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When you configure a router in transition mode, that router is prepared to send out Phase 1-compatible RTMP and NBP packets.
RTMP packets contain information about networks known to the router. By passing this information among themselves every 10 seconds, AppleTalk routers update and maintain their view of the internetwork. NBP packets are used to look up devices on the internetwork.
When the Phase 2 router is not in transition mode, it always sends out Phase 2 RTMP and NBP packets. Phase 2 RTMP packets can contain information about extended networks. A Phase 1 router cannot read the Phase 2 packets and cannot incorporate the Phase 2 information into its routing tables.
Under Phase 2, to find an entity on the internetwork, a node sends an NBP Broadcast Request packet to the closest router. If the requested entity resides on the network directly connected to that router, the NBP Broadcast Request packet is converted into a Lookup packet.
If the requested entity resides on a remote network, the router converts the NBP Broadcast Request packet into a Forward Request packet for each network containing nodes in the zone specified in the NBP Broadcast Request packet. Then the NBP Forward Request packet is transmitted to a router directly connected to the network on which the entity resides. When this router receives the NBP Forward Request packet, it converts it to a Lookup packet and broadcasts it to all nodes in the proper zone on the local network.
When the Phase 2 router is in transition mode, it sends out Phase 1-compatible RTMP packets. A Phase 2 router in transition mode also does not convert Broadcast Request packets into Forward Request packets. Rather, it sends Broadcast Request packets to a router directly connected to the network on which the requested entity resides, whether that network is local or remote.
In transition mode, to avoid extended ranges and zones lists incomprehensible to a Phase 1 router, all Phase 2 routers must observe these addressing constraints:
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