You need an active/passive design where only one VLAN attachment to a single Cloud Router is active at a time and achieve 99.9% uptime. Which BGP attribute should your on-prem router design use to influence Cloud egress?

The BGP Multi-Exit Discriminator (MED) is the correct choice for an active/passive design influencing egress from Google Cloud to on-prem. MED is a non-transitive attribute used to prefer one exit point over another when multiple paths exist from an autonomous system. A lower MED value is preferred, allowing you to designate one VLAN attachment as primary (lower MED) and the other as passive (higher MED). LOCALPREF influences ingress into Google Cloud, not egress from it. ECMP is for active/active load balancing, not active/passive. ASPATH influences path selection by making a path appear longer, which is less precise for active/passive control than MED.