Processing and Switching
In IP networks, routers make forwarding decisions based on the contents of the routing table. When a router searches the routing table, it looks for the longest match for the destination IP address. The router does this at the process level. Therefore, the search process is queued among other CPU processes, because of which, the lookup time is unpredictable and can be very long. Therefore, a number of switching methods based on exact-match-lookup have been introduced in Cisco IOS® Software.The main benefit of exact-match-lookup is that the lookup time is deterministic and very short. This has significantly shortened the time a router takes to make a forwarding decision. Therefore, routines that perform the search can be implemented at the interrupt level. This means, the arrival of a packet triggers an interrupt, which causes the CPU to postpone other tasks and handle the packet. The legacy method to forward packets is to look for a best match in the routing table. This cannot be implemented at interrupt level and must be performed at process level. For a number of reasons, some of which are mentioned in this document, the longest-match-lookup method cannot be completely abandoned, so these two lookup methods exist in parallel on Cisco routers. This strategy has been generalized, and is now also applied to IPX and AppleTalk.
For more information on Cisco IOS Software switching paths, refer to Performance Tuning Basics.
Troubleshoot Input Queue Drops
router#show interfaces ethernet 0/0
Router(conf-if)# hold-queue length inrouter#show processes CPU | i ^PID|Input
show ip traffic
Router#show buffers input-interface serial 0/0
router#show interfaces ethernet 0/0
Router(conf-if)# hold-queue length inRouter#show buffers input-interface serial 0/0
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