VLAN Tagging Vs VLAN Untagging-Difference between VLAN Tagging and VLAN Untagging
This page compares VLAN Tagging Vs VLAN Untagging and mentions difference between VLAN Tagging and VLAN Untagging.
The process of configuring port as tagged means placing or inserting 802.1q compliant tag in the ethernet frame.
Following are the specifications defined in IEEE 802.1q standard:
• It supports VLANs on ethernet.
• Defines VLAN tagging used for ethernet frames.
• Maintains procedures which are used by switches and bridges to handle and manage 802.1q frames.
• Maintains QoS priority schemes which is separately defined in IEEE 802.1p.
• Defines GARP (i.e. Generic Attribute Registration Protocol).
VLAN tag is 32 bit field which is placed between source MAC address and Ethernet type/length fields. This is known as VLAN tagging. Moreover witches use VLAN ID in order to determine which are the ports or interfaces, it need to send broadcast packet to. Due to this process of tagging, minimum frame size does not change and remains as 64 bytes while maximum frame size increased from 1518 bytes to 1522 bytes. The VLAN tag has size of 4 bytes or octets. This contains TPID and TCI fields as mentioned in table-1 below.
|VLAN tag field||Description|
|TPID||Tag protocol Identifier of size 2 bytes|
|TCI||Tag Control Information of size 2 bytes
TCI contains PCP (Priority Code Point, 3 bits);
DEI (Drop Eligible Indicator, 1 bit);
VID (VLAN Identifier, 12 bits)
VLAN Identifier specify frame is belong to which VLAN in the network. The values of 0x000 and 0xFFF are reserved while rest of the values can be used as VLAN IDs. This gives total of 4094 VLANs.
Here unlike normal switches where subnet is hardwired with physical connections, ports are grouped and configured to different VLANs. The figure-2 depicts one switch is configured for four different VLANs. This allows multiple virtual switches to be configured on a single ethernet switch. Due to this clients can be organized logically instead of limited to a subnet per physical switch. Note that one VLAN (say VLAN-1) shown is like collection of hosts configured as one similar group of ports on the ethernet switch with same VLAN ID.
VLAN is the best way to manage when the traffic on the network increases. As port can be moved from one VLAN port group to the other VLAN easily without any physical connection. This is just simple software command to be used as described later.
VLAN Tagging and VLAN Untagging
As described above process of inserting tags in VLAN is known as VLAN tagging. The process of removing tags in VLAN is known as VLAN untagging.
Figure-3 depicts typical VLAN network consisting of VLAN switches, VLAN-10, VLAN-20, hosts and servers. Hosts are configured with VLAN-10 while servers are configured with VLAN-20. In the example above, there are two switched, one has ports 1 to 10 configured as VLAN10 while other switch has ports 11 to 22 configured as VLAN20. You need PCs to be on VLAN10 while Servers to be on VLAN20. Hence inter-switch links need to be tagged while hosts and servers are to be untagged.
Here the packets moving between switches are tagged so that the next switch inline knows the destination VLAN of the packet. Following command configures interface or port 24 to both VLANs on the switch-1 and switch-2.
VLAN10 tag port 24
VLAN20 tag port 24
When a packet is forwarded between switches, the forwarding switch find out which VLAN the packet belongs to and inserts appropriate VLAN ID into packet header.
During the time packet is received with VLAN tag at switches,
➨if VLAN ID of packet is same as native VLAN ID of the port on which the packet is being forwarded, the tag is removed before the packet is forwarded.
➨if VLAN ID does not match, the packet is forwarded out of the interface, keeping VLAN ID or VLAN tag without any change.