Routing decision on azure virtual network gateway

Question

Hi,

We have two tunnels on azure vpn gateway.

Client DC1 vpn peer1: 1.1.1.1 (LNG1 - static routing)

Address space: 10.98.1.0/24

Client DC2 vpn peer1: 2.2.2.2 (LNG2 - BGP)

BGP peer: 10.98.50.10

Route received 10.98.1.0/24

Azure vpn gw: 10.53.78.30

Source: 10.98.1.10

Dest: azure vm: 10.53.78.100

Will the return traffic be routed via tunnel 1 or tunnel 2?

Answer 1

Hello, Welcome to MS Q&A

The return traffic from the Azure VM (10.53.78.100) to the source (10.98.1.10) will be routed based on the BGP routes received by the Azure VPN Gateway. Since both tunnels are advertising the same route (10.98.1.0/24), the Azure VPN Gateway will use the BGP path selection process to determine the best path.

In general, BGP path selection criteria include:

1. Highest weight (Cisco-specific)
2. Highest local preference
3. Shortest AS path
4. Lowest origin type
5. Lowest MED (Multi-Exit Discriminator)
6. eBGP over iBGP
7. Lowest IGP metric to the BGP next hop
8. Oldest path
9. Lowest BGP router ID

If both tunnels have the same BGP attributes, the Azure VPN Gateway may use ECMP (Equal-Cost Multi-Path) routing to load balance the traffic across both tunnels. However, if there are differences in the BGP attributes, the path with the preferred attributes will be selected.

To determine the exact behavior, you would need to review the BGP configuration and the attributes of the routes received by the Azure VPN Gateway.

Please let us know if any questions

Kindly accept answer if it helps

Thanks
Deepanshu