4

I am attempting to set the L3 mtu on a Nexus port. The mtu option is only available if I set "no switchport" which then prevents me configuring VLAN related settings. Am I missing something obvious? Why can't I set the interface MTU and also configure VLANs?

Thanks in advance for any assistance.

  • 1
    Why would you think you can configure layer-3 parameters on a layer-2 interface? – Ron Maupin Jan 18 at 15:46
  • What is the command you are using to try to configure the interface MTU? – Ron Maupin Jan 18 at 15:48
  • "interface nn" / "mtu 9000" – B. Austin Jan 18 at 16:08
  • @RonMaupin Thanks for the reply - so when I "sh int En/n" the mtu shows as 1500, how do I increase this then? – B. Austin Jan 18 at 16:10
  • That would be trying to set the layer-2 MTU on a specific interface, which I do not think is allowed on that particular switch model. I think you would need to use QoS MTU for that switch model. – Ron Maupin Jan 18 at 16:15
5

It appears that the Nexus 3172 can only do QoS MTU, not per port MTU.

Configure and Verify Maximum Transmission Unit on Cisco Nexus Platforms

Layer 2 MTU Configurations

Layer 2 MTU is set either through a network Quality of Service (QoS) policy or by configuring the port itself (on switches that support per-port MTU). Only the Nexus 7K, 9K, and certain 3K models support per-port MTU.

Network QoS MTU Configuration

Nexus 3K: Includes Nexus 3048, 3064, 3132Q, 3132Q-X, 3132Q-XL, 3172, and 3500-series switches

Nexus 5K: All Nexus 5000 and 5500-series switches

Nexus 6K: All Nexus 6000-series switches

In order to configure elevated MTU on these switches, create a network-qos policy or modify an existing policy in order to specify the elevated MTU. This configuration applies to all ports. This includes any Cisco Fabric Extender (FEX) ports connected to the switch. Per-port MTU is not supported.

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

Per-Port MTU Configuration

Nexus 3K: Includes Nexus 3132Q-V, 3164, 31108, 31128PQ, 3200-series, and 36180YC-R switches

Neuxs 7K: All Nexus 7000- and 7700-series switches

Nexus 9K: All Neuxs 9200-series switches (including 92xxx), 9300-series switches (including 93xxx), and 9500-series switches

In order to configure MTU on a per-port basis, the following configuration is needed:

Nexus(config)#interface ethernet 1/1
Nexus(config-if)#mtu 9216
  • It may be worth noting that Nexus 3164Q and 31128PQ are actually tuned-down 9300 Series Variants, in general, as well as in this context of configuring MTU. – Marc 'netztier' Luethi Jan 18 at 16:34
4

Usually Nexus have this config section:

policy-map type network-qos jumbo
  class type network-qos class-default
    mtu 9216

 ...
system qos
  service-policy type network-qos jumbo

While this works and enables a Nexus for jumbo frames at L2, I personally consider it a bad example of config item naming, as the term "jumbo" is misleading as to what it is or what it does: It's actually the name of a policy-map.

Note: the given (layer 2) port of the Nexus configured this way will still show 1500bytes MTU in show commands and in CDP/LLDP information. All the same, it will process packets up to 9216 in both ingress and egress directions.

Because there might be other system-wide settings which must be defined by system qos, and because system qos can only take one single service-policy/policy-map, it is not very clever to call that single policy-map "jumbo".

Whenever I have the liberty to do so, I rewrite this as:

policy-map type network-qos PMAP-NQOS-SYSTEM
 !class type CMAP-NQOS-NNNNnnn       <--- room for other classes/class-maps used by 'system qos'
 !   <action/setting>                <--- actions/settings for other system wide classes
 class type network-qos class-default
     mtu 9216

! ...
! 'system qos' can take only one and exactly one service-policy
! ...
system qos
     service-policy type network-qos PMAP-NQOS-SYSTEM

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