An integrated circuit chip device comprising: system circuitry; and monitoring circuitry for monitoring the system circuitry, the monitoring circuitry comprising units (302, 303, 304, 305, 306, 307, 308, 309, 310) connected in a tree-based structure for routing communications through the integrated circuit chip device, the tree-based structure comprising: branches extending from a root unit at the base of the tree-based structure, each branch comprising a plurality of units (302, 303, 304, 305, 306, 307, 308, 309, 310), each unit (302, 303, 304, 305, 306, 307, 308, 309, 310) connected to a single unit (302, 303, 304, 305, 306, 307, 308, 309, 310) above in the branch and a single unit (302, 303, 304, 305, 306, 307, 308, 309, 310) below in the branch, whereby communications are routable between the root unit and a destination unit of a branch via intermediate units 302, 303, 304, 305, 306, 307, 308, 309, 310) of that branch; and crosslinks (319, 320, 321, 322, 323, 324) connecting corresponding units (302, 303, 304, 305, 306, 307, 308, 309, 310) of adjacent branches at the same hierarchical level as each other, each crosslink (319, 320, 321, 322, 323, 324) configured to be enabled to route communications between the root unit and a destination unit 302, 303, 304, 305, 306, 307, 308, 309, 310) of one of the branches the crosslink (319, 320, 321, 322, 323, 324) connects via the other branch the crosslink (319, 320, 321, 322, 323, 324) connects in response to an intermediate unit 302, 303, 304, 305, 306, 307, 308, 309, 310) being deemed defective, that intermediate unit 302, 303, 304, 305, 306, 307, 308, 309, 310) being in the same branch as the destination unit 302, 303, 304, 305, 306, 307, 308, 309, 310). system circuitry; and monitoring circuitry for monitoring the system circuitry, the monitoring circuitry comprising units (302, 303, 304, 305, 306, 307, 308, 309, 310) connected in a tree-based structure for routing communications through the integrated circuit chip device, the tree-based structure comprising: branches extending from a root unit at the base of the tree-based structure, each branch comprising a plurality of units (302, 303, 304, 305, 306, 307, 308, 309, 310), each unit (302, 303, 304, 305, 306, 307, 308, 309, 310) connected to a single unit (302, 303, 304, 305, 306, 307, 308, 309, 310) above in the branch and a single unit (302, 303, 304, 305, 306, 307, 308, 309, 310) below in the branch, whereby communications are routable between the root unit and a destination unit of a branch via intermediate units 302, 303, 304, 305, 306, 307, 308, 309, 310) of that branch; and crosslinks (319, 320, 321, 322, 323, 324) connecting corresponding units (302, 303, 304, 305, 306, 307, 308, 309, 310) of adjacent branches at the same hierarchical level as each other, each crosslink (319, 320, 321, 322, 323, 324) configured to be enabled to route communications between the root unit and a destination unit 302, 303, 304, 305, 306, 307, 308, 309, 310) of one of the branches the crosslink (319, 320, 321, 322, 323, 324) connects via the other branch the crosslink (319, 320, 321, 322, 323, 324) connects in response to an intermediate unit 302, 303, 304, 305, 306, 307, 308, 309, 310) being deemed defective, that intermediate unit 302, 303, 304, 305, 306, 307, 308, 309, 310) being in the same branch as the destination unit 302, 303, 304, 305, 306, 307, 308, 309, 310). branches extending from a root unit at the base of the tree-based structure, each branch comprising a plurality of units (302, 303, 304, 305, 306, 307, 308, 309, 310), each unit (302, 303, 304, 305, 306, 307, 308, 309, 310) connected to a single unit (302, 303, 304, 305, 306, 307, 308, 309, 310) above in the branch and a single unit (302, 303, 304, 305, 306, 307, 308, 309, 310) below in the branch, whereby communications are routable between the root unit and a destination unit of a branch via intermediate units 302, 303, 304, 305, 306, 307, 308, 309, 310) of that branch; and crosslinks (319, 320, 321, 322, 323, 324) connecting corresponding units (302, 303, 304, 305, 306, 307, 308, 309, 310) of adjacent branches at the same hierarchical level as each other, each crosslink (319, 320, 321, 322, 323, 324) configured to be enabled to route communications between the root unit and a destination unit 302, 303, 304, 305, 306, 307, 308, 309, 310) of one of the branches the crosslink (319, 320, 321, 322, 323, 324) connects via the other branch the crosslink (319, 320, 321, 322, 323, 324) connects in response to an intermediate unit 302, 303, 304, 305, 306, 307, 308, 309, 310) being deemed defective, that intermediate unit 302, 303, 304, 305, 306, 307, 308, 309, 310) being in the same branch as the destination unit 302, 303, 304, 305, 306, 307, 308, 309, 310).