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The IEEE 802.1q standard provides a low-overhead method for tagging frames. Since it is an open standard, most designers select 802.1q when using non-Cisco equipment or to avoid committing to a single vendor. The 802.1q specification adds four octets of header to each frame. This header identifies the frame’s VLAN membership, but it does not include a CRC checksum for validation of the header. This is not a significant issue in most reliable networks. The reduced header, compared to ISL, and lack of CRC greatly diminishes the overhead associated with this trunking technology.

Both ISL and 802.1q may cause incorrectly configured network devices to report giants (oversized frames). These “giant” frames are beyond the specified number of octets, as per the Ethernet standard. It is important to understand that both the ISL and 802.1q specifications increase the maximum number of bytes allowed—in contrast to traditional Ethernet.


FDDI may be used as a trunking medium in VLAN networks by incorporating the 802.10 protocol, which was originally developed to provide Layer 2 security. However, the use of the Security Association Identifier, or SAID, permits assignment of a VLAN ID. SAID provides for 4.29 billion VLANs.

The 802.10 encapsulation consists of a MAC header followed by a clear header. The clear header is not encrypted and consists of the 802.10 LSAP, or Link State Access Protocol (LSAPs are defined by the IEEE and occupy the LLC portion of the frame, comprising the destination service access point, source service access point, and control byte), the SAID, and an optional Management Defined Field, or MDF. The standard provides for a protected header to follow the MDF, with data and a checksum, referred to as the Integrity Check Value, or ICV. In VLAN trunking, only the IEEE 802.10 LSAP and the SAID value are used before the data block.

To configure 802.10, the administrator must define the relationship between the FDDI VLAN and the Ethernet VLAN. The first VLAN, or default VLAN, is defined automatically.

It is important to note that 802.10 VLAN packets are valid MAC frames and may cross non-802.10 devices within the network. Also, VLAN IDs and SAID values are independent of each other—except when related in the switch table.


LAN Emulation (LANE) will be described in greater detail later in this chapter. For the moment, note that LANE is also used as a trunking technology. LANE is often introduced as the first-phase migration step to ATM in the network.

Network Design and Problem Solving

As discussed in Chapter 1, most network design projects are conceived to address one or more problems within an existing network. Consider the list of network problems and the corresponding tools noted in Table 2.3.

TABLE 2.3 Network Design Solutions

Issue Possible Solutions

Contention for the media Migrating from shared to switched media is the best solution to this problem. However, it may be necessary to segment the network with routers to reduce the number of nodes per broadcast domain.
Excessive broadcasts Network broadcast control is the responsibility of the router. The only other solution would be to reduce the number of broadcasts at the source.
Protocol issues Typically, protocols on the network are defined by the application, although designers may use tunneling and encapsulation to maintain single– protocol segments. This solution is especially applicable in WAN designs.
Addressing issues Given the logical structuring role of the address, addressing issues must include the involvement of a routing device.

Network Design in the Real World: Design Solutions

Most designers find that their solutions are the result of reactive efforts and not proactive ones. This is the nature of the beast in most large, fast-paced corporations.

Therefore, it is imperative that the designer continue to hone skills related to troubleshooting. In the largest organizations, staff in other departments may be responsible for actually connecting the protocol analyzer to the segment or generating the remote monitoring (RMON) reports, but the designer and architect will need to know what information to ask for. This arrangement can make the process more difficult—many troubleshooting efforts on very complex problems are actually solved by “That doesn’t look right” observations.

One of the best ways to avoid this situation is to generate reports that a lay person can understand. A number of products are available—my favorite is Concord Network Health, although there are others, including Cisco’s RMON tools. The designer can post the resulting reports on a Web site so that users can see the status of the network whenever they wish.

A fear that non-network designers will start to second-guess every issue in the reports is natural, and it will happen from some people. However, the reports can also provide the needed visibility to upper management to justify funding and resources. Most networks hide the problems, so they never get fixed. If you need to be convinced that disclosure is a positive step, take a look at Cisco’s Web site, The vast majority of bugs in Cisco’s software are documented and disclosed publicly. Granted, such problems can be embarrassing to the company, but the result over the past few years has been an incredible increase in market share and a vast improvement in the overall product line. Improved service should be the goal of every IT department.

Physical Topologies

The physical layout of the network is sometimes dissimilar to the logical and simple layout suggested by the hierarchical model. Consideration must be given to access, cabling, distances, shielding, and space.

Most installations use two distinct components for the intra-building configuration. These are defined as horizontal and vertical systems.

Vertical systems are typically backbone services and move up through the building. These services are usually run on fiber media, which is capable of greater bandwidth and is less susceptible to electromagnetic interference.

Horizontal systems are almost always copper, but this trend is changing as more desktops are wired for fiber. These installations usually start at a wiring closet and are fed under the floor or in plenum (ceiling). The wiring closet will typically contain a switch or hub that links the vertical connections.

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