Product(s) affected: DCN-NG systems with master & slave CCU set-up
A DCN-NG system will be installed in a building which uses different VLANs for different floors in the building. They want to be able to have interpreters on one floor (in one VLAN) and the rest of the system on another floor (in another VLAN). To this end they have connected their Master CCU to another VLAN than their Slave CCU. This set-up does not work.
Question is; should this work and if not, why not?
This cannot work in a DCN-NG system because of the way the system is designed.
The communication between the CCU’s, at start-up of the system, uses broadcast messages which are not routed by network devices. These messages have a time-to-live (TTL) of 1, which means that once they attempt to cross into another (V)LAN, the messages are not propagated by the routing (L3) switch.
The reason is that the TTL of the packet is decreased every time it traverses from one network to another. When the TTL’s value reaches 0, the packet is dropped by the routing device.
Therefore, the broadcast packet will never reach the other CCU, resulting in error messages like “No Master CCU” on the Slave CCU.
Therefore the system should be installed on the same (V)LAN. This usually does not pose any technical problems as (V)LANs can be distributed over multiple network devices without issue, it’s just that the predictability of where a device is physically located is less in this instance.