Connecting multiple expressroute circuit with a single VNet

WinTechie 171 Reputation points
2021-11-19T11:57:05.457+00:00

Hi

due to higher bandwidth requirements of ExpressRoute circuit, backend infrastructure at service provider side needs to be updated (may be at S/W Or H/W level) to support new higher bandwidth, therefore a proposal is to make one more peering connection of production VNet with new ExpressRoute circuit (to be provisioned) keeping the old connection intact.
and once service provider is done with un-linking WAN connection with current EXPR and then linking WAN with new EXPR, traffic from Azure Vnet to on-prem and vice versa should flow over this new Expressroute circuit.

Just would like to know if this approach is suitable considering whether single VNet can be peered with two circuits (allowing traffic to flow on current expr line) and after WAN connectivity changes traffic is flowing over new EXPR line and then eventually old connection can be removed.

if it's possible and approach is technically fine, would like to know if anything needs to be considering around routing weight defined in connections

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  1. GitaraniSharma-MSFT 27,251 Reputation points Microsoft Employee
    2021-11-22T14:05:13.497+00:00

    Hello @WinTechie ,

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

    Yes, it is possible. You can link a single virtual network with up to four ExpressRoute circuits in the same location or up to 16 ExpressRoute circuits in different peering locations.
    Reference : https://learn.microsoft.com/en-au/azure/expressroute/expressroute-faqs#can-i-have-one-virtual-network-connected-to-more-than-one-expressroute-circuit

    Rather, it is strongly recommended that customers setup at least two ExpressRoute circuits to avoid single points of failure. Multiple ExpressRoute circuits from different peering locations or up to four connections from the same peering location can be connected to the same virtual network to provide high-availability in the case that a single circuit becomes unavailable. You can then assign higher weights to one of the local connections to prefer a specific circuit.
    Reference : https://learn.microsoft.com/en-au/azure/expressroute/expressroute-faqs#how-do-i-implement-redundancy-on-private-peering

    Challenges of using multiple ExpressRoute circuits:
    When you interconnect the same set of networks using more than one connection, you introduce parallel paths between the networks. Parallel paths, when not properly architected, could lead to asymmetrical routing. You can influence Azure to prefer one ExpressRoute circuit over another one using one of the following techniques (listed in the order of effectiveness):

    • advertising more specific route over the preferred ExpressRoute circuit compared to other ExpressRoute circuit(s)
    • configuring higher Connection Weight on the connection that links the virtual network to the preferred ExpressRoute circuit
    • advertising the routes over less preferred ExpressRoute circuit with longer AS Path (AS Path prepend)

    Reference : https://learn.microsoft.com/en-au/azure/expressroute/designing-for-disaster-recovery-with-expressroute-privatepeering

    Once you take care of the above challenge, your approach should work without any issues.

    Kindly let us know if the above helps or you need further assistance on this issue.

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