Dell POWEREDGE M1000E - Manual (Page 165)

Managing a Switch Stack 165
NSF and VoIP
Figure 8-14 shows how NSF maintains existing voice calls during a
Management Unit failure. Assume the top unit is the Management Unit.
When the Management Unit 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 Management Unit restart.
Figure 8-14. NSF and VoIP
LAG
Phone A
Phone B