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The Existing IP and Non-IP Address Structure: Preparing for Remodeling


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The Existing IP and Non-IP Address Structure: Preparing for Remodeling



Your Multiprotocol Environment



Legacy IP



Microsoft Networking



IBM Networking



Novell IPX Networking



AppleTalk Networking



Digital Equipment Corporation Protocols for Networking



OSI and ATM Network Addressing



X.25 Networking



Taking Inventory of Your Environment



Looking Ahead

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Designing Addressing Architectures for Routing and Switching

From: Designing Addressing Architectures for Routing and Switching
Author: Howard Berkowitz
Publisher: MTP
More Information

8. The Existing IP and Non-IP Address Structure: Preparing for Remodeling

Idealism is fine, but as it approaches reality the cost becomes prohibitive.

—William F. Buckley, Jr.

The metaphor of the melting pot is unfortunate and misleading. A more accurate analogy would be the salad bowl, for, though the salad is an entity, the lettuce can still be distinguished from the chicory; the tomatoes from the cabbage.

—Carl M. Degler, Out of Our Past: The Forces That Shaped Modern America

Stand ye in the ways, and see and ask for the old paths, where is the good way, and walk therein.

—Jeremiah 6:16

Before the backbone can be designed, you need to know what is at the edges. You then need to consider how you will carry non-IP protocols, or poorly numbered IP traffic, across the backbone.

You might have an existing backbone, or perhaps you are first building one to link growing edge sites. Possibly, you might have multiple backbones for different protocol families, especially if you run IBM networking as well as IP.

Our emphasis in this chapter is on the legacy physical networks, as well as the current IP addressing structure.

Support costs are a major part of overall network cost, and legacy mechanisms tend to be among the most expensive to support. Principal among these support costs is that of network design and configuration control. Chapter 6, “Internet Failing: Details at 11,” introduces the need for conscious addressing design and management, which will be detailed further in chapters on network management and configuration control.

Your Multiprotocol Environment

A basic assumption in this book is that backbones principally use IP routing, possibly supplemented with some direct Layer 2 connectivity with Frame Relay or ATM. Nevertheless, there are perfectly valid reasons to have other routed protocols in an enterprise backbone.

Most of the sections of this chapter provide guidance on addressing considerations in different protocol families. They will prepare you to take an inventory of your present environment. You'll find worksheets for that inventory at the end of the chapter. Some protocol families, such as AppleTalk, DEC's Local Area Transport (LAT), default NetBIOS networking on Microsoft systems, and older Novell systems, simply do not scale well to the wide area. IBM networking expects to control its own network resources, but it can be far more economical to have only one set of network resources over which all protocols run. These can be made to work on an IP backbone with various forms of tunneling. Application gateways might be a better choice.

When part of an enterprise network, the key problem in NetBIOS, Novell Internetwork Packet Exchange (IPX), and AppleTalk networking is to make sure that endpoints can find one another. In these protocol families, hosts and clients search for one another by name. The result of this search is an address. In most cases, these are not IP addresses, so you need to ensure that you implement ways to map these addresses to IP addresses and provide mechanisms for the non-IP protocols to reach the IP tunneling or other mechanism.

You need to decide whether you will use ships in the night (SIN) routing for your non-IP protocols or tunnel these in IP and rely on IP routing for path determination. In SIN routing, more than one protocol family's native routing protocols coexist in the same network, but do not interact with one another.

Good examples of pure routing protocols that might run separately are the protocols for IP and for DECnet Phase IV. There also are integrated routing protocols, such as IS-IS that handles IP, OSI, and DECnet Phase V. Cisco's EIGRP handles IP, AppleTalk, and Novell.

Another range of alternatives is to use Layer 2 connectivity for non-IP protocols, such as Frame Relay with RFC 1490 encapsulation or ATM with MPOA encapsulation, discussed in Chapter 4, “Transmission System Identifiers and Logical Address Mapping: A View from the Bottom.”

LAN Protocol Multicast Resource Discovery

The major protocol families, called desktop, workgroup, or LAN protocols, historically have had much less of an idea of specialized name resolution services, such as DNS, than the IP protocol family. Their basic mechanisms have involved some type of broadcast or multicast among client and server endpoints. In some cases, routers are involved in this process of name resolution, but they mimic endpoints when doing so.

As the workgroups grow, and, more importantly, interconnect to other workgroups in a large enterprise, there have been distinct trends to add directory services to the protocol families. Directory functions can be either in native directory services or in migrating application protocols to an IP world where they can use DNS.

Static and Proxy Resource Location Services

Native, broadcast-based resource discovery protocols assume that the client and server interact directly. Routers can pass along information, but they are viewed more as passive conduits.


In routing, a black hole is a destination that appears in routing tables, but, when traffic is sent to that destination, the traffic is discarded. Black holes can be configured deliberately. Although a detailed discussion of the use of black holes in routing is beyond the scope of this book, intelligent use of them can increase network stability.

The principle here is that certain packets cannot be delivered and sending them causes a destination unreachable message to be generated for every one of these packets. Given that this message is generated by some router—a distant one if routing information about unreachability is not propagated and a local router if such information is propagated—there is a design tradeoff between the overhead of tracking detailed reachability information and the overhead of passing some traffic that could be discarded earlier if some unreachability information were known.

In large networks, it is usually better to avoid the detailed tracking overhead, which avoids the need for constant recomputation of routing tables.

Because the broadcast-based mechanisms tend to produce a lot of overhead, various strategies for reducing overhead have been developed. These tend to be alternatives to lower-overhead directory services, but have the advantage that they can be implemented in routers without disturbing a large number of hosts. The following are some of these strategies:

  • Filtering, to prevent queries or announcements from going where there will be no response or other interest.

  • Static definition, where a local router has a fixed definition of the location of the endpoint and will respond as if it were the endpoint. The router does not know if the endpoint is actually reachable and becomes a black hole if the endpoint is not reachable. Such definition can require significant configuration setup and maintenance.

  • Proxy resource resolution, where the router pays attention to the content of broadcast or multicast requests and saves the response. When subsequent queries are made, the proxy server answers them from a local cache. Again, the proxy might not have the most recent information, so black holing is a possibility. A compromise is to put a lifetime on each cache entry and periodically resolve the cached resource entry when this timer expires.

How helpful filtering can be to you depends on the structure of your communities of interest. If there are many communities of interest, and services that are only used within the community, filtering can be extremely helpful. If you have large numbers of servers that need to be seen everywhere, you cannot filter out their existence. In such cases, you want to look at more efficient resource location protocols or static/proxy mechanisms.

Black hole behavior by static or proxy responders might not be as important an operational problem as it first seems. Think about the operational realities. If a user cannot reach a resource, he calls the help desk to complain. The user behaves in the same way if the resource is actually down, if the service mechanism is pointing at it incorrectly (for example, it has moved and static definitions have not been updated), or if a proxy points at a correct location but connectivity to that location has been lost.

To diagnose a user report of this sort, the normal procedure is to test connectivity to the user. If the user is unreachable, then there is a network problem that needs to be fixed. If the server is unreachable, again there is a network problem. If the server is reachable but the service is down, a host problem needs to be solved. Independent network management tools might already have been monitoring the server, and the help desk might already know about the problem.

So, if a proxy or static definition causes user traffic to be black holed rather than getting an immediate unreachable message on the workstation, does that really slow the process of troubleshooting? If it does not, then proxy or static definition are valuable operational tools.


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