Route Preference for Express Route Gateway

Question

Hello Experts,

I have query for following scenario.

There is HUB Vnet which has Express Route GW & Express Route Circuit for Onprem connectivity

Virtual WAN HUB is deployed and there is 'Express Route Connection' added from same Express Route circuit to Exp Route GW deployed under vWAN Hub.

Customer would like to roll-out vWAN Hub & would like to remove Express Route GW & connection, however in order to avoid downtime both Exp Route GW will coexist for some time period.

Now HUB Vnet is also connected to vWAN Hub through Vnet Connections. Now On-premise IP prefixes will be learned by both Express Route Gateways ( Exp GW in HUB Vnet and Exp GW part of vWAN Hub) hence in order to reach On-prem destination which path or Express Route Gateway will be preferred.

Answer 1

Hi @Raviraj Velankar,

Welcome to Microsoft Q&A Platform. Thank you for reaching out & hope you are doing well.

1. Both ExpressRoute Gateways coexist and are connected to the same on-premises network. Azure determines the preferred path based on the Local Preference attribute and the longest prefix match algorithm, ensuring the most efficient route is selected to reach on-premises destinations
2. When both ExpressRoute Gateways are active and learning on-premises IP prefixes, Azure will prioritize routes based on local preference settings. A higher local preference value indicates a more preferred routing path. For your reference: https://learn.microsoft.com/en-us/azure/expressroute/expressroute-optimize-routing#:~:text=The%20default%20local%20preference%20is%20typically%20100.%20Higher%20local%20preferences%20are%20more%20preferred.
3. The deployment of vWAN can affect traffic routing decisions through its hub architecture. When the HUB Vnet is connected to the vWAN Hub, the virtual hub router manages route propagation and can prioritize connections based on the route settings applied in Azure. As the customer moves to the vWAN setup, it is important to consider how these routing preferences will be managed, including implementing the Hub Routing Preference (HRP) to optimize routes towards the on-premises networks For your reference: https://learn.microsoft.com/en-us/azure/virtual-wan/about-virtual-hub-routing-preference#:~:text=The%20virtual%20hub%20router%20takes,virtual%20hub%20routing%20preference.
4. As the both ExpressRoute Gateways (one in the HUB Vnet and one in the vWAN Hub) coexist, the path preference for reaching on-premises destinations will depend on the BGP (Border Gateway Protocol) route selection process, the BGP will determine the best path based on the local preferences. Configuring the local preference for routes received from each gateway is crucial. The customer can set a higher local preference for one of the gateways to ensure that it is favored for routing traffic, especially as they plan to decommission the other. For your reference: https://learn.microsoft.com/en-us/azure/expressroute/designing-for-disaster-recovery-with-expressroute-privatepeering#:~:text=Typically%2C%20local%20preference%20value%20is%20used%20to%20influence%20on%2Dpremises%20network%20to%20prefer%20one%20ExpressRoute%20circuit%20over%20others.%20Local%20preference%20is%20an%20internal%20BGP%20(iBGP)%20metric.%20The%20BGP%20route%20with%20the%20highest%20local%20preference%20value%20is%20preferred.

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Thanks,

Sai Prasanna.