AppleTalk Protocols 6-7
Each router builds a routing table that is the basis of dynamic routing
operations in an AppleTalk internet. Every ten 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 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 will remove the entry of a nonresponding router with a
really bad status from the table.
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 Zone Information Table.
Figure 6-3 represents a simple AppleTalk network and Table 6-1 shows the
corresponding routing table.