
Customizing IP Services
117356-A Rev. A 4-51
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 performs the following steps:
1. IP searches the forwarding table associated with the interface for the
destination network.
2. If the search is successful, IP dispatches the datagram to the interface noted in
the table entry.
3. 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. Configuring a
routing table too small can affect overall router performance. A check of the
number of cache hits and misses will help 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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