[c-nsp] C2960G and output drops

[Nitzan Tzelniker]

We saw similar problem and we solve it by doing etherchannel on the output
interface.

Nitzan

On Wed, Oct 1, 2008 at 11:19, Gert Doering <gert at greenie.muc.de> wrote:

> Hi,
>
> one of our switches is misbehaving, and I'm wondering whether this is a
> configuration thing, or a hardware limitation.
>
> (It's not actually a QoS thing, but it's bordering on it)
>
> Setup: WS-C2960G-24TC-L, effectively only 4 ports active:
>
> This is where stuff comes in (UDP audio streaming servers):
>
> GigabitEthernet0/8 is up, line protocol is up (connected)
>  5 minute input rate 26179000 bits/sec, 3819 packets/sec
>  5 minute output rate 3492000 bits/sec, 3295 packets/sec
> GigabitEthernet0/12 is up, line protocol is up (connected)
>  5 minute input rate 334588000 bits/sec, 41069 packets/sec
>  5 minute output rate 14453000 bits/sec, 26859 packets/sec
> GigabitEthernet0/16 is up, line protocol is up (connected)
>  5 minute input rate 27730000 bits/sec, 2940 packets/sec
>  5 minute output rate 1507000 bits/sec, 1976 packets/sec
>
> And this is where it leaves the switch:
>
> GigabitEthernet0/10 is up, line protocol is up (connected)
>  5 minute input rate 19432000 bits/sec, 32108 packets/sec
>  5 minute output rate 380792000 bits/sec, 46406 packets/sec
>
> As you see, the ports are far from saturated, and even the load from all
> "ingress" ports (380 Mbit + 27 Mbit + 26 Mbit) is far from the capacity
> of the "egress" port (G0/10).
>
> But still...
>
> GigabitEthernet0/10 is up, line protocol is up (connected)
>  Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops:
> 8731686
>
> ... and this is of course affecting the audio quality.
>
> (The output port goes to a Juniper SSG550 firewall, which has no problem
> keeping up with the load to about 950 Mbits/s - and if only one ingress
> port
> is active on the switch, we can see much higher egress load without
> noticeable
> drops)
>
>
> IOS version on the switch is 12.2(46)SE, but I had (25)SEE on it before
> that, and observed the same symptoms (except that (25)SEE does not
> display the output drops in the counters).
>
> Right now, "mls qos" is *deactivated*, because we actually don't want
> QoS as in "drop specific packets" here - we want "move ahead all packets!".
>
> If I enable "mls qos", the packet drops go way up - which I read as "the
> existing buffers, that are already not really huge, are split into 4
> smaller queues, and thus microbursts are causing much higher drops".
>
>
> My theory is that the streaming servers are micro-bursting (send out
> packets with full wire rate for 1/100s, and then do nothing for 99/100s),
> and that the switch has too small buffers to join the 4 ingress ports
> towards the egress ports.  But I'm not sure how to validate that.
>
>
> So, here comes the questions:
>
>  - how much buffer space per port does the 2960G have?
>
>  - how can I find out why the switch is dropping packets?
>
>  - what L2 switches are other people using in environments with
>   continuous high load that has "microbursts"?
>
>  - any other tricks that people are using to make servers more well-behaved
>   regarding packet sending rate?  Like "shaping traffic on the servers"
>   (to distribute the packets more evenly along the time scale)?
>
>
> We have other streaming customers, and they are directly connected to
> 6408A or 6724 ports on 7600s, and not displaying anything unusual, at
> even higher loads (multiple ingress ports running at >800 Mbit/s for
> hours, egress via port-channels).  So it's something with this 2960G...
>
> gert
> --
> USENET is *not* the non-clickable part of WWW!
>                                                           //
> www.muc.de/~gert/ <http://www.muc.de/%7Egert/>
> Gert Doering - Munich, Germany
> gert at greenie.muc.de
> fax: +49-89-35655025
> gert at net.informatik.tu-muenchen.de
>
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