Nortel Networks NN47200-503 manual Benefits, Ospf routing algorithm

28An Introduction to IP Routing Protocols

Benefits

Benefits in large networks OSPF offers the following benefits:

•Fast convergence

In the event of topological changes, OSPF recalculates routes quickly.

•Minimal routing protocol traffic

Unlike distance vector routing protocols such as RIP, OSPF generates a minimum of routing protocol traffic.

•Load sharing

OSPF provides support for equal-cost multipath routing. If several equal-cost routes to a destination exist, traffic is distributed equally among them.

•Because OSPF does not use hop count in its calculation, the routing domain is scalable.

OSPF routing algorithm

A separate copy of the OSPF routing algorithm runs in each area. Routers which are connected to multiple areas run multiple copies of the algorithm. The sequence of processes governed by the routing algorithm is as follows:

1.When a router starts, it initializes the OSPF data structures and then waits for indications from lower-level protocols that its interfaces are functional.

2.A router then uses the Hello Protocol to discover neighbors. On point-to-point and broadcast networks the router dynamically detects its neighbors by sending hello packets to the multicast address AllSPFRouters. On non-broadcast multiaccess networks, some configuration information is required in order to discover neighbors.

3.On all multiaccess networks (broadcast or non-broadcast), the Hello Protocol also elects a DR for the network.

4.The router attempts to form adjacencies with some of its neighbors. On multiaccess networks, the DR determines which routers become adjacent. This behavior does not occur if a router is configured as a passive interface, because passive interfaces do not form adjacencies.

5.Adjacent neighbors synchronize their topological databases.

6.The router periodically advertises its link-state, and also does so when its local state changes. LSAs include information about adjacencies enabling quick detection of dead routers on the network.

7.LSAs are flooded throughout the area, ensuring that all routers in an area have exactly the same topological database.

Nortel Ethernet Routing Switch 5500 Series

Configuration-IP Routing Protocols

NN47200-503 03.01 Standard

5.127 August 2007

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