Equal cost multi-path routing (ECMP)

The ECMP feature allows OSPF to add routes with multiple next-hop addresses and with equal costs to a given destination in the forwarding information base (FIB) on the routing switch. For example, if you display the IP route table by entering the show ip route command, multiple next-hop routers are listed for the same destination network (21.0.9.0/24) as shown in the following example:

Example of show ip route command output with multiple next-hop routes

switch show ip route

                            IP Route Entries

Destination        Gateway         VLAN Type      Sub-Type   Metric     Dist.
------------------ --------------- ---- --------- ---------- ---------- -----
1.0.0.0/8          10.0.8.1        1    static               1          1
10.0.8.0/21        DEFAULT_VLAN    1    connected            1          0
12.0.9.0/24        VLAN3           3    connected            1          0
15.0.0.0/8         10.0.8.1        1    static               1          1
21.0.9.0/24        162.130.101.2   2    ospf      IntraArea  2          110
21.0.9.0/24        162.130.101.3   2    ospf      IntraArea  2          110
21.0.9.0/24        162.130.101.4   2    ospf      IntraArea  2          110
127.0.0.0/8        reject               static               0          0
127.0.0.1/32       lo0                  connected            1          0
162.130.101.0/24   VLAN2           2    connected            1          0

For a given destination network in an OSPF domain, multiple ECMP next-hop routes can be one of the following types:

Intra-area (routes to the destination in the same OSPF area)
Inter-area (routes to the destination through another OSPF area)
External (routes to the destination through another AS)

Multiple ECMP next-hop routes cannot be a mixture of intra-area, inter-area, and external routes. In the example above, the multiple next-hop routes to network 21.0.9.0/24 are all intra-area.

Also, according to the distributed algorithm used in the selection of ECMP next-hop routes:

Intra-area routes are preferred to inter-area routes.
Inter-area routes are preferred to external routes through a neighboring AS.

In addition, ECMP ensures that all traffic forwarded to a given host address follows the same path, which is selected from the possible next-hop routes.

For example, in the following figure, the ECMP inter-area routes to destination network 10.10.10.0/24 consist of the following next-hop gateway addresses: 12.0.9.2, 13.0.9.3, and 14.0.9.4.

However, the forwarding software distributes traffic across the three possible next-hop routes in such a way that all traffic for a specific host is sent to the same next-hop router.

As shown in the following figure, one possible distribution of traffic to host devices is:

Traffic to host 10.10.0.1 passes through next-hop router 12.0.9.2.
Traffic to host 10.10.0.2 passes through next-hop router 13.0.9.3.
Traffic to host 10.10.0.3 passes through next-hop router 12.0.9.2.
Traffic to host 10.10.0.4 passes through next-hop router 14.0.9.4.