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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.
With ISL, an Ethernet frame is encapsulated with a header that transports VLAN IDs between switches and routers. With IEEE 802.1Q the tag is internal. This is a key advantage for IEEE 802.1Q as it means tagged frames can be sent over standard Ethernet links. ISL does add overhead to the frame as a 26-byte header containing a 10-bit VLAN ID.
Prior to the introduction of the 802.1Q standard, several proprietary protocols existed, such as Cisco Inter-Switch Link (ISL) and 3Com's Virtual LAN Trunk (VLT). Cisco also implemented VLANs over FDDI by carrying VLAN information in an IEEE 802.10 frame header, contrary to the purpose of the IEEE 802.10 standard. Both ISL and IEEE 802.1Q ...
Cases where this setup is used can be found in servers dedicated for prints, files or for segmenting different departments. An example of router on a stick usage is found in Call Manager Express installation, when the Voice over IP network and Cisco IP phone devices have a need to split. [3]
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.
To do this, VTP carries VLAN information to all the switches in a VTP domain. VTP advertisements can be sent over 802.1Q, and ISL trunks. VTP is available on most of the Cisco Catalyst Family products. Using VTP, each Catalyst Family Switch advertises the following on its trunk ports: Management domain; Configuration revision number
The Dynamic Trunking Protocol (DTP) is a proprietary link layer protocol developed by Cisco Systems for the purpose of negotiating trunking on a link between two VLAN-aware switches, and for negotiating the type of trunking encapsulation to be used. VLAN trunks formed using DTP may utilize either IEEE 802.1Q or Cisco ISL trunking protocols. [1]
This encapsulation is defined in the IEEE 802.3ac specification and increases the maximum frame by 4 octets. The IEEE 802.1Q tag, if present, is placed between the Source Address and the EtherType or Length fields. The first two octets of the tag are the Tag Protocol Identifier (TPID) value of 0x8100.