[c-nsp] Giants and input errors but no MTU mismatch 7600-to-4948

[John Neiberger]

I have an interface on a 4948 that is reporting increasing giants and
input errors. The MTU is the default 1500 and so is the interface on
the other side of the link. This is a dot1q trunk, if that is
relevant.

7600 Side:

GigabitEthernet3/3 is up, line protocol is up (connected)
  Hardware is C6k 1000Mb 802.3, address is 7081.058f.471e (bia 7081.058f.471e)
  MTU 1500 bytes, BW 1000000 Kbit/sec, DLY 10 usec,
     reliability 255/255, txload 6/255, rxload 6/255
  Input queue: 0/2000/15/0 (size/max/drops/flushes); Total output drops: 52636
  Queueing strategy: fifo
  Output queue: 0/40 (size/max)
  30 second input rate 25560000 bits/sec, 3911 packets/sec
  30 second output rate 25889000 bits/sec, 2481 packets/sec
     103422624782 packets input, 96894603872012 bytes, 0 no buffer
     Received 421340 broadcasts (390040 multicasts)
     0 runts, 0 giants, 0 throttles
     0 input errors, 0 CRC, 0 frame, 15 overrun, 0 ignored
     0 watchdog, 0 multicast, 0 pause input
     0 input packets with dribble condition detected

4948 Side:

GigabitEthernet1/46 is up, line protocol is up (connected)
  Hardware is Gigabit Ethernet Port, address is e05f.b919.522d (bia
e05f.b919.522d)
  MTU 1500 bytes, BW 1000000 Kbit, DLY 10 usec,
     reliability 255/255, txload 6/255, rxload 7/255
  Last input 00:00:00, output never, output hang never
  Last clearing of "show interface" counters never
  Input queue: 0/2000/0/0 (size/max/drops/flushes); Total output drops: 0
  Queueing strategy: fifo
  Output queue: 0/40 (size/max)
  5 minute input rate 27555000 bits/sec, 2622 packets/sec
  5 minute output rate 25030000 bits/sec, 3827 packets/sec
     103432206170 packets input, 100545576069607 bytes, 0 no buffer
     Received 401659764 broadcasts (377463573 multicasts)
     0 runts, 1892602 giants, 0 throttles
     1892602 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored
     0 input packets with dribble condition detected

Any idea how I could be getting giants inbound to the 4948 when the
interface on the 7600 is set to 1500? I could see it if both sides
were not set to dot1q, but they are:

Gi3/3               on           802.1q         trunking      1

Gi1/46             on           802.1q         trunking      1

What in the world is going on here?