The All-New Switch Book: The Complete Guide to LAN Switching Technology, Second Edition

Appendix A. APPENDIX Protocol Parsing

This appendix provides a decision tree that allows a device to determine from an incoming frame which protocol is in use, regardless of the encapsulation method used. This is particularly useful as part of a classification engine for implementing protocol-based VLANs, as discussed in Chapter 11, "Virtual LANs: Applications and Concepts." The decision tree is presented with respect to Ethernet networks only, because:

• Ethernet-based switches are by far the most common.

• Only Ethernet supports a native Type field. It is primarily because frames may be encapsulated in native Type format or LLC/SNAP formats that the protocol parsing task becomes complex. A Token Ring or FDDI device needs to implement only the subset of the decision tree presented here that deals with LLC and SNAP encapsulation.

The decision tree supports parsing of IP, IPX, DECnet, Local Area Transport (LAT), NetBIOS, and AppleTalk. Where appropriate, related protocols within each family (for example, AEP along with IP, DEC MOP with DECnet) have been included.