03 lan vlan

LAN and VLAN: some considerations

FT42823EN02GLA0 © 2010 Nokia Siemens Networks

1

Contents

1 LAN and VLAN 3 1.1 Definitions 4 1.2 Domains in a traditional LAN 5 1.3 Domains in a VLAN 9 1.4 Traffic separation by VLAN 12 1.5 Tagging 13 1.6 Scheduling algorithms 20 1.7 VLAN Aware / Unaware 24 1.8 Links Types 25



FT42823EN02GLA0

© 2010 Nokia Siemens Networks 2



3

1 LAN and VLAN


FT42823EN02GLA0


1.1 Definitions

A LAN or Local Area Network is a computer network (or data communications network) which is confined in a limited geographical location.

A Virtual (or logical) LAN is a local area network with a definition that maps workstations/PCs on some other basis than geographic location (for example, by department, type of user or primary application)



5

1.2 Domains in a traditional LAN

In a traditional Ethernet LAN, stations connected to the same media, share a domain. In this domain, every station hears broadcast frames transmitted by every other station.

As the number of stations grows, contention and broadcast traffic increase a lot.

At some point, the Ethernet becomes saturated.

To operate efficiently, the LAN must be divided into smaller pieces.

In a traditional LAN, stations are connected to each other by means of HUBS or REPEATERS.

HUB HUB

One collision Domain

One Broadcast Domain

Fig. 1 Domains in a traditional LAN (1)


FT42823EN02GLA0


A BRIDGE (or a L2 SWITCH) is able to divide one collision domain in different collision domains.




7

A BRIDGE (or a L2 SWITCH) do not forward collisions, but allows broadcast and multicast passing through.

Broadcast domain refers to a part of network where a single broadcast packet is transmitted to all segments of the network (i.e. ARP request, NETBIOS name request).

This type of traffic, affects the whole network because each device receiving a broadcast frame must analyze it.

If broadcast frames increases in frequency, available bandwidth decrease up to be exhaust (BROADCAST STORM).

SWITCH = MULTIPORT BRIDGE

L2 SWITCH



FT42823EN02GLA0


A ROUTER may be used to prevent Broadcast and Multicast from traveling through the network because it is able to segment a LAN in different Broadcast domains.

HUB HUB

Two collision Domains

Two Broadcast Domain

ROUTER

Fig. 4 Domains in a traditional LAN



9

1.3 Domains in a VLAN

VLANs allow a network manager to logically segment a LAN into different broadcast domains without using routers.

Bridging software is used to define which workstations are to be included in the broadcast domain.

VLAN 2 Broadcast Damain


VLAN 1 Broadcast Domain


L2 SWITCH L2 SWITCH

Fig. 5 Domains in a VLAN (1)


FT42823EN02GLA0


ROUTERS are necessary only to make possible communication between different VLANs.

VLAN IS A LOGICALLY DEFINED BROADCAST DOMAIN.





L2 SWITCH L2 SWITCH

ROUTER




11

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

The advantages of VLANs as regards to traditional LANs are shown in Fig. 7.

Periodically, sensitive data may be broadcast on a network. Placing only those users who can have access to have access to that data on a VLAN can reduce the chances of an outsider gaining access to the data

SECURITY

Routers are only used to interconnect different broadcast domains

REDUCED COSTS

Simply moves, adds and changesSIMPLIFIED ADMINISTRATION

Independent from the physical wiringVIRTUAL WORKGROUPS

Better control of broadcastPERFORMANCE



FT42823EN02GLA0


1.4 Traffic separation by VLAN

With VLANs it is possible to separate different logical networks on one physical infrastructure supporting the traffic separation.

Figure Fig. 8 shows a Traffic Separation Example by VLAN.

RNC

Ethernet Network

Flexi BTS Nr.1

Flexi BTS Nr.2

VLAN1 -> Voice from Flexi BTS Nr.1 to RNC

Traffic over same physical port separated by VLAN.

VLAN2 -> Data from Flexi BTS Nr.1 to RNC

VLAN4 -> Data from Flexi BTS Nr.1 to RNC

VLAN3 -> Voice from Flexi BTS Nr.2 to RNC

Fig. 8 Traffic separation by VLAN



13

1.5 Tagging

Tagging is a process used to identify the VLAN originating.

The VLAN tagging scheme in 802.1q results in four bytes of information being added to the frame following the source address and preceding the type/length field.

