“Designing enterprise cloud interconnects requires handling multi-region ExpressRoute paths without creating asymmetric routing black holes.”

At Kyndryl, I architected resilient Azure ExpressRoute hybrid connectivity across primary and secondary Azure regions.

ExpressRoute BGP Path Manipulation

To ensure optimal path selection while providing seamless failover:

• Primary Circuit (Mumbai): Advertised with standard BGP Local Preference (200).
• Secondary Circuit (Chennai): Advertised with AS-Path Prepending (3x) to enforce standby behavior.
• S2S IPSec Backup: Configured over public internet as a tertiary fallback using BGP Weighting.

# # Azure ExpressRoute Circuit Peering Config in Terraform
resource "azurerm_express_route_circuit_peering" "private_peering" {
  peering_type                  = "AzurePrivatePeering"
  express_route_circuit_name    = azurerm_express_route_circuit.er_primary.name
  resource_group_name           = azurerm_resource_group.rg_network.name
  peer_asn                      = 65000
  primary_peer_address_prefix   = "10.0.0.0/30"
  secondary_peer_address_prefix = "10.0.0.4/30"
  vlan_id                       = 100
}

The Verdict

Key Takeaway

Architect ExpressRoute Links for Asymmetric Resiliency.

Always pair Azure ExpressRoute circuits with explicit BGP Path Manipulation (Local Preference & AS-Path Prepending) to prevent asymmetric routing drops on stateful NVAs.