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Network Design in the Real World: Theoretical Issues

One of the more enjoyable aspects of network design (or any dialog in more advanced networking) is the potential for disagreement. There are many ways to design a network. Consider secondary addresses versus super-netting, for example. Neither is necessarily the right answer every time, and the really talented members of this exclusive group will be able to adapt solutions to fit the relevant business needs and technical concerns at hand.

Recently, a group of people preparing for Cisco certifications entered a lively debate regarding IP ARP (Address Resolution Protocol). A participant commented that ARP is a Layer 3 protocol, and another participant dis-agreed, contending that it is actually a Layer 2 process. (For the record, many sources, including Cisco, cite ARP as a Layer 3 protocol.)

I believe that the debate is more important than the answer. Most people can remember facts, but knowing that ARP is Layer 3 or that ARP is Layer 2 does not really show that you understand the function of the protocol. In addition, the OSI model is exactly that—a model. So long as people can argue their position (one participant contended that ARP is a Layer 7 protocol, and he provided a solid argument), I contend that learning and expertise will result.

In the context of AppleTalk, Figure 5.1 illustrates the common relationship between AppleTalk protocols and the OSI model. Clearly, arguments could be raised that impact the actual placement of the protocols within the diagram. It is unlikely, though, that you will see a question worded on an exam as, “What Layer is X protocol?” However, you should be comfortable answering such a question and defending your answer. Although the Cisco answer, for our purposes, is the right answer, that may not provide much comfort in a late-night troubleshooting session.

One additional note—surround yourself with as much talent as you can. Technology changes too fast to maintain expertise in every area all of the time. If you do this, you’re more likely to find a resource in your circle who is well-versed in the area in question. Today, for example, I discussed an Enhanced Interior Gateway Routing Protocol (EIGRP) migration for a large company with two colleagues. Everyone contributed, and all of us learned new things from the dialogue. Some of the lessons came from new ways to ask the questions rather than assuming the answer.

The following section provides additional information regarding the major AppleTalk protocols:

AppleTalk Address Resolution Protocol AARP performs two different functions in AppleTalk. First, it is responsible for mapping AppleTalk addresses to hardware addresses. This Layer 3 to Layer 2 mapping is similar to the ARP process in IP. Second, AARP handles the dynamic assignment of node addresses.
Datagram Delivery Protocol DDP provides unique addressing of all nodes on the AppleTalk internetwork and is responsible for connectionless delivery of datagrams between nodes. Also, DDP, in conjunction with AARP, provides the functions of Layer 3. DDP is responsible for connectivity to the upper-layer protocols, and AARP is tasked with connectivity to the lower layers.
Name-Binding Protocol NBP provides name-to-address resolution that is similar to DNS in TCP/IP. It also handles additional functions, including the population of names in the Chooser for resources on the network.
Routing Table Maintenance Protocol RTMP is AppleTalk’s default routing protocol. Updates are sent every 10 seconds, and routes are aged out of the table after 20 seconds, which can result in route flapping on congested segments as the RTMP updates are dropped.
Zone Information Protocol Zones are logical divisions of AppleTalk resources. ZIP maps zone names to network addresses. Although nodes belong to one zone, zones can span multiple physical networks.

When designing for the use of AppleTalk in most small- to medium-sized networks, the most significant issues will involve addressing and naming, which will be covered in this section. The next two sections will address those issues that frequently arise with larger networks—specifically, routing and scalability.

AppleTalk Addressing

The AppleTalk protocol was designed to limit the amount of technical expertise required to configure the workstation for operation on the netwok. As a result, the workstation has virtually no configuration options and obtains its address via a dynamic querying process.

In AppleTalk, the network administrator will assign a cable range, or block of addresses that the workstations will use. For our purposes, we will ignore the issues between AppleTalk phase one and phase two and assume the use of only phase two in this presentation. Recall that AppleTalk phase one does not permit cable ranges and allows for only 127 node addresses, as reflected in Table 5.1.

AppleTalk phase one is severely limited in scalability, and it is recommended that companies migrate to phase two if they have not already done so.

TABLE 5.1 Comparison of AppleTalk Phase One and AppleTalk Phase Two

AppleTalk Phase One AppleTalk Phase Two

Number of network addresses per segment 1 65,279
Number of host addresses per network 254 devices per network, however, only 127 hosts may be accommodated. 253 per network address. Virtually unlimited.
Number of zones per network 1 255

Table 5.1 presents AppleTalk phase two as being virtually unlimited in terms of host addresses. This is due to the theoretical capability of AppleTalk to consider cable range 1–65,279 as one network and 253 hosts per single cable range (cable range 1–1, for example). Thus, the true number of maximum nodes in an AppleTalk network is approximately 16 million. Although possible, this number is well beyond the broadcast and physical limitations of most networks, and most cable ranges do not span more than 10 digits (10–19, for example).

For additional information regarding AppleTalk phase one and phase two, please refer to CCNP: Cisco Internetwork Troubleshooting Study Guide (Sybex, 1999).

AppleTalk addresses are comprised of two parts: a network number and a node number. These are written in the format network.node.

The network number is defined by the cable range value for the segment and is configured on the router. Under AppleTalk phase two, multiple cable range values may be linked to a single AppleTalk network. For example, cable range 4–4 would service only 253 nodes; however, under AppleTalk phase two, the designer could define the cable range as 10–19, permitting hundreds of nodes. Note that these 10 cable ranges become a single logical network. This is comparable to expanding the mask in IP, but Apple-Talk networks do not share the concept of a separate net mask. For example, nodes on cable range 10–19 might appear as 14.91 and 17.132. In this case, both nodes are on the same network.

Cisco recommends that AppleTalk cable ranges follow some numerically significant schema, and more importantly, that administrators and designers document these numbers. Remember that the ranges cannot overlap and must remain unique within the network.

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