This increases the maximum frame size in Ethernet to 1522 bytes.

Fig. 9 reports a IEEE 802.3 untagged frame

Fig. 10and Fig. 11 explain theTAG fields.

MAC DA6 bytes

Payload46-1500 bytes

FCS4 bytes

Basic IEEE 802.3 Ethernet Frame: minimum length 64 bytes, maximum length 1518 bytes

Destination & Source MAC Addresses:The Destination MAC Address field identifies the station or stations that are to receive the frame. The Source MAC Address identifies the station that originated the frame. A Destination Address may be a unicast destined for a single station, or a "multicast address" destined for a group of stations. A Destination Address of all 1 bits refers to all stations on the LAN and is called a "broadcast address".

Length/Type:If the value of this field is less than or equal to 1500, then the Length/Type field indicates the number of bytes in the Payload field. If the value of this field is greater than or equal to 1536, then the Length/Type field indicates protocol type.

Payload (MAC Client Data):This field contains the data transferred from the source station to the destination station or stations.

Frame Check Sequence:This field contains a 4-byte cyclical redundancy check (CRC) value used for error checking.

MAC SA6 bytes

Length/Type2 bytes

VLAN tags may be added here

Preamble+SD

8 bytes

InterframeGap

12 bytes

64-1518 bytes

Fig. 9 IEEE 802.3 Untagged Frame


FT42823EN02GLA0


CFI

16 bits

TAG Protocol Identifier TPID 0x8100

1bit 12 bits3bits

Priority VLAN ID

TCI Tag Control Identifier

TPID TAG Protocol Identifier

2 bytes2 bytes

4 bytes

IEEE 802.3 Frame without VLAN Tag Header

IEEE 802.3 with 802.1Q 4-Byte VLAN Tag Header

User priority CFI (Canonical format identifier)

VLAN ID <= 4094)

4 bytes are added in the Ethernet frame between the MAC Source Address and the Type-Field.

802.1Q – VLAN (single tagged)

MAC DA6 bytes


FCS4 bytes

MAC SA6 bytes

Length/Type2 bytes

Preamble+SD

8 bytes

InterframeGap

12 bytes


FCS4 bytes

Length/Type2 bytes

InterframeGap

12 bytes

MAC DA6 bytes

MAC SA6 bytes

Preamble+SD

8 bytesTPID TCI

Fig. 10 802.1Q Single Tagged Frame (1)



15

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Is used to uniquely identify the VLAN to which the frame belongs. There can be a maximum of 212 -1 VLANs. Zero is used to indicate no VLAN ID

Vlan IDentifier

Always 0 if Ethernet.It is used to make compatibility between Ethernet and Token Ring

Canonical Format Indicator

It allows priority information to be encoded in the frame. Eight levels of priority are allowed

user Priority

It Indicates that it will follow a 802.1q TAG and not the payload; the Default TPID value in IEEE

802.1Q, is 0x8100

Tag Protocol IDentifier

DESCRIPTIONTC FIELD

Fig. 11 802.1Q Single Tagged Frame (2)


FT42823EN02GLA0


1.5.1 Class of Service (CoS) IEEE 802.1p

The IEEE 802.1p provides a standard and interoperable way to set the priority bits in a frame’s header and to map these settings to TRAFFIC CLASSES.

There are 8 TRAFFIC CLASSES (3 Bits) according to the table reported in Fig. 12.

000BEBEST EFFORT

001BKBACKGROUND

010RRESERVRD FOR FUTURE USE

011EEEXCELLENT EFFORT TRAFFIC

100CLCONTROLLED LOAD TRAFFIC

101VIVIDEO TRAFFIC

110VOVOICE TRAFFIC

111NCNETWORK CONTROL TRAFFIC

Fig. 12 Quality Of Service IEEE 802.1p (1)

WARNING Of course, network operators may choose to implement traffic differentiation on a per VLAN-ID basis rather than using the three CoS bits.



17

The TRAFFIC CLASSES are assigned to separate queues with different priorities.

Traffic classes

queues

map to

outgoing

Priority bits

Fig. 13 Quality Of Service IEEE 802.1p (2)

If a switch provides 8 queues for the 8 priorities settings, each queue will store frames with a specific priority setting to provide complete differentiated services.

Fig. 14 Switch with 8 queues; each priority has one queue


FT42823EN02GLA0


To minimize costs, however, fewer queues may be provided in such switches. Frames from several priority settings may be stored together in one queue.

