Why tag vlans

For those who can't remember, Ethernet's maximum frame size is bytes, making an ISL frame of bytes, what we call a 'giant' or 'jumbo' frame! This method allows us to optimise the root switch placement for each available VLAN while supporting neat features such as VLAN load balancing between multiple trunks. Since the ISL's header fields are covered on a separate page, we won't provide further details here. The As with all 'open standards' the IEEE In addition to the compatability issue, there are several more reasons for which most engineers prefer this method of tagging.

These include:. Amazingly enough, the The 4-byte tag we mentioned is inserted within the existing Ethernet frame, right after the Source MAC Address as illustrated in the diagram below:. Because of the extra 4-byte tag, the minimum Ethernet II frame size increases from 64 bytes to 68 bytes, while the maximum Ethernet II frame size now becomes bytes.

If you require more information on the tag's fields, visit our protocol page where further details are given. As you may have already concluded yourself, the maximum Ethernet frame is considerably smaller in size by 26 bytes when using the IEEE You may also have a look at the following articles to learn more —.

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Popular Course in this category. By clicking sign up, you agree to receive emails from Techopedia and agree to our Terms of Use and Privacy Policy. Independent logical systems can be formed accurately with the help of the VLAN tagging inside a physical network itself.

Individual domains can be created with the help of this VLAN tagging system. This helps in determining which interface or broadcast area the information packet needs to be sent to in order to receive the right information. The switches need to be configured beforehand for working properly with the process of VLAN tagging. With this system, multiple broadcast systems can be segregated into individual domains. Bridging traffic can be forwarded with the use of this system.

The port between router and switch is configured as a trunk port so that both router and switch know which packet belongs to which customer VLAN.

On that port the Ethernet frames are tagged with the In fact VLANs and tagging is nothing more than a logical separation of networks in contrast to a physical one. Now what does that mean? If there were no VLANs you would need one switch for each broadcast domain. Imagine the cabling involved and also the potential number of NICs required at the hosts.

So first, VLANs allow you to have multiple independent layer 2 constructs within the same switch. That's why they are tagged. If a port carries more than one VLAN it's also usually called a trunk. Generally you have to distinguish packets at port ingress incoming "from the cable" and egress outgoing "into the cable" :. If the switch doesn't know about tagging or about that precise VLAN, it will reject it, sometimes you have to activate some kind of ingress-filter though.

You can also force a port to accept untagged or tagged packets only. In other words:. The defacto VLAN encapsulation protocol is Its most basic function is to retain VLANs across switches.

Since VLANs are locally significant to the switch, you have to tag a frame going to near-by switches to let them know what logical grouping that frame belongs to.

VLAN is a layer 2 protocol and it segments a layer 2 network, they can only communicate in a Layer 3 device such as a router or a layer 3 switch.