Instruction Manual

IP Routing Features
Configuring OSPF
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
autonomous system.
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 Figure 5-23, 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.
Host “A”
10.10.10.1/24
Host “D”
10.10.10.4/24
Host “B”
10.10.10.2/24
Host “C”
10.10.10.3/24
Network
10.10.10.0/24
15.0.9.0/24
ECMP Router “1”
Area 8
12.0.9.0/24
13.0.9.0/24
14.0.9.0/24
Router “1”: IP Route Table
Destination Gateway
10.10.10.0/24 12.0.9.2
10.10.10.0/24 13.0.9.3
10.10.10.0/24 14.0.9.4
Router “2”
Area 9
Router “3”
Area 9
Router “4”
Area 9
Area 9
Figure 5-23. Example of OSPF ECMP Multiple Next-Hop Routing (Inter-Area)
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