NSX Alarm: MTU Mismatch Within Transport Zone

Products

• VMware NSX

Issue/Introduction

Event ID: mtu_check.mtu_mismatch_within_transport_zone

Alarm Description

• Purpose: Check if there is MTU configuration mismatch between Transport Nodes (ESXi, KVM and Edge) attached to the same Transport Zone.

• Impact: MTU values on all switches attached to the same Transport Zone not being consistent will cause connectivity issues.

Environment

VCF NSX 9.x

VMware NSX 

VMware NSX-T Data Center

Cause

The alarm triggers when it detects that MTU values are inconsistent between Transport Nodes in the same Transport Zone.

Resolution

For 3.2.0 and higher

Verify MTU settings

Using UI 

• Review which MTU are incorrect, In the NSX UI, navigate to System > Fabric > Settings > MTU Configuration Check, it should show as Inconsistent, click on the 'Inconsistent' link and it will list the DVS/Hostswitch which have inconsistent MTU's and which MTU is on which switch.

Using API

• You can review the MTU setting on NSX, either in the UI (System > Fabric >Settings > Global Fabric Settings) or using the GET API call:

  "https://<nsx-mngr-ip>/api/v1/global-configs/SwitchingGlobalConfig"

  
  
  

Workaround

1. Decide which MTU is required and which one needs to be changed.

   Note: NSX overlay traffic requires a minimum MTU of 1600, an MTU of 1700 is recommended, review Guidance to Set Maximum Transmission Unit for more details.

2. Set the same MTU value on all switches (DVS/N-VDS) attached to the same Transport Zone, (this means all transport nodes (edges/hosts) using overlay, in the same transport zone, which are configured with a host switch),

   Note: Review Host Switches for more details on Host Switches and Create Transport Zones for more details on a Transport Zone.

3. The MTU can be changed with a PUT API call, using the results of the GET API call, like above.

   1. To change the Global Config and setting the physical_uplink_mtu, sample below:

      

Or

• In the NSX UI under System > Fabric > Settings > Global Fabric Settings and set the Tunnel Endpoint value.

 

Once the MTU's are matched, trigger the MTU check under System > Fabric > Settings > MTU Configuration Check, click CHECK NOW and it should show Consistent, once it is back to Consistent, the alarm should auto resolve.

Note: Changing MTU may briefly impact network traffic which utilize the host switches where the MTU is being changed, changes are best carried out in a maintenance window.