Determinism at the Boundary: Tuning ExpressRoute BGP Steering with Local Preference

How we designed deterministic ingress and egress pathways across dual Azure ExpressRoute circuits to eliminate asynchronous routing and routing loops.

As enterprises scale workload footprints across hybrid multi-cloud environments, ensuring predictable boundary routing becomes a primary engineering challenge. Unchecked dual-homed WAN connections often lead to sub-optimal routing paths, or worse, asymmetric pathing where packets enter through one circuit and return through another. This asymmetry breaks stateful firewall inspection rules, resulting in dropped connections and phantom application latency.

The Asymmetric Loop Problem

In our primary hub architecture, we deployed two active-passive 10 Gbps ExpressRoute circuits mapping to geographically separate Azure peering locations. However, because our on-premises core routers received identical prefix weights and AS-paths from both virtual network gateways, the routers began performing ECMP. This split traffic flows arbitrarily across both circuits, causing massive packet drops at the stateful firewall boundaries.

We needed a routing structure that enforced deterministic traffic steering. The objective was simple: all egress traffic must prefer Circuit A under normal operations, and failover to Circuit B only during a physical link failure.

Injecting BGP Local Preference

To govern outbound traffic leaving our network towards Azure, we configured Local Preference on our local edge routers. By default, BGP assignees evaluate paths based on weight, local preference, origin, and AS-path length. We created a BGP route-map applying a higher local preference value (200) to prefixes received from Circuit A, and a lower preference (100) to Circuit B.

# Configure local preference on Cisco IOS XE edge router
router bgp 65001
 neighbor 10.0.0.1 route-map PEER-A-IN in
!
route-map PEER-A-IN permit 10
 set local-preference 200
# Prefixes from circuit A now score higher in routing decision table

Influencing Azure Inbound Pathing

To influence ingress traffic returning from Azure, we had to manipulate the return path routing tables. Since Azure Virtual Network Gateways respect AS-Path Prepending rules, we prepended our local autonomous system number (ASN) three times on routes advertised to Circuit B. This artificially lengthened the path of Circuit B from Azure’s perspective.

As a result, the Azure route tables immediately selected Circuit A as the shortest path for all returning flows.

Key Takeaway

[!NOTE] Hybrid cloud boundary engineering is not about bandwidth; it is about deterministic path control.

The Verdict

Always influence BGP path decisions at both ends of a hybrid link using Local Preference for outbound flows and AS-Path Prepending for inbound flows. This eliminates asymmetric routing loops and guarantees stateful packet consistency.