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correct <strong>network</strong> address information. The non-seed routers <strong>the</strong>n use<br />

this <strong>network</strong> address information <strong>to</strong> update <strong>the</strong>ir tables.<br />

Routing Table Maintenance Pro<strong>to</strong>col (RTMP) is similar <strong>to</strong> <strong>the</strong> Routing<br />

Information Pro<strong>to</strong>col (RIP) covered in Chapter 8, "TCP/IP Dynamic<br />

Routing Pro<strong>to</strong>cols," except one value is used <strong>to</strong> determine which route<br />

is <strong>the</strong> best route. This value is called a routing metric. AppleTalk uses<br />

a routing metric known as <strong>the</strong> hop count. Hop count is determined<br />

based on <strong>the</strong> number of routers that must be traversed in order <strong>to</strong><br />

reach <strong>the</strong> destination <strong>network</strong>. If a <strong>network</strong> is directly connected <strong>to</strong> a<br />

<strong>network</strong>, <strong>the</strong> hop count <strong>to</strong> reach <strong>the</strong> <strong>network</strong> is zero. An RTMP uses a<br />

technique called split horizon <strong>to</strong> prevent routing loops (discussed in<br />

Chapter 8).<br />

RTMP's goal is <strong>to</strong> have a routing table with <strong>the</strong> best single route <strong>to</strong><br />

each given <strong>network</strong>. The job of <strong>the</strong> seed router is <strong>to</strong> provide <strong>network</strong><br />

number information <strong>to</strong> routers as <strong>the</strong>y join <strong>the</strong> <strong>network</strong>. The following<br />

provides a simple example illustrating an RTMP table and<br />

corresponding <strong>network</strong>. The first zone listed for each entry is its<br />

default (primary) zone.<br />

R Net 20-20 [1/G] via 900.82, 9 sec, FastE<strong>the</strong>rnet1/0, zone<br />

Phase II SunLAN<br />

R Net 51-51 [1/G] via 900.82, 9 sec, FastE<strong>the</strong>rnet1/0, zone<br />

VaxLAN<br />

R Net 55-55 [1/G] via 900.82, 9 sec, FastE<strong>the</strong>rnet1/0, zone<br />

SunLAN-D<br />

R Net 57-57 [1/G] via 900.82, 9 sec, FastE<strong>the</strong>rnet1/0, zone<br />

UtilLAN<br />

R Net 64-64 [1/G] via 900.82, 9 sec, FastE<strong>the</strong>rnet1/0, zone<br />

PcLAN<br />

R Net 68-68 [1/G] via 900.82, 0 sec, FastE<strong>the</strong>rnet1/0, zone<br />

MediaLAN<br />

A Net 789-789 [1/G] via 0.0, 1330 sec, Tunnel1, zone GraceLan<br />

Additional zones: 'Fa<strong>the</strong>rLan','OutLAN'<br />

C Net 900-900 directly connected, FastE<strong>the</strong>rnet1/0, zone<br />

TestLAN<br />

NOTE<br />

Split horizon dictates that routing information learned from an<br />

interface cannot be sent back out in its routing update. Only <strong>network</strong>

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