Clear the neighbor cache

When there is an event such as a topology change or an address change, the neighbor cache might have too many entries to allow efficient use. Also, if an unauthorized client is answering DAD or normal neighbor solicitations with invalid replies, the neighbor cache might contain a large number of invalid entries and communication with some valid hosts might fail, the show ipv6 neighbors command output might become too cluttered to efficiently read, or both. In such cases, the fastest way to restore optimum traffic movement on a VLAN might be to statically clear the neighbor table instead of waiting for the unwanted entries to time out.