Azure hub Effective Route s2s VPN | AS path

Question

When configuring S2S VPN on Azure Virtual Hub, In some cases the output in the effective route table return different result's

for example

case 1 : (with dash)

prefix next-hop type AS path
172.31.0.0/16 VPN_S2S_Gateway 64512-64520

case 2 : (without dash , 2 lines)
prefix next-hop type AS path
172.31.0.0/16 VPN_S2S_Gateway 64512

172.31.0.0/16 VPN_S2S_Gateway 64520

I wanted to know for each case WHY the output is different

Thanks ,

Answer 1

Hello Lior Elayev,

In Case 1, the AS path 64512-64520 indicates that to reach the prefix 172.31.0.0/16, the route has to traverse through two Autonomous Systems (AS) - 64512 and 64520. This could be due to the configuration of your network where the traffic to this prefix is routed through multiple AS.

In Case 2, the AS paths are 64512 and 64520 separately for the same prefix 172.31.0.0/16. This means there are two separate routes to reach the same prefix, each traversing through a different AS. This could be due to multiple connections (like VPN or ExpressRoute) from the virtual hub to the on-premises network, each advertising a route to the same prefix through a different AS1.

https://learn.microsoft.com/en-us/azure/virtual-wan/effective-routes-virtual-hub

If the information helped address your question, please Accept the answer.

Luis