Product guide

IP Routing Features
Configuring OSPF
OSPF Equal-Cost Multipath (ECMP) for Different
Subnets Available Through the Same Next-Hop Routes
Using software prior to release E.10.xx, if different subnet destinations in an
OSPF network are reachable through a set of equal-cost next-hop routes, the
router chooses the same next-hop route for traffic to all of these destinations.
Beginning with software release E.10.xx, 5300xl routers support optional load-
sharing across redundant links where the network offers two, three, or four
equal-cost next-hop routes for traffic to different subnets. (All traffic for
different hosts in the same subnet goes through the same next-hop router.)
For example, in the OSPF network shown below, IP load-sharing is enabled
on router “A”. In this case, OSPF calculates three equal-cost next-hop routes
for each of the subnets and then distributes per-subnet route assignments
across these three routes.
Router “A”
Router “B”
Router “C”
10.1.0.0/16
Router “4”
Router “D”
Router “3”
10.2.0.0/16
10.3.0.0/16
10.32.0.0/16
10.42.0.0/16
Router “2”
Router “1”
Equal-Cost Next-Hop Routes
Workstation
Figure 11-25.Example of Load-Sharing Traffic to Different Subnets Through Equal-Cost Next-Hop Routers
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