Users Guide

Stacking 249
a stack master. Use the show switch stack-ports stack-path command to verify
the reachability of all stack units. If any units are not reachable, the stack may
split during a failover.
Checkpointing
Switch applications (features) that build up a list of data such as neighbors or
clients can significantly improve their restart behavior by remembering this
data across a warm restart. This data can either be stored persistently, as in
the case of configuration data, or the stack master can checkpoint this data
directly to the standby unit active processes, as in the case of operational data.
Use the show nsf command to view the stack checkpoint status prior to
reloading a stack member. Do not reload while a checkpoint operation is in
progress.
The NSF checkpoint service allows the stack master to communicate startup
configuration data to the standby unit in the stack. When the stack selects a
standby unit, the checkpoint service notifies applications to start a complete
checkpoint. After the initial checkpoint is done, applications checkpoint
changes to their data every 120 seconds.
Table 9-1 lists the applications on the switch that checkpoint data and
describes the type of data that is checkpointed.
NOTE: The switch cannot guarantee that a standby unit has exactly the same data
that the stack master has when it fails. For example, the stack master might fail
before the checkpoint service gets data to the standby if an event occurs shortly
before a failover.
Table 9-1. Applications that Checkpoint Data
Application Checkpointed Data
ARP Dynamic ARP entries
Auto VOIP Calls in progress
Captive Portal Authenticated clients
DHCP server Address bindings (persistent)
DHCP snooping DHCP bindings database
DOT1Q Internal VLAN assignments