System information

The Transport Layer
Book Title
9-182
The main function of AURP is to maintain accurate routing tables for the entire AppleTalk WAN by
the exchange of routing information between exterior routers. In addition, AURP encapsulates
AppleTalk data packets with the headers required by the foreign network.
AURP uses the principle of split horizons (which states that it is never useful to send information
about a route back in the direction from which the information came) to limit the propagation of
routing updates. For that reason, an exterior router sends routing information about only the
networks that comprise its local internetwork to other exterior routers connected to the tunnel.
When an exterior router becomes aware of another exterior router on the tunnel, the two exterior
routers exchange their lists of network numbers and associated zone information. Thereafter, an
exterior router sends routing information only when the following events occur:
• A network is added to the routing table.
• A change in the path to a network causes the exterior router to access that network through its
local internetwork rather than through the tunnel or to access that network through the tunnel
rather than through the local internetwork.
• A network is removed from the routing table.
• The distance to a network is changed.
When an exterior router receives AppleTalk data packets or routing information that needs to be
forwarded over the tunnel, the AURP module converts that information to AURP packets. The
AURP packets are encapsulated in the header information required by the foreign network and sent
over the tunnel to the destination exterior router, as shown in Figure 9-7.
Figure 9-7 The AURP Architectural Model
At the destination exterior router, the AURP module removes the headers required by the foreign
system from the AURP packets and sends AppleTalk data packets to their final destination. The
exterior router uses the AURP packets that contain routing information to update its routing
information tables but does not propagate that information to any other exterior router.
Note As defined by Apple Computer, AURP converts RTMP and ZIP packets into AURP packets
and vice versa. As implemented by Cisco, AURP converts Enhanced IGRP packets as well as RTMP
and ZIP packets.
AEP
AEP is an extremely simple protocol which generates packets that can be used to test the reachability
of various network nodes.
To tunnel
AURP packets
Routing information
Routing informationRouting information
AURP module
Central routing
process
To local internetwork To local internetwork