[c-nsp] Cat3560E - insufficient buffers for microbursts?

[Marian Ďurkovič]

Hi all,

  recently we replaced some of our access switches by Cat3560E models.
After the change, we noticed a significant rise of dropped packets on
outgoing interfaces.

  Closer investigation showed, that for some reason, Cat3560E is doing
significantly less output buffering than all other Cisco access switches.
While Cat2960G, Cat3550, Cat3560G and Cat3750G happily buffer any
microburst upto 100 full size (1500 byte) packets, Cat3560E only buffers
upto 64 full size packets and starts dropping beginning with 65-th packet.
This was rather bad surprise, since the Cat3560E is supposed to have
10GE uplinks - and with 10:1 or 100:1 speed downshift from uplink into
access ports decent buffering is vital for proper operation.

  Does anyone have an idea what's the reason for this behaviour - i.e.
were the ASIC buffers significantly reduced on Cat3560E models or is this
just wrong IOS defaults? Are there any CLI knobs to tune output buffering
in FIFO mode of operation (no mls qos)?


     Thanks & kind regards,

            M.


P.S. This behaviour could be reproduced by e.g. sending 1500 byte packets
     at gigabit ethernet wire rate through the switch to another device,
     which is connected at lower speed (10 or 100 Mbps).