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Chapter 4

Cisco's Diagnostic Commands

the card is a VIP/VIP2 card, then the packet will use distributed switching if
there is an entry; if not, it is processed like other packets.

The next step in the process is to copy the packet to the SP/SSP/RSP across

the CxBus/CyBus. Once the packet reaches the buffers on the SP/SSP/RSP, it
is compared to the silicon-switching cache. Two paths can be followed
from here. If an entry exists in the silicon-switching cache, the packet is
encapsulated and copied to the outgoing interface processor across the
CxBus/CyBus. The interface processor then sends out the packet. If an entry
is not found in the silicon-switching cache, the autonomous cache is checked.

If an entry exists in the autonomous cache, the packet is encapsulated by

the switch processor, copied across the CxBus/CyBus to the outgoing inter-
face processor, and sent. Again, if an entry does not exist, the next step is
taken and the optimum cache is compared.

Optimum switching follows the same exit route as autonomous switch-

ing: the packet is encapsulated by the switch processor, copied to the outgo-
ing interface, and sent. With each additional step, the packet gets closer and
closer to being process-switched.

At this point, if there is no entry in the optimum cache, the Layer 3 header

is copied, sent to the RP and then compared against the fast cache. Fast cache
can reside in two different places, depending on the router's hardware con-
figuration. If an RSP is used, the fast cache is on the same board as the opti-
mum cache. If an SP/SSP and an RP are used, the silicon and autonomous
caches reside on the switch processor, and the fast cache resides on the RP.

If the Layer 3 header does not match any entry in the fast cache, the entire

packet is copied and sent to the processor, where the processor does a lookup
in the route table. If a route exists, the packet is encapsulated and copied
back to the exiting interface. If no route exists, the packet is dropped.

The first packet of each data flow requires process switching. Once the route

lookup and proper encapsulation are made, the header is copied into the corre-
sponding switch-type cache. Subsequent packets find the entry in the correct cache
and are forwarded along the same outgoing path as the first packet.

The Route Processor

Although you don't like sending packets to the route processor, sometimes
it is unavoidable. Packets that are destined or intended for the router require
processing. Broadcasts, SNMP requests, access-list processing (in some
cases), debugging, protocol updates, error logging, and queuing all require
processor time.

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