Express Route Gateway migration not working and throwing error "Gateway subnet needs two or more address prefixes for migration"

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akinbade abiola 18,290 Reputation points

2024-05-20T08:43:51.53+00:00

Hello sunil cherukuri,

Thanks for your question.

You will need to verify the current address prefixes allocated to the gateway subnet as The gateway subnet needs two or more address prefixes for migration.

To verify you can use the Azure CLI:

https://learn.microsoft.com/en-us/cli/azure/network/vnet/subnet?view=azure-cli-latest#az-network-vnet-subnet-show-examples

az network vnet subnet show -g MyResourceGroup -n MySubnet --vnet-name MyVNet -n <GatewaySubnet>

To add this See:

https://learn.microsoft.com/en-us/azure/expressroute/expressroute-howto-gateway-migration-portal#add-a-second-prefix-to-the-gateway-subnet

Please let me know if you have further questions

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sunil cherukuri 20 Reputation points

2024-05-20T11:08:38.8+00:00

Hello @akinbade abiola ,

Thanks for your quick response. I have done testing in my Lab, and it is working fine without any issue.

Can you please suggest me on the below points:

I read a Microsoft article stating /24 CIDR Gateway Subnet is good enough for Gateway Migration and we have /24 gateway subnet but still we are getting the error "Gateway subnet needs two or more address prefixes for migration".

On the current gateway subnet, we do have one routing table with many routes. So, by adding one more address prefix on the gateway subnet, will there be any interruption or glitch while fetching the new address prefix on the gateway subnet.

As the IP is getting changed on the new gateway, ISP intervention is required here to maintain the connectivity intact between On-Prem and Azure. Correct me If I am wrong.

I believe Backup Connectivity is much recommended while doing this exercise in production. Please suggest.

akinbade abiola 18,290 Reputation points

2024-05-20T12:03:22.6466667+00:00

The CIDR subnet you have is suitable in size, but you may need to split it into two or more prefixes to satisfy the requirement.

Yes, there may be glitches, mainly as the subnet is in use

Yes you are correct, you may need ISP intervention

Yes backup Connectivity is recommended.

https://learn.microsoft.com/en-us/azure/vpn-gateway/tutorial-site-to-site-portal#vpn-gateway

https://learn.microsoft.com/en-us/azure/virtual-network/virtual-networks-udr-overview

https://azure.microsoft.com/en-us/updates/public-preview-expressroute-seamless-gateway-migration/

https://learn.microsoft.com/en-us/azure/expressroute/expressroute-howto-gateway-migration-powershell?source=recommendations
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Express Route Gateway migration not working and throwing error "Gateway subnet needs two or more address prefixes for migration"

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