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Understanding the network.pdf - Back to Home

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outers:<br />

• <strong>Back</strong>bone Routers—These routers maintain complete<br />

routing information for all <strong>the</strong> <strong>network</strong>s (all <strong>the</strong> areas or<br />

domains) that are connected <strong>to</strong> <strong>the</strong> backbone.<br />

• Area Border Routers (ABRs)—These routers connect one<br />

or more areas <strong>to</strong> <strong>the</strong> backbone area, and only maintain<br />

information about <strong>the</strong> backbone and <strong>the</strong> areas <strong>the</strong>y are<br />

attached <strong>to</strong>. Any router that has interfaces attached <strong>to</strong><br />

<strong>the</strong> backbone and at least one area is an ABR.<br />

• Internal Routers (IRs)—These routers are only involved<br />

in intra-area routing. IRs only contain information about<br />

<strong>the</strong> area <strong>the</strong>y operate in. All extra-area traffic is<br />

forwarded <strong>to</strong> <strong>the</strong> area border router because only<br />

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 Boundary Routers (ASBRs)—These<br />

routers are used <strong>to</strong> exchange routing information<br />

between OSPF and EGP. Any OSPF router that<br />

redistributes routing information is considered an ASBR.<br />

Network <strong>to</strong>pology plays a role in OSPF performance because<br />

each router maintains a database on <strong>the</strong> state of <strong>the</strong> <strong>network</strong>.<br />

OSPF was designed <strong>to</strong> be implemented in large-scale<br />

inter<strong>network</strong>s with 30 or more routers.<br />

This does not mean OSPF will not function properly in small<br />

<strong>network</strong> environments; on <strong>the</strong> contrary, it will work well.<br />

Actually, in situations where a standards-based routing<br />

pro<strong>to</strong>col is required, OSPF is a significantly better choice than<br />

RIP, especially when it comes <strong>to</strong> speed and <strong>network</strong> utilization.<br />

Ideally, <strong>the</strong> backbone routers should bear <strong>the</strong> brunt of <strong>the</strong> processing by<br />

constructing a complete <strong>to</strong>pology map from <strong>the</strong> LSAs sent by <strong>the</strong> ABRs that have<br />

established adjacencies with <strong>the</strong> IR. Therefore, <strong>the</strong> more <strong>network</strong> <strong>to</strong>pology and<br />

addressing lends itself <strong>to</strong> subdivision and summarization, <strong>the</strong> more efficient and<br />

scalable OSPF becomes. OSPF's tightly structured model, where each router class<br />

performs a function, gives OSPF <strong>the</strong> capability <strong>to</strong> efficiently manage <strong>the</strong> route<br />

processing load efficiently.<br />

Configuring OSPF<br />

Configuring OSPF is similar <strong>to</strong> setting up IGRP and EIGRP. To get <strong>the</strong> OSPF process<br />

started, it requires a local process ID in <strong>the</strong> range of 1 <strong>to</strong> 65,535 (just like IGRP and

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