here some notes about VLANS from Cisco Press Book.
A VLAN is a logical broadcast domain that can span multiple physical LAN segments.
-VLANs can include stations in a single building or multiple-building infrastructure.
-Each logical VLAN is like a separate physical bridge.
-Trunk links carry traffic for multiple VLANs.
-Each forwarding decision is based on which VLAN the packet came from.
-Each VLAN configured on the switch implements address learning, forwarding / filtering decision, and loop avoidance mechanism as if it were separate physical bridge.
-VLAN are Layer 2 implementation in our network.
-Broadcast, Multicast and Unicast traffic from the segment will be forwarded by the switch only to ports in the same VLAN.
-I Cisco IOS software, a layer 2 port is known as switch port. A switch port can either be a member of single VLAN or be configured as a trunk link to carry traffic for multiple VLANs.
-When a port is in a single VLAN, the port is called an access port.
-Static port called when assigns a single VLAN to a port.
-The IOS Catalyst switch supports the dynamic assignment of a single VLAN to a port by using a VLAN membership policy server (VMPS).
-Two forms of trunking are used for Cisco switches on Ethernet networks:
Cisco switches support IEEE 802.1Q for FE and GE interfaces. An 802.1Q trunk link provides VLAN identification by adding 4 byte tag to an Ethernet frame as it leaves a trunk port. Because the frame has been changed, a new FCS must also be computed and added to the frame.
The 4 byte tag is inserted into the frame immediately following the source address field and is composed of two separate 2 byte sessions – the tag protocol ID field and the tag control information.
The Hex value 8100 is the first 2 bytes. 0x8100 tells us an 802.1Q compliant device that is a tagged frame and to use the next 2 bytes for 802.1Q information.
- The first 3 bits of the tCI are referred to as the priority bits. These bits indicate the priority of the frame for QoS reasons.
-The next bit in the TCI field is called CFI. This is 1 bit field that, when off, indicates that the device should read the information in a field canonically (right-to-left or low order bits frame). The reason for this bit is that 802.1Q can be used for Token Ring or Ethernet frames. An Ethernet device reads canonically, but Token ring read in a non-canonically form. Ethernet frame will always be 0, but if the tag is in Token ring frame, it will be 1.
-The last 12 bits in the CFI are the VLAN ID.
B. ISL (Inter Switch Link):
ISL tagging accomplishes the same task as 802.1Q trunking but uses different frame format. ISL functions at OSI layer 2 by encapsulating a data frame with a new header and an additional CRC.