Avaya IP Telephony User Manual download pdf (Page 21)

KW Avaya IP Telephony Implementation Guide

CoS mechanisms, they are inherently used to identify specific traffic and can be used much like CoS

markings. Other non-CoS methods to identify specific traffic are to key in on source and destination IP

addresses and specific protocols (ie, RTP).

802.1p/Q

The figure below shows the IEEE 802.1Q tag and its insertion point in the Ethernet and 802.3 frames.

Note that in both cases the 802.1Q tag changes the size and format of the comprehensive Ethernet and

802.3 frames. Because of this, many intelligent switches (ones that examine the L2 header and perform

some kind of check against the L2 frame) must be explicitly configured to accept 802.1Q tagged frames.

Otherwise, these switches may reject the tagged frames.

The Tag Protocol Identifier (TPID) field has hex

value x8100 (802.1QTagType). This value alerts the switch or host that this is a tagged frame. If the

switch or host does not understand 802.1Q tagging, the TPID field is mistaken for the Type or Length

field, which results in an erroneous condition.

TPID

(2 octets)

Priority

(3 bits)

VLAN ID

(12 bits)

CFI

TCI

(2 octets)

802.1p/Q

Dest Addr

(6 octets)

Src Addr

(6 octets)

DIX Ethernet v2 frame

Type

(2)

Data

(46 to 1500 octets)

Frm Chk

Seq

(4 octets)

802.3 "Ethernet" frame

Dest Addr

(6 octets)

Src Addr

(6 octets)

Len

(2)

Data

(38 to 1492 octets)

Frm Chk

Seq

(4 octets)

DSAP

SSAP

Control

Prtcl ID or

Org Code

Ethertype

802.3 header

802.2

LLC

802.2

SNAP

8 octets total

Figure 13: 802.1Q tag

The two other fields of importance are the Priority and Vlan ID (VID) fields. The Priority field is the “p”

in 802.1p/Q and ranges in value from 0 to 7. (“802.1p/Q” is a common term used to indicate that the

Priority field in the 802.1Q tag has significance. Prior to real-time applications 802.1Q was used

primarily for VLAN trunking, and the Priority field was not important.) The VID field is used as it

always has been – to indicate the VLAN to which the Ethernet frame belongs.

Rules for 802.1p/Q Tagging

There are two questions that determine when and how to tag:

1. Is tagging required to place the frame on a specific VLAN (VLAN tagging)?

2. Is tagging required to give the frame a priority level greater than 0 (priority tagging)?

Based on the answers to these questions, tagging should be enabled following these two rules.

1. Single-VLAN Ethernet switch port (default scenario).

- On a single-VLAN port there is no need to tag to specify a VLAN, because there is only one

VLAN.

- For priority tagging only, the IEEE 802.1Q standard specifies the use of VID 0. VID 0 means

that the frame belongs on the port’s primary VLAN, which IEEE calls the “port VLAN,” and

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Avaya IP Telephony User Manual Page 21

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