[c-nsp] diagnosing output drops on a PA-GE

[Ed Ravin]

We've got a 7206 VXR NPE-400 with a PA-GE.  The Gigabit interface
is subdivided into several VLANs with subinterfaces, and we're routing
traffic between the VLANs in and out on the subinterfaces.

We're seeing a lot of output drops and input flushes - see the interface
output below.  I tried increasing the hold-queue sizes but it didn't
make much difference.

The router is moving around 160 Mb in one direction and about 10 in 
the other.  Although the graphs for the gigabit interface show that
combined, since traffic is read and written on the same physical
interface.   Max packets per second (according to the SNMP counter)
is 30k pps.  I originally thought that we were within the specs for
the router described in the Router Performance PDF, even allowing that
those numbers may be somewhat inflated.  But I'm having misgivings -
there are lots of output drops, and the router is continually sending
XOFF flow control requests ("pause output" below), suggesting that
it is overloaded.

We're not running much else on the router yet - BGP, a few access
lists, HSRP.  Are there any surprises with running traffic in and out
the same interface that I should know about?  Would we be better off with
two PA-GE interfaces for this?


Router#show run int giga2/0
Building configuration...

Current configuration : 145 bytes
!
interface GigabitEthernet2/0
 no ip address
 no ip mroute-cache
 load-interval 30
 negotiation auto
 hold-queue 100 in
 hold-queue 60 out
end

Router#show int giga2/0
GigabitEthernet2/0 is up, line protocol is up
  Hardware is WISEMAN, address is 0005.5fd0.8038 (bia 0005.5fd0.8038)
  MTU 1500 bytes, BW 1000000 Kbit, DLY 10 usec,
     reliability 255/255, txload 35/255, rxload 35/255
  Encapsulation 802.1Q Virtual LAN, Vlan ID  1., loopback not set
  Keepalive set (10 sec)
  Full-duplex, 1000Mb/s, link type is autonegotiation, media type is
unknown media type
  output flow-control is XOFF, input flow-control is XOFF
  ARP type: ARPA, ARP Timeout 04:00:00
  Last input 00:00:00, output 00:00:00, output hang never
  Last clearing of "show interface" counters 3d16h
  Input queue: 1/100/0/815 (size/max/drops/flushes); Total output drops:
2006828  Queueing strategy: fifo
  Output queue: 0/60 (size/max)
  30 second input rate 140225000 bits/sec, 25613 packets/sec
  30 second output rate 140202000 bits/sec, 25602 packets/sec
     2494806849 packets input, 2275963794 bytes, 2 no buffer
     Received 1710630 broadcasts, 0 runts, 0 giants, 2 throttles
     0 input errors, 0 CRC, 0 frame, 0 overrun, 481113 ignored
     0 watchdog, 904530 multicast, 4 pause input
     0 input packets with dribble condition detected
     2490146045 packets output, 268715192 bytes, 0 underruns
     0 output errors, 0 collisions, 34 interface resets
     0 babbles, 0 late collision, 0 deferred
     0 lost carrier, 0 no carrier, 1934442 pause output
     0 output buffer failures, 0 output buffers swapped out