Novell Internet Access Server 4.1 routing software supports the Routing Table Maintenance Protocol (RTMP) and the AppleTalk Update-Based Routing Protocol (AURP).
Routers use routing protocols to create and dynamically maintain a database of routing information called the Routing Information Table. This table contains information about all the networks on the AppleTalk internetwork. The router uses this information to forward packets to their destinations by the best possible route.
AppleTalk routing tables contain an entry for every network in the internetwork. AppleTalk learns the network ranges and/or network numbers of its directly connected interfaces through configuration of the seed router. If the router is configured as a non-seed router, the router "listens" to seed routers on the network and learns addressing information about its directly connected interfaces. An AppleTalk router learns about networks not directly connected to it through the exchange of routing information with each of its neighbors.
A router's neighbor is a router that has at least one interface on the same network. By learning its neighbor's routing information, the router learns about the entire internetwork.
AppleTalk generally uses RTMP to transmit routing information to neighboring routers. It broadcasts its routing information every 10 seconds to all directly connected networks.
To save network bandwidth, RTMP provides distance vector, split-horizon routing. In split-horizon routing, a router does not advertise routes it learns from an interface through the same interface, because neighboring routers reachable through that interface know the information already.
RTMP has a maximum hop count of 15. (Hop count is the number of routers, or hops, that must be traversed to reach the destination network.) If nodes on an AppleTalk internetwork are separated by more than 15 routers, they cannot communicate.
AURP is a tunneling and a routing protocol. It is an enhancement of RTMP. AURP provides the feature of AppleTalk tunneling in TCP/IP. This feature enables two isolated AppleTalk networks to be connected by way of a TCP/IP network. AURP provides update-based routing and reliable delivery of routing information. To reduce the amount of bandwidth, update-based routing sends updates to peer routers only when network routing information changes, rather than sending periodic broadcasts of the routing table.
Like RTMP, AURP also provides distance vector, split-horizon routing. It has a maximum hop count of 15. A tunnel counts as one hop.
NOTE: The Novell Internet Access Server 4.1 routing software supports the minimal set of AURP features defined in RFC 1504.
AURP is useful in the following scenarios:
On a stable network, AppleTalk routers using RTMP broadcast the same information to each other continuously. Because most networks are stable after initial setup, these frequent broadcasts unnecessarily consume bandwidth that could be used to transport more user data. This is particularly noticeable on slow WAN links. Unlike RTMP, AppleTalk routers using the AURP protocol exchange routing information only when a change occurs in the internetwork.
Routing information exchanged with AURP must be communicated reliably. Because only changes are communicated among AURP routers and because this information is not sent periodically (that is, on a set schedule) to peer routers, the possibility of lost information is increased. Therefore, AURP is a transport protocol that provides reliable delivery. It requires the receiving router to acknowledge the receipt of the updates.
NOTE: The AURP transport ensures reliable delivery of routing packets only. AppleTalk protocols, such as ATP, ASP, PAP, and ADSP, ensure reliable delivery of AppleTalk data packets.
For more information about using AURP for tunneling AppleTalk through IP, including configuration information, refer to Novell Internet Access Server 4.1 Routing Configuration.