Azure BGP subnet

Question

Azure BGP subnet

Handian Sudianto 5,961

When i navigate to BGP peer in Azure Virtual gateway i found strange subnet.

Actually we not use subnet 172.20 in on-prem or in the azure. Anyone know how to troubleshoot this?

10.201.0.4 and 10.201.0.5 is my azure bgp peer ip.

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G Sree Vidya 660 Reputation points Microsoft External Staff

2025-03-31T04:58:09.51+00:00

Hi Handian Sudianto

We haven’t heard from you on the last response and was just checking back to see if you have a resolution yet. In case if you have any resolution,

Please do share that same with the community as it can be helpful to others. Otherwise, will respond with more details and we will try to help.
G Sree Vidya 660 Reputation points Microsoft External Staff

2025-04-01T06:45:37.0933333+00:00

Hi Handian Sudianto

Following up to see if the below suggestion was helpful. And, if you have any further query do let us know.

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G Sree Vidya 660 Reputation points Microsoft External Staff

2025-03-31T04:58:09.51+00:00

Hi Handian Sudianto

We haven’t heard from you on the last response and was just checking back to see if you have a resolution yet. In case if you have any resolution,

Please do share that same with the community as it can be helpful to others. Otherwise, will respond with more details and we will try to help.
G Sree Vidya 660 Reputation points Microsoft External Staff

2025-04-01T06:45:37.0933333+00:00

Hi Handian Sudianto

Following up to see if the below suggestion was helpful. And, if you have any further query do let us know.

Answer 1

Hi Handian Sudianto

I could understand that Azure Virtual Gateway is learning routes from the BGP peers (10.201.0.4 and 10.201.0.5) that include the unexpected subnet 172.20.x.x.

Here are some steps to troubleshoot this issue:

Please check the BGP route table for your Virtual Network Gateway which will show you all the routes being advertised by your BGP peers and look for the 172.20.x.x subnet and identify the next hop and source.
Verify at on-premises routers (10.201.0.4 and 10.201.0.5) are not advertising the 172.20.x.x subnet unintentionally.
Check the BGP configurations for any route redistribution or static routes that might include this subnet.
Ensure that no other Azure resources (like another Virtual Network Gateway or a connected VNet) are advertising this subnet.
Enable diagnostic Logs and review the BGP logs in Azure. These logs can help you identify the source of the route advertisement.
You can find guidance on enabling diagnostic logs on the below article

Troubleshooting Azure VPN Gateway using diagnostic logs | Microsoft Learn

Hope this helps! let us know if you need assistance with anything, we will be happy to assist you.

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Azure BGP subnet

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