When Netscaler is used in L3 mode, i.e devices around Netscaler use it as a next hop or a default gateway in a IPv6 environment.

These devices loose connectivity intermittently.

Important Symptom: HA Events like missed heartbeats or interface flap issues are seen in logs, and the event log timing coincide with he IPv6 hosts losing connectivity
 

Permanent Fix : This issue is fixed in 11.1.56.x and above (Issue id #693472)

To avoid hitting this problem, identify and fix the HA related issues / interface flap issues, these events are the trigger for hitting this issue.

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Problem Cause

In case of any HA Event, like missed heart-beats (not necessarily a failover) or interface flaps - NS primary node Sends GARP for IPv4 and Unsolicited NA (Neighbor Advertisement) for IPv6, this is by design.

There is an issue with the Unsolicited NA for IPv6 that Netscaler sends,  it is missing the "router flag", so the devices which have Netscaler as a default gateway or next-hop may purge the routes pointed towards netscaler on receipt of such a an NA packet. Refer: RFC 4861, page 63 - 65,  7.2.5.  Receipt of Neighbor Advertisements.

Below is a sample ipv6 unsolicited NA packet with the router flag missing, these packets are sent twice from the link local address (FE80) and twice from the global address (2001). Notice that the router flag is missing.