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Core Issue

Buffer tuning allows you to modify the way a router allocates buffers from its available memory, and helps prevent packet drops during a temporary burst of traffic.


To see if you might benefit from buffer tuning, use the show  interfaces and show buffers commands:

Below is a partial output of show interfaces:

Output queue 0/40, 1041 drops; input queue 0/75, 765 drops 
35252345 packets input, 547082589 bytes, 940 no buffer

The input and output drops are due to the input and output queues being overrun by a burst of traffic. This is not related to a buffer problem."No buffer" represents the number of packets dropped because there is no free buffer to copy the packet. If you see "no buffer" drops, buffer tuning may be needed.

Below is a partial output of show buffers:

Middle buffers, 600 bytes (total 150, permanent 25):
  147 in free list (10 min, 150 max allowed)
  61351931 hits, 137912 misses, 51605 trims, 51730 created
  91652 failures (0 no memory)

The number of "failures" and "no memory" are the only areas you need  to worry about. Failures may occur, but these should stabilize after a while.  The router creates or trims buffers as necessary to stabilize the number of failures. If the number of failures continues to increase, then buffer tuning might be necessary.

If you do not have enough memory to create new buffers, look for a buffer  leak or a more general memory problem.

What to do if you identify a problem:

For information on potential issues and fixes, collect the output of the show interfaces and show buffers commands, or the show tech-support command, and input into the Output Interpreter tool. To use Output  Interpreter, you must be a registered  customer, be logged in, and have JavaScript enabled.

For more information, see Buffer Tuning. If you suspect that you may  be encountering a buffer leak, see Troubleshooting Buffer Leaks.

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