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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:
1 After a period of time, the RTMP changes the status of an entry from good
to suspect.
2 After an additional period of time, the RTMP changes the status of an entry
from suspect to bad.
3 After an additional period of time, the RTMP changes the status of an entry
from bad to really bad.
4 Finally, 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.