OSPF Load balancing Mechanism

[hashmia]

How does OSPF treats two default routes in its routing table that are learned from its neighbor routers. That is,

Scenario:

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Let say two ASBRs (Router 1 & 2) advertise the default route using "default originate command" to their "directly" connected neighbor Router 3. The Router 3 has two default routes one from each of Router 1 & 2.

Q1. Does Router 3 will load balance the traffic across the two default routes?

Q2. Will the load-balancing be per packet if we enable "ip cef" on Router 3?

[thisisshanky]

OSPF looks at only the cost of the routes. So as long as both routes have same cost, both will be in the routing table. The type of load balancing will be equal cost load balancing. The type of packet switching used by the router will depend on the type of switching configured for that interface (CEF, fast switching etc). By default CEF uses per source-destination pair type of Load balancing. A hash is calculated based on the source and destination address pair (from the packet) and one of the routes is selected for this source-destination pair. The route used will change for a different source-destination pair.

Per packet load sharing wont be enabled until you manually enable it with a command. Its usually undesirable to use per packet load sharing unless you have two equal cost links between two routers, and you have similar RTT on each of the links.

HTH