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IamDaMayor
Beginner

IPv6 ND failing between N7K VDC

We have run into an obscure problem on one of our Nexus 7000s, where IPv6 neighbour discovery has stopped working for link-local addresses on a specific VLAN.

NAME: "Chassis", DESCR: "Nexus7000 C7009 (9 Slot) Chassis "
PID: N7K-C7009 , VID: V01 , SN: JAF1624BBJM

We have two VDCs on this chassis connected to each other via etherchannel. There are 3 x point-to-point VLANs (one per VRF) trunked on the link, and SVIs exist at both ends, with IPv4, IPv6 global, and IPv6 link-local addresses.

Layer 2 is working - both switches can see the other's MAC address. Works on all 3 VLANs.
IPv4 is working – ARP entries and OSPF peers between them. Works on all 3 VLANs.
IPv6 global addresses are working – both can resolve the other using NDP. Works on all 3 VLANs.
IPv6 link-local NOT working on ONE of the VLANs – the two switches are unable to resolve each other via NDP, and OSPFv3 peering gets stuck in 'EXSTART' as a result. The other 2 VLANs on the link are working fine however.

We have run 'debug ipv6 nd VLAN xxxx' on both switches and the log output shows they are both attempting to solicit the other but not receiving anything back.

We've run out of things to look at on the CLI and wondered if there was anything else I should be doing? This was all working fine up until a few days ago we believe.

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