In config-sync mode, both MSFCs are active at all times; all interfaces and routing processes are available and active on both modules.
One MSFC is “designated” and maintains the master copies of startup and running configurations. The other is “nondesignated” and receives its configurations from the designated module.
The nondesignated MSFC can be monitored through an EXEC session, but doesn't allow the configuration mode.
Config-sync mode allows an immediate failover because both MSFCs are always active. All VLAN interfaces are up, and any configured routing protocols are active on both modules.
For MLS, either of two redundant MSFC modules can be involved in setting up a flow cache entry. ...