Friday, October 24, 2008

OSPF routing part VI - load balancing

Consider the scenario where router 4 and 5 are running OSPF and have 2 paths connecting each other: one over a 1.544 mb frame relay connection and the other a 100MB LAN segment.

The requirement here is to get OSPF to equally load balance traffic over both connections, but without adjusting the OSPF cost.

The OSPF default is that it will load balance over 4 paths as long as they have equal cost. In this scneraio the OSPF cost can be examined using the show ip ospf interface command. The Frame-relay cost is 64 and the LAN segment has cost 1. This is calculated by dividing the auto-cost reference bandwidth ( 100,000,000 ) by the interface bandwidth.

Herein lies the clue to enable resolution of this problem. If we are prohibited from adjusting the cost, the only other variable that can be used to influence OSPF is the interface bandwidth. This can be adjusted under the interface configuration.

Hence on R4 and R5 i use the config-if#bandwidth 100000 command on the serial frame-relay interfaces.

I then examine the routing table on R4 to check that it now has 2 paths installed in the routing table for links originated from R5.

R4#s ip route
Routing entry for
Known via "ospf 1", distance 110, metric 2, type intra area
Last update from on Serial2/0, 00:00:14 ago
Routing Descriptor Blocks:
*, from, 00:00:14 ago, via FastEthernet0/0
Route metric is 2, traffic share count is 1, from, 00:00:14 ago, via Serial2/0
Route metric is 2, traffic share count is 1

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