Infrastructure and Design
Cluster anatomy
Every Cisco UCM deployment begins with exactly one publisher - the database master and the first node installed - and scales up to 20 CUCM nodes per cluster, with subscribers holding read-only replicas. Only the publisher accepts configuration changes; if it fails, existing calls and registrations on subscribers continue in reduced functionality mode, but no adds, moves, or changes are possible until it returns.
Supporting network services
NTP is mandatory: every node must reach a common stratum source or database replication breaks. DNS is optional but recommended, and if used must be consistent across nodes. Servers receive static addressing; phones use DHCP with option 150 (a list of TFTP addresses). Option 150 is the Cisco-specific value and is the standard distractor against option 66.
Media design primitives
Two settings govern multisite design and are routinely swapped in questions. Regions decide which codec is negotiated between devices (intra-region versus inter-region relationships, plus maximum audio/video bit rates). Locations perform call admission control, subtracting each call's bandwidth from a pool. Device pools bind devices to region, location, date/time group, SRST reference, and locales.
Redundancy and sizing
Use OVA templates matched to user count, and CUCM groups (ordered node lists) for registration failover. For WAN survivability, design SRST or E-SRST at remote sites, and Cisco Expressway for firewall traversal.
Rule of thumb: if the question asks what codec the answer is Regions; if it asks how many calls fit the answer is Locations.
Traps: picking option 66 for DHCP; assuming subscribers can be edited; clustering before NTP is validated.