Equal Cost Multi-Path (ECMP)

Equal Cost Multi-Path (ECMP) is a routing technique that allows a router to use multiple paths to the same destination when they all have the same cost. Instead of keeping only one route, the router installs every equal-cost path into the routing table and shares the traffic between them.

To understand this concept, let’s use the following OSPF topology where every router advertises its directly connected networks.

Figure 1 – OSPF equal-cost paths from R1 to reach 192.168.2.0/24

In this case, R1 can reach the destination 192.168.2.0/24 using two different paths:

• R1 → R2 → R4

• R1 → R3 → R4

Before ECMP is applied, routers follow a strict order when selecting routes:

1. Prefix Length – The longest prefix match is always preferred. Example: a /24 is preferred over a /16.

   
   

2. Administrative Distance (AD) – If multiple routing protocols advertise the same prefix, the one with the lowest AD is selected. Example: OSPF (AD = 110) is preferred over RIP (AD = 120).
   
   

3. Metric – If the routes come from the same protocol and have the same AD, the lowest metric wins.
   Example: in OSPF, the cost is based on the bandwidth of the links.
   
   

4. Equal Metric – If multiple routes have the same prefix, the same AD, and the same metric, they are all installed → ECMP is applied.

Now let’s go back to our topology.