Information Technology Reference
In-Depth Information
Table 3-2
Explanation of the Fields in Example 3-2
Field
Description
total
The total number of buffers currently in the pool, both used and unused.
permanent
The minimum number of buffers that will always be present in the pool. Buffers
will not be trimmed below this number.
free list
The number of buffers that are currently available in the pool and are unused.
min
The number of buffers the router should try to keep in the free list. If the free list
drops under this number, more buffers should be created.
max allowed
The maximum number of buffers the router is allowed to keep in the free list. If the
free list increases beyond this number, the router should trim buffers.
hits
The number of buffers successfully allocated from the free list.
misses
The number of times a buffer was requested but none were available in the free list.
trims
The number of buffers trimmed from the free list because max allowed was
exceeded.
created
The number of buffers created because the free list dropped below min.
failures
The number of failures to grant a buffer under interrupt processing. The number of
failures represents the number of packets dropped because of buffer shortage.
no memory
The number of times the router tried to create a new buffer, but there was not
enough free memory.
The default values for system buffers are for an average environment. They are not optimal
for a large BGP deployment. The main area of concern is the low number of small buffers.
A value of 150 for the Permanent value results in severe packet loss with a huge influx of
TCP ACK messages on a route reflector with 50 to 100 peers. The number of small buffers
eventually will be created, but only after a significant number of TCP ACKs have been lost.
You must check the amount of free memory before changing buffer settings to ensure that
there is adequate free memory for the new buffers. This is done with the
show memory
summary
command.
You need to tune three parameters when modifying the small buffers to handle the TCP
ACKs:
•
Permanent buffers
—The number of permanent buffers should be sufficient to handle
the worst-case scenario number of TCP ACK messages. This ensures that the router
has the available buffers to handle a sudden influx of packets.
•
Min-free setting
—Increase the Min-free setting to prompt the router to create more
buffers before the free list reaches a critical level. This number should be approxi-
mately 25% of the permanent buffers.
