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VLAN cross connect (CC or VLAN-XC) is a mechanism used to create Switched VLANs, VLAN CC uses IEEE 802.1ad frames where the S Tag is used as a Label as in MPLS. IEEE approves the use of such a mechanism in part 6.11 of IEEE 802.1ad-2005 .
IEEE 802.1Q, often referred to as Dot1q, is the networking standard that supports virtual local area networking (VLANs) on an IEEE 802.3 Ethernet network. The standard defines a system of VLAN tagging for Ethernet frames and the accompanying procedures to be used by bridges and switches in handling such frames.
Cisco Inter-Switch Link (ISL) is a Cisco proprietary link layer protocol that maintains VLAN information in Ethernet frames as traffic flows between switches and routers, or switches and switches. [1] ISL is Cisco's VLAN encapsulation protocol and is supported only on some Cisco equipment over the Fast and Gigabit Ethernet links.
The technique specified by the standard is known informally as stacked VLANs or QinQ. The original 802.1Q specification allows a single virtual local area network (VLAN) header to be inserted into an Ethernet frame. QinQ allows multiple VLAN tags to be inserted into a single frame, an essential capability for implementing metro Ethernet.
Max frame size extended to 1522 bytes (to allow "Q-tag"). The Q-tag includes 802.1Q VLAN information and 802.1p priority information. 802.3ad: 2000-03 Link aggregation for parallel links, since moved to IEEE 802.1AX: 802.3-2002 2002-01 (802.3ag) A revision of the base standard incorporating the three prior amendments and errata 802.3ae: 2002-06
EtherType is also used as the basis of 802.1Q VLAN tagging, encapsulating packets from VLANs for transmission multiplexed with other VLAN traffic over an Ethernet trunk. EtherType was first defined by the Ethernet II framing standard and later adapted for the IEEE 802.3 standard. EtherType values are assigned by the IEEE Registration Authority.
Some aggregation and core switches can classify traffic by two VLANs using IEEE 802.1ad VLAN stacking, so with such aggregation devices properly placed in the center of a network, end segments and rings of single tag devices can receive only the traffic that they need. When using MPLS, Ethernet VLANs have local meaning only (like Frame Relay PVC).
As VLANs are segmented, it reduces the amount of traffic flow through a connection. By separating VLANs, it provides enhanced network security. Network administrators have direct control over multiple broadcast domains. In the event of a malicious user attempting to access any switch port, they will have limited access to the network.