Friday, July 18, 2014

Cisco Switch: Verifying MTU/Jumbo Frames

I had a situation today where a server guy came to the network department and said that something was wrong with the network.  He could not get jumbo frames to pass from one server to another.  I think the network guys had been through it already with this server guy, but essentially the network guys had told him (and verified) that jumbo frames where enabled already on the Cisco 3750-X switches.  When I was asked to get involved in this, I verified exactly what the network guy had told the server guy.  Doing a 'show system mtu" proved it.
One other place you can do this is on the interface of the Gig connection.  Below, you can see where I look at the interface where the server is connected (there are more in this case, just one for this post) and you can see what the MTU is set to for that interface.  Its a good place to look when verifying the network side of things.
ESXstack#sh int gig 1/0/35
GigabitEthernet1/0/35 is up, line protocol is up (connected)
  Hardware is Gigabit Ethernet, address is 9c4e.2061.XXXX (bia 9c4e.2061.XXXX)
  MTU 9000 bytes, BW 1000000 Kbit, DLY 10 usec,
     reliability 255/255, txload 1/255, rxload 1/255
  Encapsulation ARPA, loopback not set
  Keepalive set (10 sec)
  Full-duplex, 1000Mb/s, media type is 10/100/1000BaseTX
  input flow-control is off, output flow-control is unsupported
  ARP type: ARPA, ARP Timeout 04:00:00
  Last input never, output 00:00:25, output hang never
  Last clearing of "show interface" counters never
  Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0
  Queueing strategy: fifo
  Output queue: 0/40 (size/max)
  5 minute input rate 0 bits/sec, 0 packets/sec
  5 minute output rate 1000 bits/sec, 1 packets/sec
     45275885 packets input, 13859407503 bytes, 0 no buffer
     Received 85734 broadcasts (33608 multicasts)
     0 runts, 0 giants, 0 throttles
     0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored
     0 watchdog, 33608 multicast, 0 pause input
     0 input packets with dribble condition detected
     144105562 packets output, 37412846063 bytes, 0 underruns
     0 output errors, 0 collisions, 1 interface resets
     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


  1. OR:

    SW#sh system mtu

    System MTU size is 1500 bytes
    System Jumbo MTU size is 9198 bytes
    System Alternate MTU size is 1500 bytes
    Routing MTU size is 1500 bytes

    1. Thank you Pavel. That is correct. Thanks for your input.


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