Configuring gated

Configuring the OSPF Protocol

Multi-access networks (networks that can be accessed through two or more neighbor routers) must have one of the routers identified as a designated router.

Designated routers initiate OSPF protocol functions on behalf of the network. In Figure 3-3, you can access network 1 through the neighbor routers A, B, or C; one of these routers is elected to become the designated router for network 1.

The set of routers that exchange OSPF protocol packets between areas in an autonomous system is called the backbone. In Figure 3-3, routers C, D, E, F, G, and I form an AS backbone that allows protocol packets to travel between the three areas.

OSPF routers exchange various types of link state advertisements to build their topological databases. Most link state advertisements are flooded (sent to every router) throughout the attached area. An exception is the link state advertisement sent out by AS boundary routers that describe routes to destinations outside the AS; these advertisements are flooded throughout the AS. Table 3-1 shows the various types of link state advertisements used by the OSPF protocol.

Table 3-1

Types of Link State Advertisements

 

 

 

 

 

 

 

TypeContentOriginatedFlooded

 

 

 

By

Throughout

 

 

 

 

 

 

Router link

Router’s links to

Internal and

Area

 

 

area

area border

 

 

 

 

routers

 

 

 

 

 

 

 

Network

List of routers

Designated

Area

 

link

attached to network

router

 

 

 

 

 

 

 

Summary

Routes to

Area border

Area

 

link

destinations outside

router

 

 

 

area but within AS

 

 

 

 

 

 

 

 

AS

Routes to

AS boundary

AS

 

external

destinations outside

router

 

 

link

AS

 

 

 

 

 

 

 

AS boundary routers exchange routing information with routers in other autonomous systems. An AS boundary router can be an area border router or an internal router. It can also be a backbone router, but

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Chapter 3