EIGRP – Best Path Selection


By default EIGRP uses bandwidth and delay of the line to determine the best path to a remote network. However, if desired, EIGRP can use a composite of four metrics: bandwidth, delay, reliability, and load. Maximum transmission unit (MTU) is not used in EIGRP calculations but is a required parameter in some EIGRP-related commands. Also, by default, EIGRP will provide equal-cost load balancing across up to four links. You can disable load balancing by setting the ‘maximum-paths’ to ’1′. In this lab I use four Cisco 3725 routers [IOS 12.4(15)T10] to demonstrate EIGRP path selection behavior on Cisco routers. Below is a diagram of my setup.

EIGRP lab topology

EIGRP lab topology


My initial topology and routing table is displayed below.
Initial R1 EIGRP topology table

Initial R1 EIGRP topology table

Initial R1 EIGRP routing table

Initial R1 EIGRP routing table


As you can see from the above, since EIGRP provides equal-cost load balancing across for up to four links, both equal routes to the 12.0.0.0/30 network are in the topology and routing table. Below, I lower the bandwidth of the 10.0.13.0/30 link to 50 MB and since bandwidth is a metric used by EIGRP to determine the best path, you can see that the 10.0.13.0/30 link remains in the topology table but is removed from the routing table. Only the 11.0.0.0/30 link to the 12.0.0.0/30 network remains in the routing table.
R1 EIGRP topology table after lowering the bandwidth of the 10.0.13.0/30 link

R1 EIGRP topology table after lowering the bandwidth of the 10.0.13.0/30 link

R1 EIGRP routing table after lowering the bandwidth of the 10.0.13.0/30 link

R1 EIGRP routing table after lowering the bandwidth of the 10.0.13.0/30 link


Twitt

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