This section concentrates on common errors and misconfigurations related to AppleTalk settings, the expected symptoms of each problem, and how you can identify the exact reason for the symptoms and rectify the situation. The most common faults found in AppleTalk networks are
• Configuration mismatch—Neighboring routers do not have identical specifications for the cable range and zone name(s) of the segment they have in common.
• Duplicate cable range/network numbers or overlapping cable ranges in the network—Identical cable range/network numbers or overlapping cable ranges are assigned to distinct segments of the network.
• Phase I and Phase II incompatibility—The network is not completely upgraded to AppleTalk Phase II, and the Phase II segments are not using unary (e.g., 100-100) cable ranges or do not have a single zone per segment configuration.
• ZIP storms—Misconfigurations or software bugs cause a chain reaction on routers consisting of newly learned cable range(s) advertisements, which in turn triggers ZIP queries (from routers that want to find out the zones corresponding to those cable ranges).
• Incompatible routing timers causing unstable routing tables—Routers invalidate certain routing table entries, since they do not receive the periodic updates for those routes on time (before they are marked invalid).
• Excessive network load causing route flapping—The AppleTalk timers are not appropriate for the high load of traffic that exists in the network; in other words, congestion and overloading of network devices causes updates to be lost or missed, and hence be aged out of various routing tables.
Most of the problems experienced in AppleTalk networks are caused by one or more of the listed possible faults. The commonly reported trouble symptoms are:
• Zones are missing from Chooser.
• Users cannot see zones and/or services on remote networks.
• The zone list changes every time Chooser is opened.
• Connections to services drop.
• Old zone names appear in Chooser.
• A router port gets stuck in restarting or acquiring mode.
Configuration mismatch and duplicate cable range/network numbers can be the cause of any of the mentioned AppleTalk network's reported symptoms. Use the show appletalk interface command to see the configuration of router interfaces. If you suspect that the cable range/network number configured on an interface duplicated the cable range/network number of another segment, temporarily disable the interface and see whether the corresponding entry disappears from the routing tables. You can then find out exactly which segment's cable range is being duplicated by the suspended interface.
The Phase I/Phase II incompatibility issue may cause a lot of misbehavior, too. One common symptom of it is that AppleTalk services in a network do not appear outside that network. Use the show appletalk globals command to see if your router can support both AppleTalk Phase I and Phase II. Also, you can use the show appletalk neighbors command to see which of the router's neighbors are visible. In other words, if an AppleTalk device does not see a neighboring device, chances are good that there is Phase I/Phase II incompatibility between them.
Unstable routes can be the cause of zones and services not appearing in the Chooser or disappearing from it. They can also cause connection to services to suddenly drop. Use the show appletalk route command repeatedly, to observe appearance and disappearance of routes. The debug apple events can help you verify that the routes are added and aged correctly. Sometimes unstable routes are merely the result of inconsistency of AppleTalk timers, and the suggested action is obviously to make them consistent.
Another serious possible cause of unstable routes is the existence of routing loops. Routing loops are generally produced as a result of mishandling route redistribution, conflicting dynamic and static routes, and so on. In cases where unstable routes are due to load problems (known as route flapping), you may have to adjust routers' timers with the appletalk timers command. The default AppleTalk timers are 10, 20, and 60. RTMP updates are sent every 10 seconds; they are considered bad after 20 seconds (without updates), and they are discarded after 60 seconds. It is recommended that the first number remain as 10 and the third number be set to a value three times larger than the second number. Note that the larger the second and third number get, the slower your network convergence gets. Finally, keep in mind that consistency across the network is the most promising, desirable, and rewarding configuration.
ZIP storms may be the result of software bugs, unstable routes, or a bad ZIP reply filter on a router. Though infrequent, ZIP storms are serious because they cause excessive traffic. Cisco routers have a built-in mechanism to deal with ZIP storms: they do not report networks for which the corresponding zone(s) are not known yet. Use the show appletalk traffic command repeatedly to see if the number of ZIP requests increases rapidly. If you notice a ZIP storm, your challenge will be to fix the issue of unstable routes.
The symptom of old zone names appearing in the Chooser is usually due to a zone name change. If a zone name is changed, that change takes effect locally, but it does not trigger the other routers to take the old zone name out of their Zone Information Table until it ages out (a phenomenon known as ghosted zone). If the old zone name does not disappear eventually, it is probably still being used on some network segment. Use the show appletalk zones command to find out the cable range reported for the ghosted zone and then you can find your way (hop by hop) to the router that advertises that zone. Moreover, ghosted zones can also result from changing the case of a zone name. Although a zone information table will keep track of multiple spellings of a zone, the zone multicasting hash function actually does treat them the same. However, the misspelled zones can float around for a while until they age out of all routers.
In cases where zones are not visible through the Chooser, you should verify that on the Macintosh client the correct network adapter is selected. Finally, if services are not always visible from the Chooser, or if services are visible but users cannot connect, the reason could be excessive traffic due to ZIP storms or too many AARP broadcast frames. Make routes stable, filter and manage your network's traffic, and use generous cable ranges on your segments.
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