Here’s a question that bothered me for years till I finally gave up and labbed it: does ECMP load sharing work in an MPLS core? More specifically, will an LSP split into multiple LSPs?

The Scenario

Imagine a network with alternate paths across the network core. We know that IP traffic takes all paths across the core (usually as long as they are equal-cost, and based on the load sharing algorithm).

Does MPLS work the same way? For example, would labeled traffic from A to Z go over B-C-E and B-D-E links? Would the A-Z LSP split into two LSPs at B?

I was somewhat pleasantly surprised to figure out ECMP works with MPLS. As expected, there are two entries in the IP routing table on router B for the loopback interface of router Z:

B#show ip route 10.0.1.6                    
Routing entry for 10.0.1.6/32
  Known via "isis", distance 115, metric 40, type level-2
  Redistributing via isis
  Last update from 10.0.7.22 on Serial1/1, 00:02:39 ago
  Routing Descriptor Blocks:
    10.0.7.26, from 10.0.1.8, 00:02:39 ago, via Serial1/2
      Route metric is 40, traffic share count is 1
  * 10.0.7.22, from 10.0.1.8, 00:02:39 ago, via Serial1/1
      Route metric is 40, traffic share count is 1 

Interestingly, there are also two entries in the corresponding LFIB entry. Inbound traffic with label 1015 is split across two outbound LSPs with labels 2013 and 3015.

B#show mpls forwarding-table 10.0.1.6 detail
Local      Outgoing   Prefix           Bytes Label   Outgoing   Next Hop    
Label      Label      or Tunnel Id     Switched      interface              
1015       2013       10.0.1.6/32      0             Se1/1      point2point 
    MAC/Encaps=4/8, MRU=1500, Label Stack{2013}
    FF030281 007DD000
    No output feature configured
    Per-destination load-sharing, slots: 0
           3015       10.0.1.6/32      0             Se1/2      point2point 
    MAC/Encaps=4/8, MRU=1500, Label Stack{3015}
    FF030281 00BC7000
    No output feature configured
    Per-destination load-sharing, slots: 1

Please note that ECMP in MPLS core works only with LDP-established paths (which use IP routing table for topology discovery) – MPLS TE tunnels are unidirectional paths established and controlled by the tunnel head-end router. If you want to perform ECMP load balancing across the MPLS TE tunnels, you have to establish multiple tunnels from the head-end router (and use affinity bits or loose source routing to ensure they go across different paths in the MPLS core).