Dell PowerConnect 8100 User's Configuration Guide - Page 172

NSF and VoIP Figure 9-13 shows how NSF maintains existing voice calls during a stack master failure. Assume the top unit is the stack master. When the stack master fails, the call from phone A is immediately disconnected. The call from phone B continues. On the uplink, the forwarding plane removes the failed LAG member and continues using the remaining LAG member. If phone B has learned VLAN or priority parameters through LLDP-MED, it continues to use those parameters. The stack resumes sending LLDPDUs with MED TLVs once the control plane restarts. Phone B may miss an LLDPDU from the stack, but should not miss enough PDUs to revert its VLAN or priority, assuming the administrator has not reduced the LLDPDU interval or hold count. If phone B is receiving quality of service from policies installed in the hardware, those policies are retained across the stack master restart. Figure 9-13. NSF and VoIP 172 Managing a Switch Stack