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28 An 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 r un 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
5.1 27 August 2007
Copyright © 2005-2007, Nortel Networks
.