Handbook of Local Area Networks, 1998 Edition:LAN Interconnectivity Basics Click Here! Search the site:   ITLibrary ITKnowledge EXPERT SEARCH Programming Languages Databases Security Web Services Network Services Middleware Components Operating Systems User Interfaces Groupware & Collaboration Content Management Productivity Applications Hardware Fun & Games EarthWeb sites Crossnodes Datamation Developer.com DICE EarthWeb.com EarthWeb Direct ERP Hub Gamelan GoCertify.com HTMLGoodies Intranet Journal IT Knowledge IT Library JavaGoodies JARS JavaScripts.com open source IT RoadCoders Y2K Info Previous Table of Contents Next Protocol-Sensitive VLANs Protocol-sensitive VLANs are one of the most powerful types of VLANs. Not all switch vendors provide the capability to create this type of VLAN, so careful research must be performed to ensure switch features can be delivered. Switches that provide protocol-sensitive VLANs read and interpret the "Protocol Type" field in the MAC frame header. Packets can then be categorized into a particular VLAN based upon the value of this field. This feature allows network designers to overlay multiprotocol networks in a manner that makes sense for each protocol (see Exhibit 3-7-11). Exhibit 3-7-11.  Protocol Sensitive VLANs For example, a workgroup containing 300 (IP, IPX, and DecNet) users can be divided into 5 IP subnets, 1 IPX network and 2 DecNet networks. The broadcast domain sizes can be set for each protocol according to the applications being used, the functional/security necessities of each workgroup, and the percentage of broadcast traffic generated by each protocol (see Exhibit 3-7-12). Exhibit 3-7-12.  Protocol Sensitive VLANs — Layer 3 Switch Without protocol-sensitive VLANS, network designers are constrained by protocol limitations. In the example discussed previously, IP and DecNet parameters would have to be changed to sub-optimal values to accommodate the single large IPX (network) broadcast domain. Protocol-sensitive VLANs also allow designers to limit the number of router hops for each protocol as needed. Broadcast Domain Extension Another powerful VLAN application is the geographical extension of non-traditional broadcast domains. Large campus environments may require broadcast domains to span multiple switches and, possibly, multiple buildings. For environments where protocols such as IP are dominant and dynamic addressing services such as DHCP are not yet implemented, VLANs provide the best way to achieve continuity (see Exhibit 3-7-13). Exhibit 3-7-13.  VLANs — Broadcast Domain Extension For example, a university maintains a satellite computer science lab in the dormitories and a primary lab in the academic building. For security reasons, university administrators prefer to isolate computer science students from the general campus network. Implementing VLANs allows the computer science department to extend the same IP subnet from the academic building to the satellite lab using high speed switches rather than slower and more expensive router ports. Without VLANs, the university's network designers would have few choices. One alternative, to create a new subnet for the satellite lab which would require using an additional router port (expensive) and would impose an additional router hop (speed bump) on users in the satellite lab. Risk Factors — VLANs The majority of risks associated with VLANs have to do with over-implementing them. Designers who create too many VLANs — trying to categorize and separate every type of network user experience problems in scalability and complexity. Redundancy As broadcast domains, VLANs are, by definition, Layer 2 entities. Creating fault tolerant topologies within VLANs therefore requires the use of Spanning Tree Protocol. Each VLAN requires a distinct spanning tree. So environments that have multiple overlapping VLANs require the switches to support multiple spanning trees. For many switches, the computation associated with maintaining multiple trees is too great to provide adequate performance levels — particularly in the event of a topology change. Key Design Points With the advent of high speed switches that perform routing functions at wire speed, the necessity for VLANs is greatly reduced. Prior to Layer 3 switches, network designers attempted to reduce the number of router hops each packet had to traverse to reach its most common destinations. New Layer 3 switches, however, do not impose a performance penalty for crossing subnet boundaries, so routing can be deployed wherever needed. The key point is that networks can now be designed according to the requirements imposed by protocols and applications rather than the limitations of devices that operate strictly at Layer 2 or Layer 3. When considering VLANS, investigate the protocol mixture. Novell (IPX) environments can typically tolerate considerable larger broadcast domains than IP-based users and applications. Protocol sensitive VLANs simplify overlapping broadcast domains for different protocols. Also investigate user distribution and redundancy requirements. In many cases solutions more elegant than creating additional VLANs are available. SUMMARY Although demands on network managers are greater today than ever before, and network complexity is at a peak, network designers have more choices at hand for developing scaleable, high performance solutions. New classes of equipment allows network managers to satisfy requirements without renumbering many workstations over a weekend. New networks can be built around real-life functional requirements rather than protocol and equipment constraints. The latest wire-speed Layer 3 switches allow network designers to implement both routing and switching functions — wherever needed — without sacrificing network performance, manageability, or scalability. Previous Table of Contents Next Use of this site is subject certain Terms & Conditions. Copyright (c) 1996-1999 EarthWeb, Inc.. All rights reserved. Reproduction in whole or in part in any form or medium without express written permission of EarthWeb is prohibited. Please read our privacy policy for details.