Cisco Router Packet Forwarwding

Process Switching

The router scans the entire routing table, searching for an exit interface that is connected to the destination network. Note this is CPU intensive and no longer practiced because it would quickly bog down a router.

Fast Switching

Enabled by default on all interfaces that support fast switching. To conserve resources, the router builds a table in high speed cache to prevent process switching every packet it receives.

When a router receives a packet sent to a destination not already in cache, it will be process switched.

After finding the exit interface and determining the Next Hop IP, the router will place this information in the cache.

You can enable fast switching per interface by issuing

conf t
int fa0/0
ip route-cache

Review that fast switching has been enabled

Lab-C#show ip int fa0/0 | in IP fast
 IP fast switching is enabled
 IP fast switching on the same interface is disabled

You can view the table held in cache

show ip cache

You can review this process of placing entries in the cache by enabling packet debugging by issuing

debug ip packet detail

Keep in mind that when you have enabled packet debugging you can review process switching activity, however fast switched and CEF switched packets do not show up. So for the sake of troubleshooting it may be a good idea to disable fast switching (no ip route-cache) and CEF.


Intense School: Process Switching, Fast Switching, and CEF

Cisco Fast Switching

Cisco Verify CEF Switching