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support a path discovery method. Source routing is only supported as<br />

an option on 802.5 Token Ring. The source routing information is<br />

included in <strong>the</strong> Routing Information Field (RIF). If source routing is<br />

implemented, <strong>the</strong> bridge checks <strong>the</strong> source address field of <strong>the</strong> packet<br />

<strong>to</strong> see if <strong>the</strong> RI bit is enabled. If <strong>the</strong> bit is enabled, it reads <strong>the</strong> routing<br />

information and forwards <strong>the</strong> packet on. If source routing is enabled<br />

on <strong>the</strong> bridge and <strong>the</strong> RI bit is off or no routing information is in <strong>the</strong><br />

RIF, <strong>the</strong> packet will not leave <strong>the</strong> local ring.<br />

Path discovery is accomplished by <strong>the</strong> source packet's end-station<br />

sending out a discovery packet. These packets are sent across all<br />

possible paths, and <strong>the</strong>y send copies of <strong>the</strong>mselves back <strong>to</strong> <strong>the</strong> source<br />

when <strong>the</strong>y reach <strong>the</strong> destination host. The copies contain <strong>the</strong> path<br />

<strong>the</strong>y <strong>to</strong>ok <strong>to</strong> reach <strong>the</strong> destination. The source <strong>the</strong>n saves all <strong>the</strong><br />

possible path information <strong>to</strong> generate source route information for <strong>the</strong><br />

packets it sends <strong>to</strong> <strong>the</strong> destination. Each path, which consists of a<br />

segment or LAN ID and a bridge ID, is 2 bytes in size. Up <strong>to</strong> 15 routes<br />

are permitted, which means that any source-routed Token Ring<br />

<strong>network</strong> is limited in size <strong>to</strong> 15 bridges between any two rings. Figure<br />

6.6 illustrates a Token Ring source route <strong>to</strong>pology.

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