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Fine-tuning CEF load-sharing

In environments with a low number of IP hosts you have to fine-tune the CEF load-sharing algorithm to ensure that the traffic is spread between all parallel paths. A typical scenario is a primary-backup data center setup with pairs of replicating servers, as shown in the figure below.

In these cases, you have to try different values of seed parameter of the CEF universal algorithm.
For example, if you have two equal-cost paths between networks and ...
a1#show ip cef detail, version 33, epoch 0, per-destination sharing
0 packets, 0 bytes
via, Serial0/0/0.200, 0 dependencies
traffic share 1
next hop, Serial0/0/0.200
valid adjacency
via, Serial0/0/0.100, 0 dependencies
traffic share 1
next hop, Serial0/0/0.100
valid adjacency
... you might want the traffic between and to flow over a different link than the traffic between and The command that will help you is the show ip cef exact-route source destination. In our example, both traffic flows would go over the same serial link:
a1#show ip cef exact-route -> : Serial0/0/0.100 (next hop
a1#show ip cef exact-route -> : Serial0/0/0.100 (next hop
However, by changing the seed parameter of the ip cef load-sharing algorithm universal command, you can influence the CEF hashing function, eventually reaching a state where the traffic flows are spread between both WAN links:
a1(config)#ip cef load-sharing algorithm universal 1
a1#show ip cef exact-route -> : Serial0/0/0.100 (next hop
a1#show ip cef exact-route -> : Serial0/0/0.200 (next hop

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