BGP Multi-path.
  • When both paths are external, prefer the path that was received first (the oldest one).
  • Prefer the route that comes from the BGP router with the lowest router ID.
  • If the originator or router ID is the same for multiple paths, prefer the path with the minimum cluster list length.
  • Prefer the path that comes from the lowest neighbor address.

 Note for AS_PATH criteria
  • This step is skipped if you have configured the bgp bestpath as-path ignore command.
  • An AS_SET counts as 1, no matter how many ASs are in the set.
  • The AS_CONFED_SEQUENCE and AS_CONFED_SET are not included in the AS_PATH length.

Note for MED
  • This comparison only occurs if the first (the neighboring) AS is the same in the two paths. Any confederation sub-ASs are ignored.
  • In other words, MEDs are compared only if the first AS in the AS_SEQUENCE is the same for multiple paths. Any preceding AS_CONFED_SEQUENCE is ignored.
  • If bgp always-compare-med is enabled, MEDs are compared for all paths.
  • You must disable this option over the entire AS. Otherwise, routing loops can occur.
  • If bgp bestpath med-confed is enabled, MEDs are compared for all paths that consist only of AS_CONFED_SEQUENCE.These paths originated within the local confederation.
  • THE MED of paths that are received from a neighbor with a MED of 4,294,967,295 is changed before insertion into the BGP table. The MED changes to to 4,294,967,294.
Let's take an example for path selection in BGP 


RouteXP_R1#show ip bgp vpnv4 rd 1100:1001 10.30.116.0/23
BGP routing table entry for 1100:1001:10.30.116.0/23, version 26765275
Paths: (9 available, best #6, no table)
  Advertised to update-groups:
     1          2          3         
  (65001 64955 65003) 65089, (Received from a RR-client)
    172.16.254.226 (metric 20645) from 172.16.224.236 (172.16.224.236)
      Origin IGP, metric 0, localpref 100, valid, confed-internal
      Extended Community: RT:1100:1001
      mpls labels in/out nolabel/362
  (65008 64955 65003) 65089
    172.16.254.226 (metric 20645) from 10.131.123.71 (10.131.123.71)
      Origin IGP, metric 0, localpref 100, valid, confed-external
      Extended Community: RT:1100:1001
      mpls labels in/out nolabel/362
  (65001 64955 65003) 65089
    172.16.254.226 (metric 20645) from 172.16.216.253 (172.16.216.253)
      Origin IGP, metric 0, localpref 100, valid, confed-external
      Extended Community: RT:1100:1001
      mpls labels in/out nolabel/362
  (65001 64955 65003) 65089
    172.16.254.226 (metric 20645) from 172.16.216.252 (172.16.216.252)
      Origin IGP, metric 0, localpref 100, valid, confed-external
      Extended Community: RT:1100:1001
      mpls labels in/out nolabel/362
  (64955 65003) 65089
    172.16.254.226 (metric 20645) from 10.77.255.57 (10.77.255.57)
      Origin IGP, metric 0, localpref 100, valid, confed-external
      Extended Community: RT:1100:1001
      mpls labels in/out nolabel/362
  (64955 65003) 65089
    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.

  (64955 65003) 65089
    172.16.254.226 (metric 20645) from 172.16.224.253 (172.16.224.253)
      Origin IGP, metric 0, localpref 100, valid, confed-internal
      Extended Community: RT:1100:1001
      mpls labels in/out nolabel/362
  (65003) 65089
    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
  65089, (Received from a RR-client)
    172.16.228.226 (metric 20645) from 172.16.228.226 (172.16.228.226)
      Origin IGP, metric 0, localpref 100, valid, confed-internal
      Extended Community: RT:1100:1001
      mpls labels in/out nolabel/278

BGP selects the best path out of these 9 paths by considering various attributes that are explained in this document. In the output shown here, BGP compares the available paths and selects Path# 6 as the best path based on its lower router-ID.

Comparing path 1 with path 2:
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
Both paths are of origin IGP
The paths have different neighbor AS's so ignoring MED
Both paths are internal
  (no distinction is made between confed-internal and confed-external)
Both paths have an IGP metric to the NEXT_HOP of 20645
Path 2 is better than path 1 because it has a lower Router-ID.

Comparing path 2 with path 3:
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
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 2 is better than path 3 because it has a lower Router-ID.

Comparing path 2 with path 4:
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
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 2 is better than path 4 because it has a lower Router-ID.

Comparing path 2 with path 5:
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
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 5 is better than path 2 because it has a lower Router-ID.

Comparing path 5 with path 6:
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
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 5 because it has a lower Router-ID.

Comparing path 6 with path 7:
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
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 internal
  (no distinction is made between confed-internal and confed-external)
Both paths have an IGP metric to the NEXT_HOP of 20645
Path 6 is better than path 7 because it has a lower Router-ID.

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
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.

Comparing path 6 with path 9:
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
Both paths are of origin IGP
The paths have different neighbor AS's so ignoring MED
Both paths are internal
  (no distinction is made between confed-internal and confed-external)
Both paths have an IGP metric to the NEXT_HOP of 20645
Path 6 is better than path 9 because it has a lower Router-ID.

Conclusion: The best path is #6