Configuring AppleTalk Routing Protocols

As indicated in the discussion of dynamic routing protocols for TCP/IP in Chapter 4, many factors affect the decision of which routing protocol to implement on your network. These factors ”network topology, scalability, ease of implementation, and convergence speed ”are just as important in your decision of what dynamic routing protocol to use for AppleTalk as they are for IP.

The Cisco IOS offers two dynamic AppleTalk routing protocols: Routing Table Maintenance Protocol (RTMP) and AppleTalk EIGRP. Unlike TCP/IP, AppleTalk has a default dynamic routing protocol, RTMP, that operates without manual configuration by the network administrator. AppleTalk EIGRP can be implemented on a network's IOS devices on a segment-by-segment basis. However, because only Cisco IOS devices support EIGRP, network segments with non-IOS AppleTalk routers or the segments with a mixture of both continue to require RTMP for AppleTalk to operate properly.

The sample ZIP network uses both RTMP and EIGRP. EIGRP is implemented to reduce the bandwidth consumption on WAN links, which is caused by RTMP. If your AppleTalk network is sufficiently small in size and has sufficient WAN bandwidth, RTMP alone likely will suffice, eliminating the need to configure the additional commands required to implement EIGRP.

In the following sections, we examine RTMP and EIGRP routing configuration.

Configuring AppleTalk RTMP

RTMP is the default AppleTalk dynamic routing protocol; it is similar in function to IP Routing Information Protocol (IP RIP). AppleTalk RTMP is a distance vector routing protocol that establishes and maintains AppleTalk routing tables between AppleTalk routers. We examined the properties of distance vector routing protocols and IP RIP in Chapter 4. AppleTalk RTMP is an interior gateway protocol (IGP). AppleTalk does not use exterior gateway protocol (EGP) routing protocols because AppleTalk runs on intranets , not over the public Internet. AppleTalk RTMP is enabled on all AppleTalk interfaces by default when you use the global configuration command appletalk routing .

As the first dynamic routing protocol for AppleTalk networks, RTMP lacks some of the advanced features of newer dynamic routing protocols, primarily in the areas of scalability and reduced bandwidth consumption. One of the primary failings of RTMP is the extremely "chatty" nature of the routing protocol-it sends routing updates every 10 seconds. As we will see in the next section, more recently developed dynamic routing protocols solve some of these issues.

AppleTalk RTMP uses a hop count metric similar to IP RIP. The hop count is the measure of the number of router hops a packet must traverse to go from the packet source to the destination. AppleTalk RTMP supports a maximum hop count of 30. Any route farther than 30 hops is marked as inaccessible. In the output of show appletalk route from the SF-2 router, we can see that the route to AppleTalk network 11-100 has a metric of 1 hop, shown as [1/G] in the AppleTalk routing table:

 SF-2#  show appletalk route  Codes: R - RTMP derived, E - EIGRP derived, C - connected, A - AURP         S - static  P - proxy  5 routes in internet  The first zone listed for each entry is its default (primary) zone.  C Net 1-10 directly connected, Fddi0, zone SF Zone  C Net 101-150 directly connected, Ethernet1, zone Sales  C Net 151-200 directly connected, Ethernet0, zone Marketing  R Net 11-100 [1/G] via 2.12, 10 sec, Fddi0, zone Operations  S Net 40000-40000 [1/G] via 5.10, 315 sec, Fddi0, zone SF Zone 

By default, only one route to an AppleTalk network is kept in the routing table at any given time. This behavior is different than in IP routing, in which the router automatically retains multiple equal-cost paths. To enable the router to place equal-cost paths in its AppleTalk routing table, use the global configuration command appletalk maximum-paths . For example, the command appletalk maximum-paths 2 allows the router to learn about two equal-cost paths for a given AppleTalk network destination. The number of equal-cost paths you enable on your router depends on your AppleTalk network topology. When multiple equal-cost paths are retained, the router load shares on a per-packet basis over all parallel equal-cost paths for an AppleTalk network destination.

Configuring AppleTalk EIGRP

AppleTalk EIGRP is an enhanced version of Cisco's original Interior Gateway Routing Protocol (IGRP) adapted for use in AppleTalk networks. AppleTalk EIGRP uses the same transport mechanism, DUAL update algorithm, and neighbor discovery process as used by EIGRP for IP, discussed in Chapter 4. EIGRP for AppleTalk offers features found in link state protocols, such as partial incremental updates and decreased convergence time. EIGRP sends routing updates only when changes occur in the network topology, so it uses less bandwidth than RTMP, which sends frequent complete routing table updates. Implementing EIGRP, on WAN links, particularly on those of limited bandwidth, can result in better network performance for the traffic traversing those links.

Configuring the AppleTalk EIGRP routing process consists of two steps: enabling the router to run EIGRP, and identifying which interfaces are included in the EIGRP routing process.

