142 Managing a Switch Stack
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
DHCP server and DHCP snooping store their bindings database, or the
Management Unit can checkpoint this data directly to the standby unit.
Persistent storage allows an application on a standalone unit to retain its data
across a restart, but since the amount of storage is limited, persistent storage
is not always practical.
The NSF checkpoint service allows the Management Unit to communicate
certain data to the backup unit in the stack. When the stack selects a backup
unit, the checkpoint service notifies applications to start a complete
checkpoint. After the initial checkpoint is done, applications checkpoint
changes to their data.
Table 8-1 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 backup unit has exactly the same data
that the Management Unit has when it fails. For example, the Management Unit
might fail before the checkpoint service gets data to the backup if an event occurs
shortly before a failover.
Table 8-1. 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
DOT1S Spanning tree port roles, port states, root bridge, etc.
DOT1X Authenticated clients
DOT3ad Port states
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