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james21970
Beginner

4948 Queueing

All,

In troubleshooting an issue with output drops on one of our switches attached to a customer, one of our engineers used the information in the 'show interface gix/x' as a guide for the queue size. The section that i'm describing is this:

GigabitEthernet1/30 is up, line protocol is up (connected)

<output omitted>

  Queueing strategy: fifo

  Output queue: 0/40 (size/max)

The customer asked our engineer if he could increase that value of 40. I came across the ticket, and lent a hand and explained that the 4948 uses hardware queues, and we can see these here:

clt3-access-13#sh platform hardware interface g1/30 tx-queue

Switch Phyport Gi1/30 Tx-Queue Hardware State

Phyport TxQ   High     Shape      Share     Shape        Share          Curr

            Priority Mant. Exp. Mant. Exp. NextTime     NextTime        Time

-------------------------------------------------------------------------------

Gi1/30  0   Disabled  0    0     45   16  0x11B1D00D4   0x11B1D00D4   0xAB97B6383

Gi1/30  1      -      0    0     45   16  0xAA3506F73   0xAA3507324   0xAB97BAAB8

Gi1/30  2      -      0    0     45   16  0x11B1F99F3   0x11B1F99F3   0xAB97BEF88

Gi1/30  3      -      0    0     45   16  0xAB97B7DCD   0xAB97B8B12   0xAB97C3327

Phyport TxQ  Head    Tail   Full   Not   Num    BaseAddr   Size

                                  Empty Packets

---------------------------------------------------------------

Gi1/30  0   0x0000  0x0000  False False   0      0x20800   2080

Gi1/30  1   0x03C5  0x03C5  False False   0      0x21020   2080

Gi1/30  2   0x0000  0x0000  False False   0      0x21840   2080

Gi1/30  3   0x0070  0x0070  False False   0      0x22060   2080

My question is two part really. First off, why does the output in the show interface have the output queue as 40 when it's obviously not being used(if I remember correctly, this had to do with router interfaces)?

My other question, is that the QoS documentation for the 4948 is here (apparently the 4500 docs are what is supposed to be used):

http://www.cisco.com/en/US/docs/switches/lan/catalyst4500/12.2/15.02SG/configuration/guide/qos.html#wp1463316

But I can find no refereence to the timeframe that these queues get serviced. This is important to me because when I look at the queue's for the interface:

clt3-access-13#sh qos interface g1/30

QoS is enabled globally

Port QoS is enabled

Administrative Port Trust State: 'untrusted'

Operational Port Trust State: 'untrusted'

Trust device: none

Default DSCP: 0 Default CoS: 0

Appliance trust: none

Tx-Queue   Bandwidth   ShapeRate   Priority   QueueSize

             (bps)       (bps)                (packets)

  1        250000000   disabled    N/A        2080    

  2        250000000   disabled    N/A        2080    

  3        250000000   disabled    normal     2080    

  4        250000000   disabled    N/A        2080

I'm thinking that that packet count can easily get overrun. If we do the math real quick, and say we use a 1500 byte packet, and use the formula (1500*2080)8, we end up getting 24960000, which is much lower than 250000000. So I'm a bit confused on the correlation between the queue size regarding packets, and the bandwidth. This is why I was asking about the timeframe that the above rates are referencing, because it would seem to me that you would have to have quite a bit more than 2080 (standard size packets) to accumulate 250 Mbps of traffic, not to mention that a queue limit of 2080 packets may seem like alot, but we have customer links that are showing pps well in excess of 4000 with no output drops, which again leads me to believe that the packet count shown above is for a timeframe that is shorter than a second.

Does this make sense?

Any help is appreciated,

James

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