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• Internal routers—These routers are involved only in routing<br />

<strong>the</strong>ir internal area (intra-area routing) and only maintain<br />

information about <strong>the</strong> area in which <strong>the</strong>y operate. All<br />

out-of-area traffic is forwarded <strong>to</strong> <strong>the</strong> border router, because<br />

only <strong>network</strong> summaries of o<strong>the</strong>r areas within <strong>the</strong> <strong>network</strong> are<br />

maintained locally.<br />

• Au<strong>to</strong>nomous system (AS) boundary routers—These routers<br />

have interfaces on <strong>the</strong> <strong>network</strong> that are outside <strong>the</strong><br />

inter<strong>network</strong> routing domain. These routers commonly<br />

redistribute internal and external routing domain information.<br />

Their job is <strong>to</strong> ensure that external routing information is<br />

distributed through <strong>the</strong> internal routing domain so that traffic<br />

destined for <strong>network</strong>s outside <strong>the</strong> domain is delivered in<br />

accordance with <strong>the</strong> EGP routing policy.<br />

OSPF Network Types<br />

Along with router classification, OSPF also designates <strong>the</strong> different<br />

types of <strong>network</strong>s it operates on. These designations dictate how<br />

OSPF sends messages. There are three types of <strong>network</strong>s that OSPF<br />

recognizes:<br />

• Broadcast <strong>network</strong>s—These <strong>network</strong>s allow multiple routers <strong>to</strong><br />

be connected <strong>to</strong> <strong>the</strong> same physical <strong>network</strong> segment. The<br />

<strong>network</strong> must also support broadcast and multicast capabilities.<br />

OSPF uses multicast addresses 224.0.0.5 and 224.0.0.6 <strong>to</strong> send<br />

and receive messages. All LAN media types and some WAN<br />

media types support <strong>the</strong>se capabilities.<br />

• Point-<strong>to</strong>-point <strong>network</strong>s—A point-<strong>to</strong>-point Layer 2 link is used<br />

<strong>to</strong>connect a pair of routers. A serial line connection is a common<br />

example of a point-<strong>to</strong>-point <strong>network</strong>.<br />

• Nonbroadcast multi-access <strong>network</strong>s—These <strong>network</strong>s allow<br />

multiple routers <strong>to</strong> be connected <strong>to</strong> <strong>the</strong> same <strong>network</strong> segment,<br />

but do not support <strong>the</strong> broadcast and multicast capabilities<br />

needed <strong>to</strong> send OSPF messages. In <strong>the</strong>se cases, messages are<br />

sent using unicast, and additional configuration is needed so<br />

routers know who <strong>to</strong> exchange messages with.<br />

A typical multi-area OSPF implementation will consist of mostly<br />

broadcast <strong>network</strong>s and nonbroadcast multi-access <strong>network</strong>s. In<br />

large WAN inter<strong>network</strong>s, it is common for <strong>the</strong> backbone area <strong>to</strong> be a<br />

collection of meshed Frame Relay PVCs (permanent virtual circuits)<br />

with <strong>the</strong> WAN/LAN gateway routers acting as area border<br />

routers/backbone routers.

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