VLAN Tagging Concept

VLAN Tagging Concept:

VLAN trunking creates one link between switches that supports as many VLANs. switches treat the link as if it were a part of all the VLANs.At
the same time, the trunk keeps the VLAN traffic separate, so frames in VLAN 10 would not
go to devices in VLAN 20, and vice versa, because each frame is identified by VLAN number
as it crosses the trunk.

The use of trunking allows switches to forward frames from multiple VLANs over a single
physical connection by adding a small header to the Ethernet frame.To flood the frame,
switch SW1 needs to forward the broadcast frame to switch SW2. However, SW1 needs to
let SW2 know that the frame is part of VLAN 10, so that after the frame is received, SW2
will flood the frame only into VLAN 10, and not into VLAN 20. So, as shown at Step 2,
before sending the frame, SW1 adds a VLAN header to the original Ethernet frame, with the
VLAN header listing a VLAN ID of 10 in this case.

When SW2 receives the frame, it understands that the frame is in VLAN 10. SW2 then removes
the VLAN header, forwarding the original frame out its interfaces in VLAN 10

The 802.1Q and ISL VLAN Trunking Protocols 

Cisco has supported two different trunking protocols
Inter-Switch Link (ISL)
 IEEE 802.1Q.

Inter-Switch link is cisco proprietary and IEEE 802.1Q is standard but Today IEEE 802.1Q more popular trunking protocol, with Cisco not even bothering to support ISL in many of its switch models
today.
While both ISL and 802.1Q tag each frame with the VLAN ID, the details differ. 802.1Q insert an extra 4 byte 802.1Q VLAN header into the original frame's ethernet header. as for the field in 802.1Q header , only 12 bit VLAN ID field inside  the 802.1Q header matters for topics discussed in this book. This 12-bit field supports a theoretical maximum of 212 (4096) VLANs but in practice it supports a maximum of 4094. (Both 802.1Q and ISL use 12 bits to tag the  VLAN ID, with two reserved values [0 and 4095].)
Cisco switches break the range of VLAN IDs (1–4094) into two ranges: the normal range and
the extended range. All switches can use normal-range VLANs with values from 1 to 1005.
Only some switches can use extended-range VLANs with VLAN IDs from 1006 to 4094.
The rules for which switches can use extended-range VLANs depend on the configuration
of the VLAN Trunking Protocol (VTP).

802.1Q also defines one special VLAN ID on each trunk as the native VLAN (defaulting to use
VLAN 1). By definition, 802.1Q simply does not add an 802.1Q header to frames in the native
VLAN. When the switch on the other side of the trunk receives a frame that does not have an
802.1Q header, the receiving switch knows that the frame is part of the native VLAN. Note that
because of this behavior, both switches must agree on which VLAN is the native VLAN.

The 802.1Q native VLAN provides some interesting functions, mainly to support connections

to devices that do not understand trunking. For example, a Cisco switch could be

cabled to a switch that does not understand 802.1Q trunking. The Cisco switch could send

frames in the native VLAN—meaning that the frame has no trunking header—so that the

other switch would understand the frame. The native VLAN concept gives switches the

capability of at least passing traffic in one VLAN (the native VLAN), which can allow some

basic functions, like reachability to telnet into a switch.