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TCP/IP Tutorial and Technical Overview - IBM Redbooks

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► Hello <strong>and</strong> dead intervals: The routers must specify the same timer intervals<br />

used in the Hello protocol. We describe this protocol further in “OSPF packet<br />

types” on page 203.<br />

► Stub area flag: The routers must agree that the area is configured as a stub<br />

area. We describe stub areas further in 5.6.5, “OSPF stub areas” on<br />

page 210.<br />

After two routers have become neighbors, an adjacency relationship can be<br />

formed between the devices. Neighboring routers are considered adjacent when<br />

they have synchronized their topology databases. This occurs through the<br />

exchange of link state information.<br />

Designated <strong>and</strong> backup designated router<br />

The exchange of link state information between neighbors can create significant<br />

quantities of network traffic. To reduce the total b<strong>and</strong>width required to<br />

synchronize databases <strong>and</strong> advertise link state information, a router does not<br />

necessarily develop adjacencies with every neighboring device:<br />

► Multi-access networks: Adjacencies are formed between an individual router<br />

<strong>and</strong> the (backup) designated router.<br />

► Point-to-point networks: An adjacency is formed between both devices.<br />

Each multi-access network elects a designated router (DR) <strong>and</strong> backup<br />

designated router (BDR). The DR performs two key functions on the network<br />

segment:<br />

► It forms adjacencies with all routers on the multi-access network. This causes<br />

the DR to become the focal point for forwarding LSAs.<br />

► It generates network link advertisements listing each router connected to the<br />

multi-access network. For additional information regarding network link<br />

advertisements, see “Link state advertisements <strong>and</strong> flooding” on page 201.<br />

The BDR forms the same adjacencies as the designated router. It assumes DR<br />

functionality when the DR fails.<br />

Each router is assigned an 8-bit priority, indicating its ability to be selected as the<br />

DR or BDR. A router priority of zero indicates that the router is not eligible to be<br />

selected. The priority is configured on each interface in the router.<br />

200 <strong>TCP</strong>/<strong>IP</strong> <strong>Tutorial</strong> <strong>and</strong> <strong>Technical</strong> <strong>Overview</strong>

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