A method of broadcasting event messages in a system-on-chip having system circuitry and monitoring circuitry for monitoring the system circuitry, the monitoring circuitry comprising units connected in a tree-based structure for routing communications through the system-on-chip, the tree-based structure comprising branches extending from a root unit, each branch comprising a plurality of units, each unit connected to a single unit above in the branch and a single unit below in the branch, whereby each unit routes communications to and from individually addressable entities above that unit in its branch, the tree-based structure further comprising crosslinks connecting corresponding units of adjacent branches, the method comprising: if an event is generated at an event unit or its local subsystem, routing an event message directly from that event unit to: any adjacent unit above the event unit in the event unit's branch, any adjacent unit below the event unit in the event unit's branch, and any corresponding unit of an adjacent branch to which the event unit is connected via a crosslink.