CCIE or Null!

VN-Tag, typically was a technology only seen in the Data Center (When using Nexus 2000 series FEXs)  however this has started changing. If you check out Cisco’s new 6800’s series Catalyst switch you’ll see they are now pushing a new ‘instant access’ model. This new model allows us to deploy ‘dumb’ switches that are centrally managed by the main 6800 Chassis essentially making those switches act as ‘external linecards’. Now in the Data Center this is nothing new, sounds just like the Nexus 2000 Series Fabric Extenders (FEX’s) right? Well, they are a little more similar then you think deep down. VN-Tag is the technology used by Nexus 2k FEX’s with their upstream parent switch, that same VN-Tag is also being utilized in the Cisco 6800 Series with its downstream external switches.

The VN-Tag technology itself simply adds an additional header to the packet as it traverses between the ‘Instant Access’ (Or FEX) and the it’s parent switch, where all the switching occurs. It is important to call-out the ‘Instant Access’ switches do NOT do any local switches (once again just like Nexus 2k FEX) all the information is forwarded up the 6500 or 6800 chassis where the packet will get switched or routed accordingly.

Let’s take a quick look at the VN-Tag information itself:

Surprisingly it’s nothing more than an additional 6-bytes. The fields are as follows:

• EtherType [16-bits] = 0x8926
• Destination Bit [1-bit] – Indicates which direction the frame is flowing.
• Pointer bit [1-bit] –
• Destination VIF – [14-bits] – Identifies the destination port.
• Looped – Identifies the source vNIC, ment to identify multicast frames to ensure it is not forwarded back to where it originated.
• Reserved [2-bits] – For future use.
• Source VIF [12-bits] – vif_id of the downstream port.
• Version [2-bits] – Set to 0

Expanding the VN-Tag to products outside of the Nexus line is definitely a great move, and it definitely has great use-cases in many large enterprises that want a more consolidated approach for management, after all this easy way to cut down on your Layer-2 spanning-tree topology. You can also cut down on configuration/implementation time, since this technology allows you to pre-provision the ports on an Instant Access before it is actually connected to the chassis.

Additional Info:

Cisco Instant Access White Paper

IEEE  802.1Q Whitepaper

P.S. – This was a blog post I had start middle/late last year when the 6800 & Instant Access were first announced. Finally got around to finishing it better late than never right?