Express route cross connect BGP configuration

Question

Hi Microsoft team,

We have two Azure regions(West US and East US), these regions are connected to our on-premises datacenters using express route as shown below.

West US --> Seatle

East US --> Washington

We want do express route cross connect as shown below, and I don't see any documentation on how to do the BGP configuration when express route cross connect is implemented.

For example when we do the BGP configuration on express route for West US we define two /30 networks and first IP address is used by on prem routers and second IP by Microsoft.

Q1. How the BGP peering is done between West US and Washington DC when express route cross connect is implemented?

Q2. For Express route cross connect to West US what IP should I use for BGP peer on Washington DC routers?

West US --> Seatle

West US --> Washington DC

East US --> Washington DC

East US --> Seatle

Answer 1

@Veerraju Gopalli

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

I understand that you are trying to configure Azure ExpressRoute Cross Connections.

While creating an ExpressRoute connection, the physical connection and the BGP is established with your CE (Customer Edge Router) and MSEE (Microsoft Enterprise Edge ExpressRoute router) in the same region.

• This MSEE is represented as Circuit in Azure Portal
• Just doing the above does not complete the configuration.
• It is in this step, we specify a Primary and Secondary subnet that subsequently becomes the IP address used by the routers
• Refer : ExpressRoute circuit routing configuration
• Here, only one such ExR Physical connection is established with the help of a ISP and this cannot span across regions.

For connecting to a VNet, you will be required to

• Deploy a Gateway in the VNet
• And Connect this gateway to the Circuit
• In this step, we do not configure any subnet(s)
• This step is Linking virtual networks to ExpressRoute circuits
• Here, However, multiple such VNet Links can be established and can also span across regions depending upon the SKU and Size of the Gateway and Circuit

Your scenario is documenter here

Or in other words,

• You will not establish a physical connection between West US VNet and Washington DC OnPrem
• Instead, you will establish a Vnet Link between West US Gateway and East US ExpressRoute circuit.
• This means, you will not be required to add any additional subnets to your current configuration for BGP.

P.S :

Cross Region connectivity requires you use a Premium SKU Circuit

I hope I was able to make this clear.

Thanks,

Kapil

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