3Com 2500 manual The network range The distance in hops to the destination network

AppleTalk Protocols 7-7

Each router builds a routing table that is the basis of dynamic routing operations in an AppleTalk internet. Every 10 seconds, each router sends an RTMP data packet to the network. Routers use the information that they receive in the RTMP broadcasts to build their routing tables. Each entry in the routing table contains these items:

■The network range

■The distance in hops to the destination network

■The interface number of the destination network

■The state of each port (good, suspect, bad, really bad)

The router uses these items to determine the best path along which to forward a data packet to its destination on the network. The routing table contains an entry for each network that a datagram can reach within 15 hops of the router. The table is aged at set intervals as follows:

1After a period of time, the RTMP changes the status of an entry from good to suspect.

2After an additional period of time, the RTMP changes the status of an entry from suspect to bad.

3After an additional period of time, the RTMP changes the status of an entry from bad to really bad.

4Finally, the router removes from the table the entry of a nonresponding router with a really bad status.

The data in the routing table is cross-referenced to the Zone Information Table (ZIT). This table maps networks into zones. The section on the session layer protocols includes information about the ZIT.

Figure 7-3 illustrates a simple AppleTalk network and Table 7-1 shows the corresponding routing table.