HP Routing Services -UX 11i v2 manual Configuring the Ospf Protocol

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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
	Type	Content	Originated	Flooded
			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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