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Chapter 7. OSPF | 86

7. OSPF

Open Shortest Path First

This chapter provides the following examples:

• Inter-area Router on page 87

• OSPF on a Border Router on page 92

• Stub Areas on page 98

• nssa Areas on page 107

• VLAN Routing OSPF on page 116

• OSPFv3 on page 122

For larger networks Open Shortest Path First (OSPF) is generally used in preference to RIP.

OSPF offers several benefits to the administrator of a large or complex network:

• Less network traffic:

- Routing table updates are sent only when a change has occurred.

- Only the part of the table which has changed is sent.

- Updates are sent to a multicast, not a broadcast, address.

• Hierarchical management, allowing the network to be subdivided.

The top level of the hierarchy of an OSPF network is known as an autonomous system (AS) or

routing domain, and is a collection of networks with a common administration and routing

strategy. The AS is divided into areas: Intra-area routing is used when a source and destination

address are in the same area, and inter-area routing across an OSPF backbone is used when

they are not. An inter-area router communicates with border routers in each of the areas to which

it provides connectivity.

The M4100 and M7100 Managed Switch operating as a router and running OSPF determines

the best route using the assigned cost and the type of the OSPF route. The order for choosing a

route if more than one type of route exists is as follows:

• Intra-area.

• Inter-area.

• External type 1: The route is external to the AS.

• External type 2: The route was learned from other protocols such as RIP.

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