Nexus 5K MTU question

[MathieuVHM]

Hardware: Cisco Nexus 5548 + 4x Nexus 2248 FEX
Software: 7.0(8)N1(1)

While troubleshooting some network issues, I noticed several interfaces listed jumbo frames on our Nexus 5K. Even after clearing the counters, they were still increasing in quite large numbers. However the Nexus and all of our devices should be using the default 1500 MTU so I have no idea where the jumbo packets are coming from. 

While investigating, I noticed the show interface command shows an MTU of 1500 however the show queuing interface command shows an MTU of 1600. Can anyone explain me the difference?

dcaaccsrv-021# show interface eth100/1/48
Ethernet100/1/48 is up
  Hardware: 10/100/1000 Ethernet, address: 34a8.4ea0.9e31 (bia 34a8.4ea0.9e31)
  Description: eth100/1/48 grbfwlphyp01-new eth5
  MTU 1500 bytes, BW 1000000 Kbit, DLY 10 usec
  reliability 255/255, txload 1/255, rxload 2/255
  Encapsulation ARPA
  Port mode is trunk
  full-duplex, 1000 Mb/s
  Beacon is turned off
  Input flow-control is off, output flow-control is on
  Switchport monitor is off
  EtherType is 0x8100
  Last link flapped 6week(s) 6day(s)
  Last clearing of "show interface" counters never
  13 interface resets
  30 seconds input rate 5056912 bits/sec, 565 packets/sec
  30 seconds output rate 476912 bits/sec, 323 packets/sec
  Load-Interval #2: 5 minute (300 seconds)
    input rate 7.93 Mbps, 823 pps; output rate 602.34 Kbps, 411 pps
  RX
    3491887905 unicast packets  39723129 multicast packets  115572016 broadcast packets
    3647183050 input packets  4150594046181 bytes
    1438642279 jumbo packets  0 storm suppression bytes
    0 runts  0 giants  0 CRC  0 no buffer
    0 input error  0 short frame  0 overrun   0 underrun  0 ignored
    0 watchdog  0 bad etype drop  0 bad proto drop  0 if down drop
    0 input with dribble  0 input discard
    0 Rx pause
  TX
    2100297504 unicast packets  57924075 multicast packets  50663231 broadcast packets
    2208884810 output packets  667699075454 bytes
    109003466 jumbo packets
    0 output error  0 collision  0 deferred  0 late collision
    0 lost carrier  0 no carrier  0 babble 0 output discard
    0 Tx pause

dcaaccsrv-021# show queuing interface eth100/1/48
if_slot 32, ifidx 0x1f630bc0
Ethernet100/1/48 queuing information:
  Input buffer allocation:
  Qos-group: 0
  frh: 2
  drop-type: drop
  cos: 0 1 2 3 4 5 6
  xon       xoff      buffer-size
  ---------+---------+-----------
  34560     39680     48640

  Queueing:
  queue    qos-group    cos            priority  bandwidth mtu
  --------+------------+--------------+---------+---------+----
  2        0            0 1 2 3 4 5 6   WRR       100      1600

  Queue limit: 66560 bytes

  Queue Statistics:
  queue  rx              tx
  ------+---------------+---------------
  2      3647182778      2208270408

  Port Statistics:
  rx drop         rx mcast drop   rx error        tx drop         mux ovflow
  ---------------+---------------+---------------+---------------+--------------
  0               0               0               0                InActive

  Priority-flow-control enabled: no
  Flow-control status:
  cos     qos-group   rx pause  tx pause  masked rx pause
  -------+-----------+---------+---------+---------------
  0              0    xon       xon       xon
  1              0    xon       xon       xon
  2              0    xon       xon       xon
  3              0    xon       xon       xon
  4              0    xon       xon       xon
  5              0    xon       xon       xon
  6              0    xon       xon       xon
  7            n/a    xon       xon       xon

Cheers,

Mathieu

[Kirk J]

Greetings.

L3 interfaces have the MTU specifically applied to them.

For L2 interfaces, it is QOS policies that define what allowed MTU sizes are.

switch(config)#policy-map type network-qos jumbo
switch(config-pmap-nq)#class type network-qos class-default
switch(config-pmap-c-nq)#mtu 9216
switch(config-pmap-c-nq)#exit
switch(config-pmap-nq)#exit
switch(config)#system qos
switch(config-sys-qos)#service-policy type network-qos jumbo

The mtu size shown in the show int eth x/y output does not reflect real MTU.

It is the show queuing int eth x/y output that shows the different queuing classes and allowed MTUs for each class.

Thanks,

Kirk...