Configuring MTU on Nexus 5000

Unanswered Question
Mar 9th, 2010

Based on the Cisco documentation I added these lines into the config of the switch:

policy-map jumbo
  class class-default
    mtu 9216
system qos
  service-policy jumbo

When I run a show int though I'm still seeing the MTU set as 1500.  Any ideas?  Is there more to it than this?

switch# show int e1/2
Ethernet1/2 is up
  Hardware is 1000/10000 Ethernet, address is 000d.ecd3.2dc9 (bia 000d.ecd3.2dc9
)
  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
  Input flow-control is off, output flow-control is off
  Auto-mdix is turned on
  Rate mode is dedicated
  Switchport monitor is off
  Last clearing of "show interface" counters never
  5 minute input rate 27 bytes/sec, 0 packets/sec
  5 minute output rate 0 bytes/sec, 0 packets/sec
  Rx
    105549568 input packets 104585363 unicast packets 944820 multicast packets
    19385 broadcast packets 0 jumbo packets 0 storm suppression packets
    80312619994 bytes
    0 No buffer 0 runt 0 Overrun
    0 crc 0 Ignored 0 Bad etype drop
    0 Bad proto drop

I have this problem too.
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Jon Marshall Tue, 03/09/2010 - 11:15

What version of software are you using ?

There are known issues with some versions of the software showing the incorrect MTU on the physical interface.

Jon

m.winward Tue, 03/09/2010 - 11:43

Right now it's 4.0(1a)N1(1) but I'm getting ready to upgrade to the newest code.  I also opened a case with the TAC; they sent me a link to documentation with a different (slightly) set of commands

Thanks for the reply.

Ganesh Hariharan Tue, 03/09/2010 - 20:40

Based on the Cisco documentation I added these lines into the config of the switch:

policy-map jumbo
  class class-default
    mtu 9216
system qos
  service-policy jumbo

When I run a show int though I'm still seeing the MTU set as 1500.  Any ideas?  Is there more to it than this?

switch# show int e1/2
Ethernet1/2 is up
  Hardware is 1000/10000 Ethernet, address is 000d.ecd3.2dc9 (bia 000d.ecd3.2dc9
)
  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
  Input flow-control is off, output flow-control is off
  Auto-mdix is turned on
  Rate mode is dedicated
  Switchport monitor is off
  Last clearing of "show interface" counters never
  5 minute input rate 27 bytes/sec, 0 packets/sec
  5 minute output rate 0 bytes/sec, 0 packets/sec
  Rx
    105549568 input packets 104585363 unicast packets 944820 multicast packets
    19385 broadcast packets 0 jumbo packets 0 storm suppression packets
    80312619994 bytes
    0 No buffer 0 runt 0 Overrun
    0 crc 0 Ignored 0 Bad etype drop
    0 Bad proto drop

Hi,

As per the below link from cisco in open caveteis section they have bug id with your MTU case and some of the resolved caveteas also mentioned in version 4.0(1a)N1(1).

CSCsl21529

Symptom: An incorrect MTU value is displayed in the show interface command output. The Nexus 5000 Series switch only supports class-based MTU. Per-interface level MTU configuration is not supported. The switch supports jumbo frames by default. However, the show interface command output currently displays an incorrect MTU value of 1500 bytes.

Workaround: None.

http://www9.cisco.com/en/US/docs/switches/datacenter/nexus5000/sw/release/notes/Rel_4_0_1a_N1_1/Nexus_5000_Release_Notes_4_0_1a_N1_1.html#wp144071

Remember to rate the helpful post

Ganesh.H

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