Dell PowerConnect M6220 Configuration Manual page 82

Link A
Link B
Router A
Figure 4-7. Forwarding Without ECMP
With ECMP, Router A can forward traffic to some destinations in Network D via Link A and traffic to
other destinations in Network D via Link B, thereby taking advantage of the bandwidth of both links. A
hash algorithm is applied to the destination IP addresses to provide a mechanism for selecting among the
available ECMP paths.
ECMP routes may be configured statically or learned dynamically. If a user configures multiple static
routes to the same destination but with different next hops, then those routes will be treated as a single
route with two next hops. For example, given the network in Figure 4-8, if the user configures the
following two static routes on Router A, the routing table will contain a single route to 20.0.0.0/8:
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
Router A
Figure 4-8. Next Hop with Two Static Routes
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.
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Routing Configuration
Next hop 1
Next hop 2
Router B
10.1.1.2
10.1.2.2
Router B
Network D
20.0.0.0/8