To enable EIGRP for AppleTalk, use the IOS global configuration command appletalk routing eigrp . This command takes as a parameter a process identifier, which is often the autonomous system number used in configuring IP EIGRP or IP BGP. In the following example, we enable AppleTalk EIGRP on the Singapore router using autonomous system number 25000:

 Singapore#  configure  Configuring from terminal, memory, or network [terminal]?  Enter configuration commands, one per line.  End with CTRL+Z.  Singapore(config)#  appletalk routing eigrp 25000  Singapore(config)#  ^Z  

After AppleTalk EIGRP is enabled, you must identify which interfaces of the router should be included in EIGRP routing updates. The IOS interface configuration subcommand appletalk protocol is used to instruct the router which AppleTalk dynamic routing protocol to use on the particular interface. The command takes as a parameter the keyword eigrp or rtmp . Within the ZIP network, we have enabled AppleTalk EIGRP on all WAN interfaces. The following is an example of configuring EIGRP as the routing protocol on the WAN interface of the ZIP Singapore router:

 Singapore#  configure  Configuring from terminal, memory, or network [terminal]?  Enter configuration commands, one per line.  End with CTRL+Z.  Singapore(config)#  interface serial 0.100  Singapore(config-if)#  appletalk protocol eigrp  Singapore(config-if)#  ^Z  

Because RTMP is configured by default on all AppleTalk interfaces, both EIGRP and RTMP routing updates are sent on interfaces on which EIGRP is enabled. This can be verified using the show apple interface command, as seen on the ZIP Singapore router:

 Singapore#  show appletalk interface serial 0.100  Serial0.100 is up, line protocol is up    AppleTalk cable range is 2902-2902    AppleTalk address is 2902.2, Valid    AppleTalk zone is "WAN Zone"    Routing protocols enabled: RTMP & EIGRP    AppleTalk address gleaning is not supported by hardware    AppleTalk route cache is not initialized 

Note that after AppleTalk EIGRP has been enabled on the router, automatic redistribution of routing information between AppleTalk EIGRP and RTMP is performed. This process ensures that routes learned by either dynamic routing protocol are mutually exchanged so that EIGRP and RTMP routers are aware of all network addresses available. The IOS global configuration command appletalk route-redistribution is automatically inserted into the configuration of the router configured for AppleTalk EIGRP to invoke the redistribution. Intentionally disabling the automatic redistribution can cause EIGRP routers to be unaware of RTMP-derived routes and vice versa, potentially making some network resources unavailable to some users.

On interfaces in which only Cisco IOS AppleTalk routers are present, RTMP routing can be disabled to eliminate the redundant advertisement of routing updates. It is important not to disable RTMP on any interface in which there are AppleTalk workstations, servers, printers, or non-IOS-based AppleTalk routers. Disabling RTMP on interfaces with those devices prevents them from accessing AppleTalk network services. In the ZIP network, we have disabled RTMP on all WAN interfaces in which only Cisco IOS AppleTalk routers are present. To disable RTMP, use the IOS interface configuration subcommand no appletalk protocol rtmp . The following is an example of disabling RTMP on the WAN interface of the ZIP Singapore router:

 Singapore#  configure  Configuring from terminal, memory, or network [terminal]?  Enter configuration commands, one per line.  End with CTRL+Z.  Singapore(config)#  interface serial 0.100  Singapore(config-if)#  no appletalk protocol rtmp  Singapore(config-if)#  ^Z  

As just noted, RTMP cannot be completely disabled on AppleTalk interfaces where other AppleTalk end stations and non-IOS AppleTalk routers reside. However, if only AppleTalk end stations are attached to the network segment (which is known as a stub network), the advertisement of complete RTMP routing updates can be suppressed in favor of a modified short routing update. The short form of the routing update is sufficient to allow AppleTalk servers, workstations, and printers to continue to operate, and it does not have the added network overhead of sending complete routing table updates via RTMP.

The IOS interface configuration subcommand appletalk rtmp-stub configures the router to send only the stub update on the interface on which the command is configured. Although we have chosen not to implement this feature on the ZIP network, the following is an example of configuring the command if we chose to implement it on the Singapore router's Ethernet interface:

 Singapore#  configure  Configuring from terminal, memory, or network [terminal]?  Enter configuration commands, one per line.  End with CTRL+Z.  Singapore(config)#  interface ethernet  Singapore(config-if)#  appletalk rtmp-stub  Singapore(config-if)#  ^Z  

The operation of both AppleTalk EIGRP and RTMP can be verified with the previously examined show appletalk route command. To further assist you in verifying AppleTalk EIGRP configuration and operation, additional IOS EXEC commands can be used, as indicated in Table 5-3.

Table 5-3. AppleTalk EIGRP IOS EXEC Commands

IOS EXEC EIGRP Command	Function
show appletalk eigrp interfaces	Displays information about interfaces configured for AppleTalk EIGRP.
show appletalk eigrp neighbors	Displays the AppleTalk EIGRP- discovered neighbors.
show appletalk eigrp topology	Displays the AppleTalk EIGRP topology table.
show appletalk eigrp traffic	Displays the number of packets sent and received for AppleTalk EIGRP process(es).