Fig. 15 Switch with less than 8 queues; more than one priority in one queue



19

When 4 queues are available, like in the FlexiPacket ODU, the 8 CoS codes could be associated to four priority values as reported in Fig. 16 (FlexiPacket ODU default).

37

26

25

24

13

12

01

00

Queue PriorityValue

CoS

Fig. 16 FlexiPacket ODU Priority Code Point Configuration

When 5 queues are available, like in FlexiPacket HUB 2200/1200, the 8 CoS codes could be associated to five priority values as reported in Fig. 16 (HUB 1200/2200 configuration).

47

36

25

24

13

12

01

00

Queue PriorityValue

CoS

Fig. 17 FlexiPacket HUB (2200/1200) Priority Code Point Configuration


FT42823EN02GLA0


1.6 Scheduling algorithms

By means of scheduling algorithms is possible to decide which frames forward first based on its priority and how to manage the shared available bandwidth in case of congestion.

Four strategies can be considered:

1) Without QoS management: FIFO (First In First Out) queuing (Fig. 18)

• Only one queue

• Frames are transmitted in the same order they arrive

In case of congestion:

• All frames experience queue delay irrespective of their class of service

• Frames may be discarded irrespective of their class of service

First In First Out

Fig. 18 FIFO Queuing



21

2) Strict priority queuing (Fig. 19)

• One queue for each class

• Queues are processed in descending order (highest to lowest).

• Queues assigned as high priority are serviced until they are empty.

Low priority queues potentially can be starved; in order to avoid it, high priority traffic should be kept small.

Strict Priority Queuing (SPQ)

• SPQ Uses multiple queues

• Allows prioritization

• Always empties higher priority queue before going to the next queue:– Empty Queue Q3– If Queue Q3 empty, then dispatch from Queue no. 2– If both Queue Q3 and Queue Q2 empty, then dispatch from Queue Q0…

1 3 6 6 7 7 7

Queues

Until Queue 3 is emptied

Direction of Data flow

7 7 7

6

3

Q3

Q2

Q1

1Q0

6

Queue Priority

Q3 7

Q2 6,5

Q1 3,4

Q0 1,2

Fig. 19 Strict Priority Queuing


FT42823EN02GLA0


3) Weighted Fair Queuing (Fig. 20)

• Each queue has a percentage of capacity.

• The weight is used to ensure that more important queues get serviced more often than other less important queues

• The choice of the weights also depends on the amount of traffic in each class

Weighted Fair Queuing (WFQ)

•The scheduler will empty all four queues simultaneously according to their weights.

•Queuing algorithm shares the bandwidth fairly among flows.

•Weights are introduced to give proportionately more bandwidth to flows with higher CoS

7 7

Queue Priority Weight BW

Q3 7 8 80 Mbps

Q2 5,6 4 40 Mbps

Q1 3,4 2 20 Mbps

Q0 1,2 1 10 Mbps

80 Mbps

40 Mbps

1 10 Mbps

3 20 Mbps

66WFQScheduler

7

66

Q3

Q2

1

3Q1

Q0

77 7

Fig. 20 Weighted Fair Queuing



23

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

4) Multi stage: Weighted Fair Queuing + Strict Priority (Fig. 21)

The last two strategies can be combined together.

SP High

W%

SP high

SP low

Z%

W%

SP Low

Z%

Fig. 21 Weighted Fair Queuing + Strict Priority


FT42823EN02GLA0


1.7 VLAN Aware / Unaware

VLAN AWARE

If the data is to go to a device that knows about VLAN implementation (VLAN Aware), the VLAN identifier is added to the data.

VLAN UNAWARE

If it is to go to a device that has no knowledge of VLAN implementation (VLAN Unaware), the BRIDGE sends the data without the VLAN identifier.

Fig. 22 VLAN Aware/Anaware



25

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1.8 Links Types

Devices on a VLAN can be connected in three ways based on whether the connected devices are VLAN Aware or VLAN Unaware as reported in Fig. 23, Fig. 24, Fig. 25 and Fig. 26.

Recall that a VLAN aware device is one which understands VLAN memberships (i.e. which users belong to a VLAN) and VLAN formats.

This is a combination of the previous two links. This is a link where both VLAN aware and VLAN Unaware devices are attached.A hybrid link can have both tagged and untagged frames, but all the frames for a specific VLAN must be either tagged or untagged.

