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Checking SNMP polling intervals, switch counters and IOS XE bugs is a good starting point. Comparing CLI data with SNMP results should help identify the cause.
The debug output suggests the VPNv6 packet is reaching the ASBR with the expected label, but the next-hop forwarding state is missing. Since VPNv4 works, it may be worth checking the VPNv6 label binding and LFIB programming on the receiving PE/ASBR.
The missing 4.2(x) and 5.1(x) images are likely in restricted/legacy access. Please check the Cisco ACI upgrade path tool or raise a TAC case to confirm the correct supported path and obtain the required images.
With RSTP, root bridge changes usually converge in under a second to a few seconds on a clean link. It only takes longer if legacy STP or more complex topologies are involved.
