Fast Switching

Fast Switching
Like process switching, fast switching has been available on all Cisco platforms for many years,
including the ubiquitous 2500 series. Fast switching involves the use of a cache on the route processor
where forwarding information is maintained. The first packet in a conversation is passed
to the route processor, matched against routes, and process switched. The fast switching cache
is updated, and subsequent packets have only the header matched in the cache. The result is that
the rest of the conversation is forwarded without being passed to the route processor.
Forwarding information is stored in the form of a binary tree, which allows bit-by-bit decision
making to be carried out regarding the next hop. This binary tree may require up to 32 levels of
comparison to fully match a route, but the decision is often reached much more quickly, and is
considered to be a very efficient lookup mechanism.
Entries in the fast cache are created at the beginning of a conversation, and therefore suffer
the perennial problems of caches—how do updates to other information, such as the ARP
cache, affect the cached information? And the answer is that they don’t, leaving the possibility
that changes in the ARP cache may leave the fast cache with out-of-date and incorrect information.
In that case, the cache must be recreated. The second problem with fast switching is that
the cache can construct only a single route to a destination, so any load sharing must be on a
conversation-by-conversation basis (sometimes caller per-destination load sharing) with a
cache entry for each conversation.
Nonetheless, fast switching is perhaps 10 times faster than process switching and is widely used.
The fast switching tree is shown in Figure 18.10. Each bit in the destination address is compared
with the table, and because each possibility is either a 1 or a 0, a single match is gained
with every pass.