Abstract: A packet based telecommunication system, such as ATM, comprises a main data path and at least one bypass path, for bypassing a portion of the data path, the portion and the respective bypass defining a protection domain, the system comprising nodes for each of the domains, for monitoring respective domains and for issueing alarms in the form of packets, to other nodes downstream, with a domain identifier indicating the respective domain in which the alarm originated. At a given one of the nodes an alarm issued from a node upstream of the given node and a corresponding identifier, are detected. At the given node it is determined whether to trigger a rerouting of the data along one of the bypass paths which bypasses the domain monitored by the given node, on the basis of the detected domain identifier. Using the identifier, downstream nodes can determine more easily whether the alarm is caused by a domain which has a bypass path triggered by another node upstream.