MTU Mismatch Within Transport Zone Alarm in NSX Manager

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

VMware NSX 
VMware NSX-T Data Center

Resolution

Steps to Resolve:

For 3.2.0 and higher

Recommendation:

1. 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.
2. You can review the MTU setting on NSX, either in the UI (System > Fabric > Setting > Global Fabric Settings) or using the GET API call  "https://<nsx-mngr-ip>/api/v1/global-configs/SwitchingGlobalConfig"
   
3. Decide which MTU is required and which one the change needs to be applied on, NSX overlay traffic requires a minimum of 1600, 1700 is recommended, please review Guidance to Set Maximum Transmission Unit for more details.
4. Set the same MTU value on all switches 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, review Host Switches for more details on Host Switches and Create Transport Zones for more details on a Transport Zone.
5. This can be changed using a PUT with the above API to change the Global Config and setting the physical_uplink_mtu, sample below:
   
6. Or in the NSX UI under System > Fabric > Setting > Global Fabric Settings and set the Tunnel Endpoint value.
7. Once the MTU's are fixed and 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 chnaged, changes are best carried out in a maintenance window.