Background: Know: Recognize: 802.1q, Prerequisites - frames, Ethertype, IP address
Associating frames with VLANsEdit
A virtual local area network, virtual LAN or VLAN, is a group of services which allow to NetOp to control if they are attached to the same domain but they are not located on the same network switch.
Protocols and designThe most common standard of tody is 802.1Q by the IEEE 802.1 work group work for carring traffic for multiple VLANs on single trunking interface between different switches in different physical location. Unlike InterSwitch Link (ISL) , 802.1Q does not encapsulate the original Ethernet frame.The process of inserting the 802.1q tag into an Ethernet II frame results in the original Frame Check Sequence(FCS) field to become invalid since we are altering the frame, hence it is essential that a new FCS is recalculated, based on the new frame now containing the IEEE 802.1Q field. This process is automatically performed by the switch, right before it sends the frame down a trunk link.
How to work
802.1Q inserts a new 4-byte field between the SA and TYPE fields of the original Ethernet frame. The 802.1Q specification defines two 2-byte fields (tag protocol identifier [TPID] and tag control information [TCI]) that are inserted within an Ethernet frame following the source address field. The TPID field is currently fixed and assigned the value 0x8100.
Description each of the fields in the 802.1Q header illustrated in figure 1:figure 1
- TPID (Tag Protocol Identifier)(2 Bytes)— This indicates to the receiver that an 802.1Q tag follows. The value for the TPID is a hexadecimal value of 0x8100.
- Priority(User priority bits)(3 bits)— This is the 802.1p priority field. Eight priority levels are defined in 802.1p and are embedded in the 802.1Q header. These three proiority bits can be used to mark packets which to identify the packets blong to which Cos and allows Ethernet frame to e marked with eight levels of priority(values 0-7). </strong>
- CFI (Canonical format indicator)(1 bit) —This single bit indicates whether or not the MAC addresses in the MAC header are in canonical (0) or non-canonical (1) format.
- VID (VLAN Identifier)(12 bits)— This indicates the source VLAN membership for the frame.The 12-bit field allows for VLAN values between 0 and 4095. However, VLANs 0, 1, and 4095 are reserved.For most enterprise customers, 4096 VLANs is adequate. For service provider applications, 4096 VLANs may not be enough.
- DA— A 40-bit multicast address with a value of 0x01-00-0C-00-00 that indicates to the receiving Catalyst that the frame is an ISL encapsulated frame.
- CRC---It is part of the frame check swquece(FCS). An optional frame padding is provided by the extension field.
Use in: Cisco VLAN Trunking Prrotocol (VTP) or IEEE802.1Q
One disadvantage is using CoS markings is that frames lose their CoS markings when transiting a 802.1Q to a non-802.1p link. Trunking with 802.1Q must be enabled before the CoS field even exists. As soon as the packet encounters Layer 3 forwarding, either with a router or a Layer 3 switch, the old LAN header gets discarded and the CoS field will be lost. Therefore, a ubiquitous permanent marking should be used for network transit. This is typically accomplished by translating a CoS marking into another marker or simply using a different marking mechanism.
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