[c-nsp] mitigate output drops on Cisco 2960G platform

[Martin T]

Hi,

I have three WS-C2960G-24TC-L switches which all have about ten
servers connected. Each switch has a single 1GigE uplink. Those 1GigE
uplink ports on all switches experience occasional OutDiscards. I
would like to increase the outgoing buffer on uplink ports in order to
mitigate the egress memory buffer overflow.

Increasing the output hold-queue(for example "hold-queue 4096 out")
does not help as this affects only traffic from switch CPU(probably
SNMP, syslog etc traffic). According to documentation, "hold-queue"
should allocate system memory for interface I/O buffers, but I wasn't
able to confirm this with "sh memory" after configuring "hold-queue
4096 out" on all the interfaces in the switch..

Looks like the only way to modify port ASIC egress buffers on 2960G
are with MLS(Multi-Layer Switching) QOS. 2960G family seems to have
four output queues:


2960G#sh mls qos queue-set
Queueset: 1
Queue     :       1       2       3       4
----------------------------------------------
buffers   :      25      25      25      25
threshold1:     100     200     100     100
threshold2:     100     200     100     100
reserved  :      50      50      50      50
maximum   :     400     400     400     400
Queueset: 2
Queue     :       1       2       3       4
----------------------------------------------
buffers   :      25      25      25      25
threshold1:     100     200     100     100
threshold2:     100     200     100     100
reserved  :      50      50      50      50
maximum   :     400     400     400     400
2960G#

..but as I have QoS disabled(according to "sh mls qos"), then all
those queues should be aggregated into one large queue. There probably
is a point to configure QoS if there are output drops on some ports
facing the servers. I mean one could group certain ports facing the
servers and allocate more egress buffer for those while at the same
time decrease egress buffer for some other ports facing the servers
which have very little traffic. In case of uplink port experiences
output buffer drops, the single large queue is probably the best
option?

All the servers and uplink ports are on the same linerate. 5min
average egress traffic on those uplink ports is <100Mbps.

Any suggestions other than reduce the traffic on switch uplink ports?


regards,
Martin