[c-nsp] jumbo MTU on WS-X6704-10GE

[A.L.M.Buxey at lboro.ac.uk]

hi,

run into an interesting one, wonder if someone can help. 

we have several 10GbE links, run from WS-X6704-10GE modules
on Sup720 powered 6509e chassis.  2 of them dont have any requirement
to carry jumbo frames...and on those we see no problem. however,
one of the links is taking over from a 1GbE link (fed from WS-X6724-SFP
modules) which had 9000 MTU traffic running through it. from the
specs, the 10GbE module has a max of 9216..which is what the 
system jumbomtu has been set for..as well as an instruction on the
interface to handle...so, the config for the interface looks like:

interface TenGigabitEthernet2/1
 description 10GbE jumbomtu interlink
 mtu 9216
 no ip address
 logging event link-status
 switchport
 switchport trunk encapsulation dot1q
 switchport trunk native vlan 708
 switchport trunk allowed vlan 100,120,280,708,736
 switchport mode trunk

however, in the counters, we see this:

32 bit counters:
 0.                   rxCRCAlignErrors = 0
 1.                   rxUndersizedPkts = 0
 2.                    rxOversizedPkts = 203234
 3.                     rxFragmentPkts = 0
 4.                          rxJabbers = 0
 5.                       txCollisions = 0
 6.                         ifInErrors = 37
 7.                        ifOutErrors = 0
 8.                       ifInDiscards = 37
 9.                  ifInUnknownProtos = 37
10.                      ifOutDiscards = 0
11.            txDelayExceededDiscards = 0
12.                              txCRC = 0
13.                         linkChange = 0
14.                   wrongEncapFrames = 37


on our other links, which dont have jumbo/mtu , all these values are
a nice , beautiful '0'..which is exactly what we like to see :-)

can anyone shed some info on this peculiarity?  IOS is 12.2(17d)SXB
on all involved routers (ie those with happy 10GbE and those with errors)

alan