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  1. Member
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    #1

    Default Native VLAN and reserved VLANs

    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.
    Last edited by xagreus; 04-27-2017 at 07:44 PM.
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  3. Junior Member Neil86's Avatar
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    #2
    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.
    Last edited by Neil86; 04-28-2017 at 02:24 AM.
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  4. Senior Member dontstop's Avatar
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    #3
    It's definitely a typo. Classic mistake 4096 VLANs made from the numbers 0 - 4095 inclusive.
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    #4
    @Neil86: Good work with the explanation.
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  6. Member
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    #5
    @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!!
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  7. Junior Member Neil86's Avatar
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    #6
    Thanks, and you're welcome, glad to help.

    Good luck with your studies.
    Last edited by Neil86; 04-28-2017 at 04:47 PM.
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