Avaya Configuring IP Services Bedienungsanleitung PDF herunterladen (Seite 125)

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Configuring and Customizing IP

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Specifying the Maximum Size of the Forwarding Table

To minimize the amount of time it spends looking up routes, IP creates and

maintains a cache of frequently used routes -- called a forwarding table or cache --

for each IP interface.

A forwarding table is a first in first out (FIFO) buffer. When a datagram arrives on

an IP interface for forwarding, IP searches the forwarding table associated with

the interface for the destination network.

If the search is successful, IP dispatches the datagram to the interface noted in the

table entry.

If the search is unsuccessful, IP consults the routing table to get the same

information, dispatches the datagram to the appropriate interface, and caches the

information in the appropriate forwarding table -- either by appending information

to the table (if the table is not full) or by overwriting the oldest, first-in table entry

(if the table is full).

If IP flushes a route from the routing table, it also removes the route from the

forwarding tables, thus ensuring that invalid routing information is not retained in

interface-specific caches.

An interface that receives packets that are destined for a large number of different

destinations may benefit from a larger forwarding table. The larger the number of

entries, the more likely it is that the destination will already be in the forwarding

table and the faster the route lookups will be for those destinations.

Keep in mind that configuring a forwarding table size that is larger than necessary

reduces the total amount of memory usable by other applications. On the other

hand, configuring a routing table too small can affect overall router performance.

Check the number of cache hits and misses to determine the optimal size of the

forwarding table. For debugging purposes, if you see the

wfIpInterfaceCacheMisses statistic going up at a rapid rate, consider increasing

the table size. However, an occasional cache miss does not warrant an increase in

table size.

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