Tuesday, November 23, 2010

Tuning Cisco Router Buffers

Tuning buffers is usually not necessary and doing so incorrectly can cause serious performance problems.

Use the show buffers command to tell if your buffers need adjusting.

Public Buffers:

Router>show buffers

Small buffers, 104 bytes (total 50, permanent 50) :
50 in free list (20 min, 150 max allowed)
265016 hits, 0 misses, 0 trims, 0 created
0 failures (0 no memory) <-- Look for failures

If you see failures you might need to alter your buffer parameters. Failures indicate the number of times the router has attempted to allocate a buffer and failed, causing it to drop the packet. The no memory counts the number of times a failure occurred because no memory was available which usually is resolved by adding more memory to the router.

Tune public buffer pools example:

Router#configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
Router(config) #buffers big initial 100
Router(config) #buffers big max-free 200
Router(config) #buffers big min-free 50
Router(config) #buffers big permanent 50
Router(config) #end
Router#

Interface Buffers:

Router>show buffers

Ethernet0 buffers, 1524 bytes (total 32, permanent 32) :
8 in free list (0 min, 32 max allowed)
24 hits, 0 fallbacks
8 max cache size, 8 in cache
30963 hits in cache, 0 misses in cache
As long as misses and fallbacks low, no need to adjust interface buffers.

Tune interface buffer pools example:

Router#configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
Router(config) #buffers Ethernet0 initial 200
Router(config) #buffers Ethernet0 max-free 300
Router(config) #buffers Ethernet0 min-free 50
Router(config) #buffers Ethernet0 permanent 50
Router(config) #end
Router#

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