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قديم 02-18-2012, 01:03 AM   #7
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من كتاب The Bryant Advantage

If a router has only one path to a destination, obviously that will be the path taken. When multiple paths exist, the router will consider each route and then make a decision based on the longest match or on administrative distance.

Consider this routing table:



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If a packet arrives on a router interface destined for 191.168.32.1, which route would the router choose? It depends on the prefix length, or the number of bits set in the subnet mask. Longer prefixes are always preferred over shorter ones when forwarding a packet.



In this case, a packet destined to 191.168.32.1 is directed toward 10.1.1.1, because 191.168.32.1 falls within the 191.168.32.0/26 network (191.168.32.0 to 191.168.32.63). It also falls within the other two routes available, but the 192.168.32.0/26 has the longest prefix within the routing table (26 bits vs. 24 or 19 bits).
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What if the routes for a destination are all the same? Consider the same table, but with subnet masks of the same length for each protocol.


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The longest-match rule will not suffice in this situation. When routes are received via different protocols, and the longest-match rule does not result in selection of a route, administrative distance is then considered.

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