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Chapter 4
Cisco's Diagnostic Commands
VIP distributed switching on VIP interfaces. The following features aren't
supported by CEF:
ATM dixie
Token Ring
Multipoint PPP
access-lists on the GSR
Policy routing
NAT
SMDS
Nevertheless, CEF does many things--even load balancing is possible
through FIB. If there are multiple paths to the same destination, the IP route
table knows about them all. This information is also copied to the FIB, which
CEF consults for its switching decisions.
Load balancing can be configured in two different modes. The first
mode is load balancing based on the destination (called per-destination
load balancing
); the second mode is based on the packet (called per-
packet load balancing). Per-destination load balancing is on by default,
and must be turned off to enable per-packet load balancing.
Accounting may also be configured for CEF, thus furnishing you with
detailed statistics about CEF traffic. Two specifications can be made when
collecting CEF statistics:
To collect information on traffic that's forwarded to a specific destination
To collect statistics for traffic that's forwarded through a specific
destination
CEF was designed for large networks--if reliable and redundant switch-
ing paths are necessary, CEF is the way to go. Keep in mind that its hardware
requirements are significant, however, and it lacks support for many Cisco
IOS features.
Packet Flow--High-End Routers
You now have all of the pieces of the puzzle, and you understand the router
architectures and the switching methods. With this knowledge, you can eas-
ily follow a packet as it transits a router. Don't confuse this packet flow with
the OSI comparison given earlier in the chapter.
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