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Jumbo frames in Nexus 5000

When we connect storage devices or some special requirements we need to change port MTU size. In Cisco 4500 and Cisco 6500 we have a option to enable MTU size per interface basis. But in Nexus 5000 it is only enable on full switch. This means all ports will be effected by MTU size. I don't know why this advance Nexus 5000 doesn't have option to change MTU per interface basis. There are simple commands to activate the same :

In this below configuration we are changing MTU size to 9216 :


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



We need to create QOS policy which can filter MTU with size 9216. After you apply this policy, All ports will use 9216 MTU. You doesn't need to restart the switch. There is one catch in it :

switch# show interface ethernet 1/7
Ethernet1/7 is up
 Dedicated Interface
  Hardware: 1000/10000 Ethernet, address: 002a.6abe.850e (bia 002a.6abe.850e)
  MTU 1500 bytes, BW 10000000 Kbit, DLY 10 usec
  reliability 255/255, txload 1/255, rxload 1/255
  Encapsulation ARPA
  Port mode is access
  full-duplex, 10 Gb/s, media type is 10G
  Beacon is turned off
  Input flow-control is off, output flow-control is off
  Rate mode is dedicated
  Switchport monitor is off
  EtherType is 0x8100
  Last link flapped 20:06:59
  Last clearing of "show interface" counters never
  30 seconds input rate 1960 bits/sec, 0 packets/sec
  30 seconds output rate 11272 bits/sec, 5 packets/sec
  Load-Interval #2: 5 minute (300 seconds)

I have configured MTU size 9216 in this switch. You will see above interface is still showing 1500 MTU. This is a know bug of Nexus 5000. To check actual MTU size which is configured of the switch we need to issue this command :


switch# show queuing interface ethernet 1/7
Ethernet1/7 queuing information:
  TX Queuing
    qos-group  sched-type  oper-bandwidth
        0       WRR            100

  RX Queuing
    qos-group 0
    q-size: 469760, HW MTU: 9216 (9216 configured)
    drop-type: drop, xon: 0, xoff: 469760
    Statistics:
        Pkts received over the port             : 68888
        Ucast pkts sent to the cross-bar        : 19077
        Mcast pkts sent to the cross-bar        : 49811
        Ucast pkts received from the cross-bar  : 19518
        Pkts sent to the port                   : 535812
        Pkts discarded on ingress               : 0
        Per-priority-pause status               : Rx (Inactive), Tx (Inactive)

  Total Multicast crossbar statistics:
    Mcast pkts received from the cross-bar      : 516294

Can you notice "HW MTU: 9216 (9216 configured)" ? Now it is confirmed that MTU is configured 9216.

Please be sure when you enable Jumbo frame on Nexus 5000 because it will enable on all ports. This configuration can break your network if you do not know what you are doing. Thats all for this post. Thanks for reading my post.

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