BGP Multipath Load Sharing

Below shows the IP routing table on RT1 after implemented the maximum-paths 2 command to perform BGP Multipath Load Sharing across the parallel EBGP paths over RT2 and RT3. Note that this command affects only the number of routes being inserted into and maintained in the IP routing table; not the number of best paths as to be selected by the BGP process. BGP still designates one of the paths as the best path based on the best path selection algorithm, as indicated by the > symbol; and this is the path that the router (RT1) advertises to both its IBGP and EBGP peers.

RT1#sh ip bgp
BGP table version is 4, local router ID is 192.168.1.1
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
              r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete

   Network          Next Hop            Metric LocPrf Weight Path
*> 192.168.1.0      0.0.0.0                  0         32768 i
*> 192.168.2.0      13.13.13.3               0             0 200 i
*                   12.12.12.2               0             0 200 i
RT1#do sh ip route

Gateway of last resort is not set

     12.0.0.0/24 is subnetted, 1 subnets
C       12.12.12.0 is directly connected, Serial1/0
C    192.168.1.0/24 is directly connected, FastEthernet0/0
     13.0.0.0/24 is subnetted, 1 subnets
C       13.13.13.0 is directly connected, Serial1/1
B    192.168.2.0/24 [20/0] via 12.12.12.2, 00:01:32
                    [20/0] via 13.13.13.3, 00:00:16
RT1#

Note: Load balancing and load sharing are actually different concepts!

The maximum-paths {num} BGP router subcommand enables EBGP multipath; while the maximum-path ibgp {num} BGP router subcommand enables IBGP multipath. The paths to the same destination must have the same weight, local preference, AS path length (in fact the actual value), Origin, and MED attributes in order to become multipath candidates.

Below are some additional requirements for EBGP multipath:

• The path should be learned from an external or confederation-external neighbor (EBGP).
  

Below are some additional requirements for IBGP multipath:

• The path should be learned from an internal neighbor (IBGP).
  
• The IGP metric towards the BGP next-hops for the paths is same (equal-cost IGP paths); unless the routers are configured for Unequal-Cost IBGP Multipath Load Balancing.