Process Switching

Process Switching
When packets are process-switched, the complete packet is forwarded across the internal architecture
to the route processor. This is the “heart” of the router, and is a busy place to be! Often
accessed via two buses—the Cbus and the systems bus—it involves a long trip through the router
and out to the forwarding interface for the whole packet. At the route processor, the forwarding
interface and the MAC header rewrite information is applied. Delay is considerable, but there are
some advantages: If the routing table holds multiple paths of equal cost to the destination, then
load balancing can be carried out on a per-packet basis.
The routing process is shown in Figure 18.9. This diagram illustrates the linear nature of
process-switching, where a packet travels right through the “heart” of the router, resulting in
slow forwarding.
FIGURE 1 8 . 9 Process switching flow
Route
Input interface Processor
Output interface Rewrite MAC