Information Technology Reference
In-Depth Information
Using the destination IP address in the packet header, the process first checks the RIB
to determine the outbound interface. It then consults the ARP cache to build the Layer 2
header.
3
At this point, the packet is rewritten with the new Layer 2 header and is copied back
into the packet memory or system memory for forwarding to the outbound interface.
4
Figure 2-3
IP Process Switching
Buffers
ARP
RIB
IP Input
Processor
Packet
Memory
Inbound
Interface
Outbound
Interface
Process switching is CPU-intensive and might result in low system performance if a large
number of packets need to be examined at the process level. The following CPU-intensive
tasks are involved in process-switching an IP packet:
•
Memory copy of the packet from the receive buffer to a shared memory system buffer.
•
Routing table lookup. This task has generally become less of a problem over the years
because of more-efficient algorithms used to store information.
•
Memory copy of the packet from the shared memory system buffer to the transmit
buffer.
The limitation of process switching is exacerbated if the router needs to handle large
numbers of packets in an unstable network, such as the environment on the Internet. Process
switching is also not an efficient switching mechanism, because packet information is never
reused. Process switching involves performing prefix lookups directly into the RIB, which
is not optimized for route table lookups.
