VRRP/HSB Upgrade Causes AP Reboots
A campus has two AC6805s in VRRP hot standby with HSB. AC1 is master, AC2 backup, both on V200R019C10. There are 600 APs: 180 AP4050DN and 420 AP7060DN. The upgrade plan is: upgrade AC2 to V200R020C00, force a VRRP switchover, upgrade AC1, then switch back. When AC2 reboots into the new version, HSB shows inactive because the peers are on different versions. The engineer proceeds. After switchover, the 180 AP4050DN reboot and join AC2; the 420 AP7060DN remain on AC1. After AC1 is upgraded, the 420 AP7060DN reboot. Voice clients see about 6 minutes of outage. The customer asks for a zero-AP-reboot upgrade next time and asks whether dual-link backup would have prevented the reboots. The AC pair is managed by a single NMS, and no AP firmware pre-upgrade was performed.
1. Explain why HSB became inactive and why the reboot set split by AP model. What does this tell you about the AP firmware package on AC2 and the AP group configuration?
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HSB is a stateful hot-standby relationship; it requires compatible software and synchronized configuration. When AC2 came up on V200R020C00 while AC1 was still V200R019C10, the pair could not sync, so AC2 had no AP state, no AP group state, and no synchronized AP firmware package view. The split reboot set is a clue: the 180 AP4050DN were the model whose firmware package on AC2 differed from the running AP image, or whose AP group/dual-link configuration made them re-home to AC2; the 420 AP7060DN either had a compatible image or stayed on AC1 because their CAPWAP sessions were still anchored there. Do not treat the 180 as a random failure: compare AP model, AP firmware version, AP group, and dual-link/primary-secondary AC settings for the rebooted versus non-rebooted sets.
2. Recommend an upgrade sequence and state whether dual-link backup solves the reboot problem. Justify the answer.
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Zero AP reboot is not achievable when the AC software requires an AP firmware change and the APs cannot do an in-service image upgrade. Dual-link backup would not have prevented these reboots: it protects against an AC failure, but it does not make an incompatible AP firmware image compatible.
Recommended sequence: keep HSB only between identical versions; stage AP firmware first, or use an AC release that supports the existing AP image; upgrade AC2; verify HSB only after both ACs are on the same version; then switch VRRP master; upgrade AC1; switch back. If AP firmware must change, schedule per AP group and use dual-link backup only to keep the other group online, accepting that the upgraded group will reboot. Validate HSB sync, VRRP preempt, CAPWAP timers, and AP group/AP firmware package consistency before the maintenance window.