Review AS-path rewriting when Route Server connects dual-homed ExpressRoute networks

Source facts

In Microsoft’s dual-homed ExpressRoute design, the route servers share autonomous-system number 65515. BGP loop prevention discards a received path containing the receiving server’s own AS number. The documented design therefore requires the intervening NVA to apply as-override when peering with each route server so routes from the other server are not discarded for that reason. Microsoft Learn.

Applicability

Use this review for the documented hub-and-spoke design with Route Server in both locations and ExpressRoute integration. Identify each BGP session and appliance vendor. Do not apply an AS-path change to an unrelated topology solely because it contains Route Server.

DSE recommendation

DSE recommends drawing the expected AS path at every peering boundary before editing the appliance. Ask the routing owner to map Microsoft’s requirement to the vendor’s supported configuration. Record the intended prefixes and existing policy so a rejected route can be traced through the actual topology. Arrange a controlled change window and an explicit restoration plan.

Verification

Inspect received and advertised paths on both sides of the NVA during the approved test. Confirm the expected prefix reaches the intended route server and workload route table. Exercise representative traffic and its return path separately. Preserve sanitized before-and-after path evidence rather than treating an established BGP session as proof of route propagation.

Official references

Microsoft Learn: About dual-homed networks with Azure Route Server.