Load Balancing over 2 different paths

Unanswered Question
Nov 29th, 2007

If I have an IP routing enabled 3750G that has 2 different L3 paths to a destination and OSPF recognizes these 2 equal cost paths, does CEF automatically load balance on these links or does OSPF use some load-balancing mechanism?

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Edison Ortiz Thu, 11/29/2007 - 08:49

Small correction on how CEF makes forwarding decision.

CEF uses a FIB (Forwarding Information Base) not RIB (Routing Information Base) to make IP destination prefix-based switching decisions.

The FIB contains the prefixes from the IP routing table structured in a way that is optimized for forwarding. When routing or topology changes occur in the network, the IP routing table is updated, and those changes are reflected in the FIB. The FIB maintains next-hop address information based on the information in the IP routing table.

Because there is a one-to-one correlation between FIB entries and routing table entries, the FIB contains all known routes and eliminates the need for the route cache maintenance that is associated with switching paths such as those used in fast switching and optimum switching.

This is a good document to read on CEF

http://www.cisco.com/univercd/cc/td/doc/product/software/ios124/124cg/hisw_c/ch05/ceb_ov.htm#wp1053721

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