interconnect/WARN/2025_001

VLAN attachment MTU matches VPC MTU

Product: Interconnect
Rule class: WARN - Something that is possibly wrong

Description

This rule verifies that the Maximum Transmission Unit (MTU) of your VLAN attachment matches the MTU of your VPC network.

Consistent MTU settings across your VPC network, VLAN attachments, and on-premises networks prevent packet fragmentation or drops, ensuring optimal network performance. Mismatched MTUs can lead to performance issues like increased latency or packet loss, as packets exceeding the smaller MTU will be dropped or fragmented.

For details on how mismatched MTUs are handled and best practices, see Google’s pages about Mismatched MTUs and MTU best practices.

Remediation

There are two main ways to ensure MTU consistency between a VPC network and VLAN attachments in Google Cloud:

  1. Change the MTU of the VLAN attachment.: You can adjust the Maximum Transmission Unit (MTU) setting of your Virtual Private Cloud (VPC) network to match the MTU of your VLAN attachments. This can be done by using following commands:

For each zone in the network, stop all VMs in that zone.

gcloud compute instances stop <INSTANCE_NAMES...> \
    --zone=<ZONE>

Update network MTU.

gcloud compute networks update <NETWORK> \
    --mtu=<MTU>

For each zone in the network, start all VMs in that zone.

gcloud compute instances start <INSTANCE_NAMES...> \
    --zone=<ZONE>
  1. Change the MTU of the VPC network. : Alternatively, you can configure the MTU of your VLAN attachments to align with the MTU of your VPC network. This can be done by using this command:
gcloud compute interconnects attachments dedicated update <NAME> \
    --region=<REGION> \
    --description=<DESCRIPTION> \
    --bandwidth=<BANDWIDTH> \
    --mtu=<MTU>

Further information

Please reference the following documents for MTU update command details.