Hybrid Link

An access link connects a VLAN Unaware device to the port of a VLAN Aware Bridge.Access Link

All the devices connected to a trunk link, including workstations, must be VLAN Aware.All frames on a trunk link must have a special header attached. These special frames are called TAGGED FRAMES.

Trunk Link

DESCRIPTIONLINK TYPE

Fig. 23 Link Types


FT42823EN02GLA0


L2-SwitchL2-Switch

Trunk Link

Trunk Link

VLAN-aware Workstation

VLAN-aware Bridge/L2-Switch


Fig. 24 Trunk Link



27

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

L2-Switch

Access Link

VLAN-unaware Device


Fig. 25 Access Link


FT42823EN02GLA0


VLAN yellow

VLAN gray

VLAN red

VLAN yellow

VLAN green

VLAN red

VLAN anawareStations

VLAN anawareSwitch

VLAN awareSwitches

and Stations

Server for allVLAN

configure VLAN-ID

per port

configure VLAN-ID perport

SiSi

S1

S2

S3

Server yellow

Server redServer gray

Server green

IEEE 802.1QVLAN-ID VLAN red

IEEE 802.1Q

Fig. 26 VLAN Scenario



29

1.8.1 Q-in-Q

In the VLAN tag field defined in IEEE 802.1Q, only 12 bits are used for VLAN IDs, so a device can support a maximum of 4,094 VLANs.

In actual applications, however, a large number of VLAN are required to isolate users, especially in metropolitan area networks, and 4,094 VLANs are far from satisfying such requirements.

The so called Q-in-Q (IEEE 802.1ad) feature enables the encapsulation of double VLAN tags within an Ethernet frame, with the inner VLAN tag being the customer network VLAN tag while the outer one being the VLAN tag assigned by the service provider to the customer.

In the backbone network of the service provider (the public network), frames are forwarded based on the outer VLAN tag only, while the customer network VLAN tag is shielded during data transmission.

The Q-in-Q feature enables a device to support up to 4,094 x 4,094 VLANs.

DA SA

DA SA

DA SA

LEN/Etype Data FCS

TPID TAG LEN/Etype Data FCS

TPID TAG LEN/Etype Data FCSTPID TAG

Untagged Ethernet Frame

Service Provider Tagging

Customer Tagging

6 6 2 446 to 1500

2 2

2 2

Bytes

Single Tagged Ethernet Frame

Double Tagged Ethernet Frame

Fig. 27 Untagged, Single Tagged and Double Tagged Ethernet Frames


FT42823EN02GLA0




31

Double Tag Example

S-VLAN 2

C-VLAN 2

A

DC

E

B

23

4

S-VLAN 2

C-VLAN 2

Swap outer with 4 and forward to D- port 2

221

Forwarding DecisionVLAN Outer Tag

VLAN Inner Tag

A-Port

S-VLAN 2

C-VLAN 2

x

1S-VLAN 4

C-VLAN 2

1

2

S= Service Provider

C= Customer

Fig. 28 Double TAG Example


FT42823EN02GLA0


1.8.1.1 Q in Q TPID

The QinQ frame contains the modified tag protocol identifier (TPID) value of VLAN Tags. By default, the VLAN tag uses the TPID field to identify the protocol type of the tag. The value of this field, as defined in IEEE 802.1Q, is 0x8100. The device determines whether a received frame carries a service provider VLAN tag or a customer VLAN tag by checking the corresponding TPID value. After receiving a frame, the device compares the configured TPID value with the value of the TPID field in the frame.

If the two match, the frame carries the corresponding VLAN tag. For example, if a frame carries VLAN tags with the TPID values of 0x88a8 and 0x8100, respectively, while the configured TPID value of the service provider VLAN tag is 0x88a8 and that of the VLAN tag for a customer network is 0x8200, the device considers that the frame carries only the service provider VLAN tag but not the customer VLAN tag.

In addition, the systems of different vendors might set the TPID of the outer VLAN tag of QinQ frames to different values.

For compatibility with these systems, you can modify the TPID value so that the QinQ frames, when sent to the public network, carry the TPID value identical to the value of a particular vendor to allow interoperability with the devices of that vendor.

The TPID in an Ethernet frame has the same position with the protocol type field in a frame without a VLAN tag.

03 lan vlan

Documents