Allied Telesis AR450S Instrukcja Użytkownika Pobierz pdf (Strona 7)

Switching on the AR440S, AR441S and AR450S 3-7

Software Release 2.6.6

C613-03079-00 REV A

When the router is first powered up, a “default” VLAN is created and all ports

are added to this VLAN. In this initial unconfigured state, the router broadcasts

all packets it receives to the default VLAN. This VLAN has a VID of 1 and an

interface name of vlan1. This VLAN cannot be deleted, and ports can only be

removed from it when they also belong to at least one other VLAN. If all

devices on a physical LAN are to belong to the same logical LAN; that is, in the

same broadcast domain, then the default settings are acceptable and no

additional VLAN configuration is necessary.

VLAN Tagging

An Ethernet packet can contain a VLAN tag, with fields that specify VLAN

membership and user priority. The VLAN tag is described in IEEE 802.3ac, and

is four octets that can be inserted between the Source Address and the Type/

Length fields in the Ethernet packet. To accommodate the tag, IEEE 802.3ac

also increased the maximum allowable length for an Ethernet frame to 1522

octets (the minimum size is 64 octets). IEEE 802.1q specifies how the data in the

VLAN tag is used to switch frames. VLAN-aware devices are able to add the

VLAN tag to the packet header. VLAN-unaware devices cannot set or read the

VLAN tag.

The format of VLAN data in an Ethernet frame is shown in Figure 3-1 on

page 3-7. Twelve bits of the tag are the VLAN Identifier (VID), which indicate

the VLAN to which the packet belongs.

Figure 3-1: Format of user priority and VLAN data in an Ethernet frame.

The meaning and use of the fields in the Ethernet frame are listed in Table 3-2

on page 3-7.

Table 3-2: Fields in the Ethernet frame for QoS and VLAN switching.

Field Length Meaning and use

TPID 2 octets The Tag Protocol Identifier (TPID) is defined by IEEE Standard

802.1q as 0x81-00.

User Priority 3 bits The User Priority field is the priority tag for the frame, which

can be used by the router to determine the Quality of

Service to apply to the frame. The three bit binary number

represents eight priority levels, 0 to 7.

TPID

16 bits

1 bit

3 bits

12 bits

User

Priority

CFI VID

0x81-00

Preamble

Destination

Address

Source

Address

Type/

Length

Frame Data

CRC

64 bits

48 bits 48 bits

bits

368-12000

32 bits

SWITCH6

1 2 3 4 5 6 7 8 9 10 11 12 ... 47 48

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