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. You don’t need to know OSPF yet: it is a dynamic routing protocol covered later, and here it simply gives R1 two routes with the same cost.

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.

R1 learns two equal-cost routes to reach 192.168.2.0/24 through OSPF. Both routes have AD = 110 and a metric of 3, so according to the route selection process, ECMP can be applied here and both routes can be installed simultaneously in the routing table.

Figure 2 – Equal-cost OSPF routes to 192.168.2.0/24

We can confirm this by looking at the routing table:

R1# show ip route

Gateway of last resort is not set

     192.168.1.0/24 is variably subnetted, 2 subnets, 2 masks
C       192.168.1.0/24 is directly connected, GigabitEthernet0/0
L       192.168.1.1/32 is directly connected, GigabitEthernet0/0
O    192.168.2.0/24 [110/3] via 192.168.10.2, 00:05:54, GigabitEthernet0/1
                    [110/3] via 192.168.20.2, 00:05:54, GigabitEthernet0/2
//output ommited

R1 can now use 2 different next hops to send traffic to 192.168.2.0/24.

Thanks to ECMP, the traffic is now load-balanced across the two paths, allowing R1 to make better use of the available bandwidth and distribute the load.

Figure 3 – ECMP applied: both OSPF routes installed in the routing table

All IGPs (RIP, OSPF, IS-IS) and static routes support ECMP by default.
EIGRP also performs ECMP by default. Its variance command goes further and allows unequal-cost load balancing, which is outside the CCNA scope.

On Cisco IOS, up to 4 equal-cost routes can be installed by default (this can be increased on some platforms using the maximum-paths command).

Equal Cost Multi-Path (ECMP) is a fundamental routing feature.
When multiple paths have the same prefix, administrative distance, and metric, they are all installed in the routing table and used simultaneously.

This provides two main benefits:

• Traffic load is distributed across multiple paths.

• Available bandwidth is better utilized, making the network more efficient.

At the CCNA level, remember:
Routers can use ECMP with IGPs such as RIP, OSPF, IS-IS, as well as with static routes.