Standard VLANs vs Extended VLANs

[chhayheng]

HI Guys,

My brain come up with one question relate the VLANs.
The question is:
 1. Why they are separate Standard VLANs and Extended VLAN ? 
 2. Why they not put them (10bits or 12bits) in only one type as Standard VLANs or Extended VLANs ?

Anybody knows the story behind this?
I hope everybody help to share the story of them. :)

Best Regards,
CCH

[Giuseppe Larosa]

Hello CCH,

Cisco acquired catalyst company to have switches in his product portfolio.

These first era switches were able to support only standard vlans 1-1001 with vlans 1002-1005 with special meaning for source based bridging on token ring or FDDI.

Catalyst swiches were running CatOS a set / clear command line totally different from IOS.

Cisco introduced ISL encapsulation for trunking with 26 bytes header and 4 bytes trailer totally encapsulating the original ethernet frame. An ISL frame is not an ethernet frame but it has an ethernet frame inside it.

Later IEEE introduced first version of 802.1Q standard with 12 bits for vlan-id and introduced the 802.1Q vlan tag that is only 4 bytes. (with 3 bits 802.1p of CoS 1 bit for canonical non canonical order of bits in the mac addresses 12 bits for vlan-id and 16 bits for the ethertype). This was called internal tagging.

From that era comes the distinction between standard Vlans and extended Vlans: for 802.1Q vlans 1-4094 can be used for user traffic, in original catalyst/cisco implementation the vlan-id was shorter and usable vlan were 1-1001.

So Cisco added support for vlans following the 802.1Q standard, but for many years an important limitation was present with extended Vlans 1006-4094: they could not be created and propagated by Cisco VTP Vlan trunking protocol.

To create a vlan in extended range the switch had to be configured for VTP transparent mode.

This is the most important technical fact  about the distinction between standard Vlans and extended Vlans.

To be noted most Cisco multilayer switches implement a routed port ( a L3 port) by picking a Vlan in the extended range and reserving it for internal usage. In this way the switch hardware logic could emulate a routed port by using an SVI and a physical port that acts as the only access port in the corresponding L2 domain.

In the long run Cisco ISL trunking protocol was left over as 30 bytes overhead are too much in comparison with the 4 bytes of an 802.1Q Vlan tag.

ISL was modified to support a 12 bit vlan field and 3 bit of CoS.

Also because new ideas were introduced like 802.1Q tunneling: means having two or more 802.1Q Vlan tags between the ethernet header and the packet payload. This is called Q in Q and it was the start of the metro ethernet age.

These are the reasons for this distinction.

Hope to help

Giuseppe

[kousikdutta]

Really Helpful info.

Thanks,

[chhayheng]

Hi Giuseppe Larosa,

Thank for your answered.

For the first Switch era vlan 1-1005. So when we have 3 vlan, we need cross 3 links between switch. Am I right ?

Later on, IEEE release 802.1Q with 4byte tage (12bits Vlan, 3 bits 802.1p of CoS 1 bit for canonical non canonical )

Can you explain me what are they ?
"3 bits 802.1p of CoS 1 bit for canonical non canonical"

Thank you

 

[lpassmore]

For the first Switch era vlan 1-1005. So when we have 3 vlan, we need cross 3 links between switch.

Not quite.  Cisco had ISL which enabled multiple VLAN support over a single link.

"3 bits 802.1p of CoS 1 bit for canonical non canonical"

The 3 bits of CoS (or more correctly Priority Code Point - PCP) enables 802.1p which allows packet prioritisation over the link. Values are typical 0 for best effort, 1 for background to 7 for network critical.

The 1 bit for canonical is an indicator of whether the MAC address in the packet is in canonical or not.  Ethernet MAC addresses are canonical which means that they are transmitted over the wire left to right.  Non-canonical protocols such as Token Ring (802.5) transmit each byte right to left.  

This is seldom used now and since the introduction of 802.1ad, this bit has been changed to become a drop eligibility.