Understanding the Output from Diagnostic Commands
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The output queue works in the same manner. Output queues reside on the
RP as well, and once a packet is processed, it must be sent to the exiting inter-
face. The output queue is used to hold the packet until the packet can be copied
to the buffers on the SP/SSP; from there, it is forwarded to the specified inter-
face processor. The interface processor then sends the packet to the interface
queue/buffer from which the packet will be sent to the destination.
Packets can be dropped from the output queue, as well. If the other pro-
cesses on the router are so busy that they can't accept the outgoing packet,
they are dropped. These drops are considered normal because the Layer 3
protocol is in the process of slowing down to avoid any further drops.
Buffers and Queues in Low-End Routers
As with switching caches, all buffers are located in shared memory (with the
exception of the hardware buffer). The hardware buffer resides on the phys-
ical interface.
The difference between the 4000/2500 series routers and the 7000/7500
series routers is that there are no intermediate buffers. A packet is copied
directly from the hardware buffer to the system buffers in shared memory.
Let's look at the show buffers results from a 2514 router. You should be
able to see the difference between it and the buffer output from the Cisco 7507:
Router_A#show buffers
Buffer elements:
499 in free list (500 max allowed)
28753345 hits, 0 misses, 0 created
Public buffer pools:
Small buffers, 104 bytes (total 90, permanent 50):
50 in free list (20 min, 150 max allowed)
8454394 hits, 0 misses, 0 trims, 0 created
0 failures (0 no memory)
Middle buffers, 600 bytes (total 25, permanent 25):
23 in free list (10 min, 150 max allowed)
1676303 hits, 51 misses, 153 trims, 153 created
0 failures (0 no memory)
Big buffers, 1524 bytes (total 50, permanent 50):
50 in free list (5 min, 150 max allowed)
1255286 hits, 0 misses, 0 trims, 0 created
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