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98 Routing Configuration
Routing protocols can also be configured to compute ECMP routes. For example, referring to Figure 4-8,
if OSPF were configured in on both links connecting Router A and Router B, and if Router B advertised
its connection to 20.0.0.0/8, then Router A could compute an OSPF route to 20.0.0.0/8 with next hops of
10.1.1.2 and 10.1.2.2.
Static and dynamic routes are all included in a single combined routing table. This routing table accepts
ECMP routes; however, the routing table will not combine routes from different sources to create ECMP
routes. Referring to Figure 4-8, assume OSPF is configured on only one of the links between Router A
and Router B. Then, on Router A, assume that OSPF reports to the routing table a route to 20.0.0.0/8
with a next hop of 10.1.1.2. If the user also configures a static route to 20.0.0.0/8 with a single next hop of
10.1.2.2, the routing table will not combine the OSPF and static routes into a single route to 20.0.0.0/8
with two next hops. All next hops within an ECMP route must be provided by the same source.
An ECMP route contains only next hops whose paths to the destination are of equal cost. Referring to
Figure 4-8, if OSPF were configured on all links, but Router A's interface to the 10.1.1.x network had an
OSPF link cost of 5 and its interface to the 10.1.2.x network had an OSPF link cost of 10, then OSPF
would use only 10.1.1.2 as the next hop to 20.0.0.0/8.
Example 1: Configuring an ECMP Route
In the following example, two static routes to the same destination are configured to use different next
hops (e.g., for load balancing purposes). Note that the preference metric is not specified, so both routes
assume the default static route preference of 1.
console#Config
ip route 20.0.0.0 255.0.0.0 10.1.1.2
ip route 20.0.0.0 255.0.0.0 10.1.2.2
exit
The following command adds a third route with a preference value of 5. This route will be used only
when the first two are unreachable:
ip route 20.0.0.0 255.0.0.0 10.1.3.2 5