Tuesday, June 8, 2010

enabling jumbo MTU on Nexus

In our data center, enabling jumbo MTU has proven to result in significant throughput improvements. It should have been straightforward, but unfortunately not with Nexus. This note is meant to fill in a gap where existing documentation may be lacking.

In a typical Nexus 7000/5000/1000v architecture, use these steps to enable jumbo MTU end to end, and then verify.

1. System jumbomtu
It defines the maximum MTU size for the switch, which must be configured on ALL devices.
Note the max value depend on hardware, for example it’s 9000 on Nexus 1000v, and 9216 elsewhere.

2. On Nexus 5000, must configure with system QoS policy
Note it can not be configured on the interface! This is quite different from any other device I am aware of. To make it even more confusing, the command varies between releases.

For example, with 4.0.1:
policy-map jumbo
 class class-default

  mtu 9216
system qos 
 service-policy jumbo

With 4.1, 4.2 it becomes:
policy-map type network-qos jumbo
 class type network-qos class-default
  mtu 9216
system qos
 service-policy type network-qos jumbo

3. On Nexus 1000v, configure on port channel
You will see some documentation showing configuration on physical interface, and disappointed when you actually try to do it. No, it won’t take the command. You must configure on the port channel! And then, magic, it will appear on the physical interface.

More detailed explanation on Nexus1000v is here.

4. On Nexus 7000, configure on the interface
Don’t forget “system jumbomtu” on ALL devices

So how to verify? Assuming you have jumbo traffic, then you will see the difference before and after with “show interface … counter detail”

Nexus5000# sh int e1/1 count detail
Rx Packets: 95396987709
Rx Unicast Packets: 95396962128
Rx Multicast Packets: 21481
Rx Broadcast Packets: 4100
Rx Jumbo Packets: 89336029550
Rx Bytes: 136394308168545
Rx Packets from 0 to 64 bytes: 6028626455
Rx Packets from 65 to 127 bytes: 611554
Rx Packets from 128 to 255 bytes: 221439
Rx Packets from 256 to 511 bytes: 188679
Rx Packets from 512 to 1023 bytes: 579970
Rx Packets from 1024 to 1518 bytes: 30730062
Rx Trunk Packets: 95396966228
Tx Packets: 2613558
Tx Unicast Packets: 257688
Tx Multicast Packets: 2355702
Tx Broadcast Packets: 168
Tx Bytes: 202281759
Tx Packets from 0 to 64 bytes: 128120
Tx Packets from 65 to 127 bytes: 2472242
Tx Packets from 128 to 255 bytes: 8732
Tx Packets from 256 to 511 bytes: 4391
Tx Packets from 512 to 1023 bytes: 63
Tx Packets from 1024 to 1518 bytes: 10
Tx Trunk Packets: 2307778

Known Issues

Last but not the least, be aware of some known issues (depending on version):

CSCsl21529, Symptom: An incorrect MTU value is displayed in the show interface command output. The Cisco 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.
Also, VEM may not retain jumbo MTU setting after reboot. Just something to watch and monitor.


  1. Sean,

    Minus disclaimer and YMMV; did you enable the system jumbo mtu on a production 7K? I'm running Layer 2 and Layer 3 interfaces and am hesitant to increase the system MTU if interfaces could assume the jumbo MTU and kill neighbor relationships.


  2. Chris, in the case of 7k, system jumbomtu must be set to support jumbo on layer 2,

    In fact, the system jumbo MTU default size is 9216 bytes. In other words, it's likely already set on your system to maximum. Verify with "show run all". You still need to set mtu on individual interfaces.