Hey Chris,
Couldn't help but ask, how do you find your Juniper switches? Do you
have any special probes that work with them?
Thanks
Dennis
At 6:09 PM +1000 9/8/11, Chris Kawchuk wrote:
The SNMP-polled discards are absolutely correct form what I've seen.
(Sorry to hear you've been having issues w/this dennis - our Cisco
3750s report drops/errors completely correctly from what we've seen
- have about 20 or so of these in the network). Note that we
primarily use a combination of Juniper EX switches, MX core routers
and the like. Whenever we see a transmit discard it's usually (as
Pedro said):
1. QoS Settings. We have the wrong forwarding classes setup on the
interface, and/or haven't applied the scheduler-map (HW queues)
correctly. Note that this is 90% of what I normally see. I compare
what IM says, and then check for drops on each of the forwarding
queues; and yep - the numbers line up perfectly. This one is by far
the most useful use of the "Tx Discards".
2. Saturated Links - Absolutely. Gotta drop somewhere; shaper or no
shaper - you'll eventually tail-drop at >100% utilization.
3. Oversized frames/MTU problems, esp if the DF bit is set. Can't
push a 9K frame down a 4K L2 pipe.
4. Downstream Customer is rate-limited, however we can usually shape
it w/a deep enough buffer so that it's not dropped (just really
delayed). =). If we want to police, then we do that via a hard
policer in a firewall filter on the outbound IP interface; which
goes into the polcier's counters and doesn't contribute to the
interface's counters. (Nice and silent if we want it to be).
When I telnet to the switch and show stats on the interface the only errors
I see are 16 FCS errors. Not quite the 4000+ I expected to see.
I still wonder, what does IM see that the hardware won't show me directly?
And since IM is polling the hardware to get info to display bubbles - what
is actually happening - and what is to be believed?
Thats because the frames are not errors. Somehow the "show
interface" command being used isn't reporting the Tx discards -
Somehow it's counting them, since Intermapper is reading this
information from what the switch is telling it. =) (i.e. Intermapper
is not making this up - it's just reading the Iface's SNMP Tx
discard value, which is populated by the device).
i have learned that if you are interested in discards choose your
device wisely.
some devices is just a mystery about discards..
Agreed on that =)
- Chris.
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