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Chapter 3

VLANs

The VLAN Tag Protocol Identifier (TPID) is globally assigned and uses an

EtherType field value of 0x81-00. The Tag Control Information (TCI) is a
16-bit value and has three fields contained within:

User Priority A 3-bit field used to assign up to eight layers of priority.
The highest priority is 0, and 7 is the lowest (specified in 802.1q).

Canonical Format Indicator (CFI) A 1-bit field that is always a 0 if run-
ning an 802.3 frame. This field was originally designed to be used for
Token Ring VLANs, but it was never implemented except for some pro-
prietary Token Ring LANs.

VLAN ID (VID) The actual VLAN number the frame is assigned upon
entering the switch (12 bits). The reserved VLAN IDs are as follows:

0x0-00 Null, or no VLAN ID, which is used when only priority
information is sent

0x0-01 Default VLAN value of all switches

0x-F-FF Reserved

Because Ethernet frames cannot exceed 1,518 bytes and ISL and 802.1q

frames can be up to 1,522 bytes, the switch may record the frame as a baby
giant frame.

Trunking

runk links are point-to-point, 100 or 1000Mbps links between two

switches, between a switch and a router, or between a switch and a server.
Trunked links carry the traffic of multiple VLANs, from 1 to 1,005 at a time.
You cannot run trunked links on 10Mbps links.

Cisco switches use the Dynamic Trunking Protocol (DTP) to manage

trunk negation in the Catalyst switch engine software release 4.2 or later,
using either ISL or 802.1q. DTP is a point-to-point protocol and was created
to send trunk information across 802.1q trunks. Dynamic ISL (DISL) was
used to support trunk negation on ISL links only before DTP was released in
software release 4.1, and before DISL, auto-negotiation of trunk links was
not allowed.

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