Troubleshooting guide

11-11
ATM and Layer 3 Switch Router Troubleshooting Guide
OL-1969-01
Chapter 11 Troubleshooting Layer 3 Network Connections
System Architecture
Cisco Express Forwarding
Cisco Express Forwarding (CEF) evolved to best accommodate the changing network dynamics and
traffic characteristics resulting from increasing numbers of short-duration flows typically associated
with Web-based applications and interactive multimedia sessions. Other Layer 3 switching paradigms
use a route-cache model to maintain a fast lookup table for destination network prefixes (see
Figure 11-5). The route-cache entries are traffic driven, in that the first packet to a new destination is
routed via routing table information, and as part of that forwarding operation, a route-cache entry for
that destination is added. This process allows subsequent packet flows to that same destination network
to be switched based on a route-cache match. These entries are periodically aged out to keep the route
cache current and can be immediately invalidated if the network topology changes.
Figure 11-5 Route-Cache and Distributed Routing Comparison
This “demand-caching” scheme used by other Layer 3 switches is optimized for networks where the
majority of traffic flows are associated with a subset of destinations. Since the traffic profiles at the core
of the Internet (and potentially within some large enterprise networks) no longer resemble this model,
CEF was introduced. CEF eliminates the increasing cache maintenance problem resulting from growing
numbers of topologically dispersed destinations and dynamic network changes.
Route-Cache Switch
Distributed Catalyst 8500 CSR Switch
Line card
CPU
Line card
Shared
memory
fabric
Routing table/
route cache
Ingress
port
Egress
port
49986
Line
card
FIB
table
Line
card
Route
Processor
Routing table/
central FIB
table
Shared
memory
fabric
Ingress
port
Egress
port
FIB
table