IEEE 802.1Q

The standard IEEE 802.1Q provides a mechanism of encapsulation very widespread and established in many equipment of different marks, as well as in the Commutateurs virtual network of VMWARE ESX 3. This standard succeeds the encapsulation ISL owner CISCO. the heading of screen is supplemented by a beacon of 4 bytes.

The standard IEEE 802.1Q defines the contents of the beacon of VLAN (VLAN tag) with which one supplements the heading of screen Ethernet. The format of the Ethernet screen modified with the 4 additional bytes is presented below.

Contents of the Ethernet screens

Weave Ethernet

Weave Ethernet modified

Contents of the field " Tag"

It should be noted that field FCS is recomputed after the insertion of the beacon of VLAN.

Tag protocol to identify, TPID, EtherType

The 16 first Bit S are used to identify the protocol of the inserted beacon. In the case of the beacon 802.1Q the value of this field is fixed at 0x8100.

Priority

This field of 3 Bit S refers to the standard IEEE 802.1p. On 3 bits one can code 8 levels of priorities from 0 to 7. The concept of priority in VLANs is without relationship with the mechanisms of priority IP. These 8 levels are used to relative fix a priority at the screens of a VLAN in other VLANs.

Canonical Format To identify, CFI

This field coded on 1 bit ensures compatibility between the addresses MAC Ethernet and Token-ring. A Commutateur Ethernet will always fix this value at 0. If a port Ethernet receives a value 1 for this field, then the screen will not be propagated since it is intended for a port “without beacon” (untagged port).

VLAN Id, VID

This field of 12 bits is used to identify the virtual lan (VLAN) to which belongs the screen. It is possible to code 4096 VLANs with this field.

Specificities

Activation on Cisco

In the switch Cisco, here the order allowing to use 802.1Q in the place of final ISL in configuration of interface to be parameterized.

switchport trunk encapsulation dot1q

Activation on Linux

From versions 2.6 and higher of the kernel, the taggés VLAN 802.1q are supported. The order allowing their configurations is

vconfig

Traffic 802.1Q analyzes

The software Tcpdump or Wireshark makes it possible to analyze traffic VLAN; example:

/usr/sbin/tcpdump - v - I eth0 vlan

Appendices

Internal bonds

IEEE 802.1

External bonds

Cisco, Configuring ISL and 802.1q Trunking Between has CatOS Switch and year external Router (InterVLAN Routing)
linux-France, inetdoc.LINUX

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