How paths form and change
A neighbour link is what a device hears, and a route is what a message uses. This lesson unplugs a router, refreshes the map and shows that repair takes time and is not guaranteed.
Heard is not used
A router keeps a neighbour table: the devices it hears well enough to list. A message does not follow that table. It follows a route, a path of forwarding devices to its destination, and a device can hear many neighbours while using one of them.
The house on the bench draws both. Thin dashes are reported links. The bold line is the route the wardrobe sensor's report takes, through its parent and on to the coordinator. A wall changes what a link costs and never becomes a hop.
Finding and repairing a route
Zigbee PRO discovers routes on demand and keeps many-to-one routes towards a concentrator, which is usually the coordinator. It is not a walk along a corridor. What matters to a reader is the cost. Route discovery sends frames, so it takes airtime, and it takes time.
When a router on the path disappears, a device can find another route if another router is in reach. In the house the sensor moves to a different router because one exists. In a house with no second router in reach it would have no route, and a map would say so. Repair is possible, takes time, and is not guaranteed.
A map is a snapshot
The routing table on the bench carries a last updated time because a map is collected, not streamed. Zigbee2MQTT warns that a topology scan can take from ten seconds to two minutes and that the network can respond more slowly while it runs. Until the scan finishes, the map shows what it last found.
That is why the plug can be unplugged and still appear on the map. Both the plug and the map are real, and only one of them is current.
The next lesson follows a message to a device that is asleep: sleep, poll, deliver.
Common mistakes
- Reading the neighbour links as the route. A link says a device was heard. The route is the single path a message is using.
- Trusting a map without its time. A topology map is a snapshot. A device that is gone can still be drawn until the next scan.
- Refreshing the map over and over. A scan costs airtime and can slow the network while it runs, so run one when you need it.
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