[c-nsp] ISR G2 Interface RX Performance

[Nathanael Law]

Hello all,

We're having some issues with a 3925 and real-time UDP traffic bursts.  The bursts
are approximately 1500 packets long and are sent in 5.7 ms for an effective rate
of ~250 kpps (~375 Mbps).  The steady state traffic on this connection is < 10kpps.

Physical Topology
=================

+------+                      +------+
|      |                      |      |
| 3750 | gi2/0/2 ------ gi0/0 | 3925 | 
|      |                      |      |
+------+                      +------+

Packet captures have shown that the 3750 gi2/0/2 interface has no issues sending
the entire burst; however, packet captures on both the receiving host and the 
3925 shows that only about 1/3rd of the packets show up on the gi0/0 interface.
The overrun counter increases slightly with each burst.

3925#show interfaces gi0/0
GigabitEthernet0/0 is up, line protocol is up
  Hardware is PQ3_TSEC, address is 70ca.9bb5.7a80 (bia 70ca.9bb5.7a80)
  Description: Uplink to core switch 2
  MTU 1500 bytes, BW 1000000 Kbit/sec, DLY 10 usec,
     reliability 255/255, txload 12/255, rxload 1/255
  Encapsulation 802.1Q Virtual LAN, Vlan ID  1., loopback not set
  Keepalive set (10 sec)
  Full Duplex, 1Gbps, media type is RJ45
  output flow-control is unsupported, input flow-control is unsupported
  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 never
  Input queue: 0/75/176/0 (size/max/drops/flushes); Total output drops: 0
  Queueing strategy: fifo
  Output queue: 0/40 (size/max)
  5 minute input rate 5331000 bits/sec, 2511 packets/sec
  5 minute output rate 48693000 bits/sec, 5037 packets/sec
     40386885831 packets input, 7413412230506 bytes, 0 no buffer
     Received 3800689 broadcasts (0 IP multicasts)
     0 runts, 0 giants, 111 throttles
     82202 input errors, 0 CRC, 0 frame, 82202 overrun, 0 ignored
     0 watchdog, 3800681 multicast, 0 pause input
     80802723127 packets output, 110122764930458 bytes, 0 underruns
     0 output errors, 0 collisions, 1 interface resets
     0 unknown protocol drops
     0 babbles, 0 late collision, 0 deferred
     0 lost carrier, 0 no carrier, 0 pause output
     0 output buffer failures, 0 output buffers swapped out

The only other statistic of note seems to be the rx_overflow_err on the gi0/0
interface:

Internal Driver Information:
 throttled=111, enabled=111, disabled=0
 rx_coalesce_failed=0, rx_framing_err=0, rx_overflow_err=1726086, rx_buffer_err=65
 rx_no_enp=0, rx_discard=0
 tx_one_col_err=0, tx_more_col_err=0, tx_no_enp=0, tx_deferred_err=0
 tx_underrun_err=0, tx_late_collision_err=0, tx_loss_carrier_err=0
 tx_exc_collision_err=0, tx_buff_err=0, fatal_tx_err=0

>From this it seems that the actual gigabit physical interfaces on the 3925 cannot
handle even 11% of line rate (2 Mpps for a 1 Gbps connection @ 64 byte packets).
I knew that the processor can't handle that, but I expected the interface itself
to come at least a little closer given that CEF on the 3925 can supposedly handle
833 kpps without any features turned on.

Does my analysis seem accurate?  If not, any pointers in the right direction would
be appreciated.  If so, what Cisco routing hardware would be minimally required to
support line-rate 1 Gbps input (the 3925 is a WAN router that basically passes
traffic off to one of our MPLS providers)?  Would the interfaces on an ASK1k (ESP5)
 do the job?

We do have a TAC case open for this issue, but they have been unable to provide 
documentation on the limitations of the physical interface so far.

Documents that seem related:
 - https://supportforums.cisco.com/docs/DOC-2613 (doesn't reference anything as new
   as the ISR G2s, but I figured it may still apply)

Thank you,

Nathanael Law