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Answer: The ASNs being used on the on-premises routers are different.
The issue arises because BGP (Border Gateway Protocol) uses the AS path as one of its route selection criteria. For traffic to load-balance across multiple VPN connections using ECMP (Equal-Cost Multi-Path), the routes must have the same priority and equivalent attributes, including the AS path length. In this scenario, the on-premises routers use unique ASNs, causing their advertised routes to have different AS paths. This difference prevents ECMP, as BGP selects only the "best" route (based on AS path and other attributes), leading to only one route being added to the routing table. Using the same ASN for both on-premises routers (if part of the same Autonomous System) would ensure identical AS paths, enabling load-balancing.
Author: LeetQuiz Editorial Team
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Your on-premises data center has two routers connected to your Google Cloud environment via separate VPNs (one per router). All applications are functioning correctly, but traffic is only flowing through a single VPN instead of being load-balanced across both connections as intended.
During troubleshooting, you observe the following:
What is the most likely cause of this issue?
A
The on-premises routers are configured with the same routes.
B
A firewall is blocking the traffic across the second VPN connection.
C
You do not have a load balancer to load-balance the network traffic.
D
The ASNs being used on the on-premises routers are different.
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