Native VLAN and reserved VLANs

xagreus

Hello, community-

In addition to Cisco's CCENT/CCNA OCG, I've been studying Paul Browning's CCNA in 60 Days (it cost me $2.99 a while back), which has been a pretty good book so far. On pp. 121-122, it states that

the native VLAN can be manually changed to any valid VLAN number (except for 0 and 4096, because these are in the reserved range of VLANs).

However, on a Cisco Learning Network thread, it states that 0 and 4095 are reserved. That seems to make more sense, since VLANs 1-4094 are allowed on a trunk, but after scouring the OCG and searching online, I can't seem to find anything definitive. So I thought I'd appeal to you more experienced networkers.

Thanks for any insight you can provide.

Edit: I also looked for a pertinent thread on the Techxams forums, but couldn't find one.

Neil86

The VLAN ID field (12-bits) inside the trunking protocol header theoretically supports 4096 VLANS ( 2¹² = 4096). These are VLANS 0-4095, inclusive, which is a total of 4096. Therefore, VLANS 0 and 4095 are reserved.

It must be a typo, because 0-4096, inclusive, would actually equal 4097 VLANS, which you cannot get from 12 bits.

Look at the bits and their values. There are 12 bits in the VLAN ID field:

Bit: 1 - 2 - 3 - 4 - 5 - 6 - 7 - 8 - 9 - 10 - 11 - 12
Value: 1 - 2 - 4 - 8 - 16 - 32 - 64 - 128 - 256 - 512 - 1024 - 2048

When you add the values, it equals 4095 (4096 if you include the 0 VLAN, all bits off).

If all bits are off, that equals 0, which is a reserved VLAN. The same applies if all bits are on, which equals 4095, which is also a reserved VLAN. Any combination between those two values serves as a permitted VLAN ID.

However, certain VLANs and VLAN ranges are used for certain purposes:

0 & 4095 -> reserved
1 -> Cisco default
2-1001 -> used for ethernet VLANS
1002-1005 -> FDDI & Token Ring
1006-4094 -> extended VLANS for other uses

Hope that clarifies things.

dontstop

It's definitely a typo. Classic mistake 4096 VLANs made from the numbers 0 - 4095 inclusive.

rob42

@Neil86: Good work with the explanation.

xagreus

@Neil86 - thanks for taking the time to write up the detailed explanation - that definitely does clarify a lot!

So it's similar to the usable IPs in a subnet: The first and last addresses are reserved for network and broadcast (.0 and .255 in a /24). That will help me remember.

Much appreciated!!

Neil86

Thanks, and you're welcome, glad to help.

Good luck with your studies.