FHRP
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HSRP Troubleshooting: Both Routers Active and Flapping States
Dual-active HSRP is invisible from one router: each end looks like a healthy Active gateway until you put both outputs side by side and see Standby=unknown on each. Captured live on IOS XE 17.18.2 in CML, plus the preempt behaviour that silently inverts your design.