Information Technology Reference
In-Depth Information
It is also common practice to enable SPD aggressive mode. If the input queue has become
congested, maintaining malformed IP packets is a poor use of system resources. The
standard recommended SPD configuration is as follows:
•
ip spd mode aggressive
•
ip spd headroom 1000
•
ip spd queue min-threshold 998
•
ip spd queue max-threshold 999
You can verify the SPD parameters with the command
show ip spd
, as shown in
Example 3-1.
Command Output for
show ip spd
Example 3-1
Router#show ip spd
Current mode: normal.
Queue min/max thresholds: 73/74, Headroom: 100, Extended Headroom: 10
IP normal queue: 0, priority queue: 0.
SPD special drop mode: none
The SPD Headroom is not a queue of its own, just like the input hold queue is not an actual
queue, but a counter. The SPD Headroom and SPD Extended Headroom are extensions to
the input hold queue counter.
System Buffers
The last component of the data path to the process level is the system buffers. These buffers
are where the actual data is stored for the processor. System buffers are created and destroyed
as needed. The memory used to allocate system buffers is the main processor memory. The
system buffer information on the router is shown with the
show buffers
command in Exec
mode. A sample portion is provided in Example 3-2 for only the small buffers. The small
buffers are the system buffers that are used for packets less than 104 bytes.
System Buffer Information for Small Buffers
Example 3-2
Small buffers, 104 bytes (total 150, permanent 150):
140 in free list (30 min, 250 max allowed)
564556247 hits, 148477066 misses, 16239797 trims, 16239797 created
29356200 failures (0 no memory)
The small buffers are of most interest from a BGP tuning perspective. The handling of a
large influx of TCP ACK messages is the main thrust of queue optimization. A TCP ACK
is 64 bytes, so it is stored in the small buffers. Table 3-2 explains each field shown in
Example 3-2.
