Local-Area Networks and LAN Switching - CCIE Routing and Switching Exam Certification Guide - page 154

IEEE 802.1q Frame Format
As shown in Figure 4-32, IEEE 802.1q inserts a 4-byte tag after the Source Address field
(before the Type/Length field) of Ethernet frames.

IEEE 802.1q Frame Format Figure 4-32

16 bits 3 bits 1 bit 12 bits

IEEE 802.1q tag TPID priority CFI VID

Ethernet v2
or IEEE frame

Type/
Length

DA SA tag data FCS

The tag fields are described in Table 4-11.

IEEE 802.1q Tag Fields Table 4-11

IEEE 802.1q Field Description

TPID 16-bit Tag Protocol Identifier, which indicates that an 802.1q tag follows.

Priority 3-bit IEEE 802.1p priority, which provides 8 levels of prioritization.

CFI Canonical Format Indicator, which indicates whether the MAC addresses are in

canonical (0) or noncanonical (1) format.

VID 12-bit VLAN identifier that allows 4096 unique VLAN values; VLAN numbers 0,

1, and 4095 are reserved.

IEEE 802.1q Configuration
On router interfaces, subinterfaces are created for each VLAN. On each subinterface, the
encapsulation is set to IEEE 802.1q to enable trunking to the switches. As shown in Example
4-15, each subinterface is configured with a different VLAN number with the encapsulation
dot1q vlan-number command.

Example 4-15 IEEE 802.1q Configuration Example

interface fastethernet 1/1.1
encapsulation dot1q 10
ip address 192.168.1.1 255.255.255.0
!
interface fastethernet 1/1.2
encapsulation dot1q 20
ip address 192.168.2.1 255.255.255.0