Global Router MTU Too Big Alarm

Products

• VMware NSX

Issue/Introduction

Title: Alarm for mtu_check.global_router_mtu_too_big
Event ID: mtu_check.global_router_mtu_too_big

Alarm Description

• Purpose: Check if the global router MTU configuration is bigger than the MTU of switches in overlay Transport Zone which connects to Tier0 or Tier1.
• Impact: Global router MTU value should be less than all switches' MTU value by at least 100, as we require a 100-quota for Geneve encapsulation. Otherwise, it may cause connectivity issues.

Environment

VMware NSX-T Data Center

Cause

This alarm is generated when the global router MTU configuration is bigger than the MTU of switches in the overlay Transport Zone, which connects to Tier 0 or Tier 1.

Resolution

Steps to Resolve

For 3.2.0 and higher

Recommendations:

1. Navigate to System | Fabric | Settings | MTU Configuration Check | Inconsistent on the NSX UI to check more mismatch details.
2. Set the bigger MTU value on switches by navigating to System | Fabric | Profiles | Uplink Profiles on the NSX UI, or by invoking the NSX API PUT /api/v1/host-switch-profiles/ with mtu in the request body, or API PUT /api/v1/global-configs/SwitchingGlobalConfig with physical_uplink_mtu in request body.
3. Or set the smaller MTU value of global router configuration by navigating to Networking | Global Networking Config | Global Gateway Configuration | Gateway Interface MTU on the NSX UI, or by invoking the NSX API PUT /api/v1/global-configs/RoutingGlobalConfig with logical_uplink_mtu in the request body.
4. The Tier0 uplink interface MTU when connected to VLAN networks do not have to be smaller than the overlay switching MTU or the same as the Global router MTU; this MTU should match the MTU of the BGP peer.