BCMSN Buffs - Help me understand something here..

cisco_troopercisco_trooper Too manyMember Posts: 1,442 ■■■■□□□□□□
How many vlans can an 802.1q trunk support?

According to Cisco Press BCMSN it is 4096. Why then can you only assign an access port to VLANs 1 - 4094? What are the other two VLANs that an 802.1q trunk can support?

Comments

  • networker050184networker050184 Mod Posts: 11,962 Mod
    4096 VLANs 0-4095. 0 and 4095 are reserved for system use which leaves 1 - 4094 available for use.
    An expert is a man who has made all the mistakes which can be made.
  • cisco_troopercisco_trooper Too many Member Posts: 1,442 ■■■■□□□□□□
    4096 VLANs 0-4095. 0 and 4095 are reserved for system use which leaves 1 - 4094 available for use.


    Do these VLANs ( 0 and 4095 ) actually traverse the trunk though? Looking out on my buddy google it looks like 4095 is used by CPU only and assigned to invalid VLANs. If it is an invalid VLAN I would have to assume this is not actually going to be forwarded out a trunk port...and would be only locally significant. icon_confused.gif
  • kryollakryolla Member Posts: 785
    what ever vlans are turned up and allowed will traverse the trunk links
    Studying for CCIE and drinking Home Brew
  • cisco_troopercisco_trooper Too many Member Posts: 1,442 ■■■■□□□□□□
    kryolla wrote:
    what ever vlans are turned up and allowed will traverse the trunk links

    So I can do:

    switchport trunk allowed vlan add 0, 4095

    ..and that will be a valid configuration on a trunk port?

    Sorry for these silly questions but I do not have switches in front of me at the moment to just knock this out...
  • kryollakryolla Member Posts: 785
    vlan 0 & 4095 is reserved for system use.

    vlan 1 default
    2-1001 ethernet vlan normal use
    1006-4094 ethernet vlan extended range refer to Cisco DOC
    Studying for CCIE and drinking Home Brew
  • cisco_troopercisco_trooper Too many Member Posts: 1,442 ■■■■□□□□□□
    kryolla wrote:
    vlan 0 & 4095 is reserved for system use.

    vlan 1 default
    2-1001 ethernet vlan normal use
    1006-4094 ethernet vlan extended range refer to Cisco DOC

    I know, but Cisco says a trunk will support 4096 VLANs, implying 0 and 4095 can traverse a trunk link, which I believe is inaccurate....
  • kryollakryolla Member Posts: 785
    check the RFC for 802.1Q
    ISL it is 1024 vlans
    Studying for CCIE and drinking Home Brew
  • dtlokeedtlokee Member Posts: 2,378 ■■■■□□□□□□
    kryolla wrote:
    vlan 0 & 4095 is reserved for system use.

    vlan 1 default
    2-1001 ethernet vlan normal use
    1006-4094 ethernet vlan extended range refer to Cisco DOC

    I know, but Cisco says a trunk will support 4096 VLANs, implying 0 and 4095 can traverse a trunk link, which I believe is inaccurate....

    I don't really know what your hangup is on this, are you just trying to prove a point about the book being wrong. If you look at the output "show interface trunk" it will say "vlans allowed on trunk 1-4094" so 0 and 4095 are excluded, end of story.
    The only easy day was yesterday!
  • cisco_troopercisco_trooper Too many Member Posts: 1,442 ■■■■□□□□□□
    dtlokee wrote:
    kryolla wrote:
    vlan 0 & 4095 is reserved for system use.

    vlan 1 default
    2-1001 ethernet vlan normal use
    1006-4094 ethernet vlan extended range refer to Cisco DOC

    I know, but Cisco says a trunk will support 4096 VLANs, implying 0 and 4095 can traverse a trunk link, which I believe is inaccurate....

    I don't really know what your hangup is on this, are you just trying to prove a point about the book being wrong. If you look at the output "show interface trunk" it will say "vlans allowed on trunk 1-4094" so 0 and 4095 are excluded, end of story.

    My biggest problem right now is I don't have a switch in front of me which is beginning to drive me mad, because this is simple stuff right here. The command either works or it doesn't. And IF it does, which I suspect it DOESN'T, but IF it does, it wouldn't be hard to figure out if that traffic would go over the trunk. That's the jist of it.
  • cisco_troopercisco_trooper Too many Member Posts: 1,442 ■■■■□□□□□□
    dtlokee wrote:
    kryolla wrote:
    vlan 0 & 4095 is reserved for system use.

    vlan 1 default
    2-1001 ethernet vlan normal use
    1006-4094 ethernet vlan extended range refer to Cisco DOC

    I know, but Cisco says a trunk will support 4096 VLANs, implying 0 and 4095 can traverse a trunk link, which I believe is inaccurate....

    I don't really know what your hangup is on this, are you just trying to prove a point about the book being wrong. If you look at the output "show interface trunk" it will say "vlans allowed on trunk 1-4094" so 0 and 4095 are excluded, end of story.

    My biggest problem right now is I don't have a switch in front of me which is beginning to drive me mad, because this is simple stuff right here. The command either works or it doesn't. And IF it does, which I suspect it DOESN'T, but IF it does, it wouldn't be hard to figure out if that traffic would go over the trunk. That's the jist of it.

    It's official. BCMSN is crap. You can only assigned 1 - 4094 to the trunk just now confirmed on a 2950, making the 4096 number incorrect. I wonder how it is on the test....
  • EdTheLadEdTheLad Member Posts: 2,111 ■■■■□□□□□□

    It's official. BCMSN is crap. You can only assigned 1 - 4094 to the trunk just now confirmed on a 2950, making the 4096 number incorrect. I wonder how it is on the test....

    Enough already, an 802.1q trunk will support 4096 vlans due to a 12bit field in the vlan tag.Cisco's implementation allows 4094, is the BCMSN incorrect or is your Interpretation incorrect.Does anyone really care?
    Networking, sometimes i love it, mostly i hate it.Its all about the $$$$
  • cisco_troopercisco_trooper Too many Member Posts: 1,442 ■■■■□□□□□□
    EdTheLad wrote:

    It's official. BCMSN is crap. You can only assigned 1 - 4094 to the trunk just now confirmed on a 2950, making the 4096 number incorrect. I wonder how it is on the test....

    Enough already, an 802.1q trunk will support 4096 vlans due to a 12bit field in the vlan tag.Cisco's implementation allows 4094, is the BCMSN incorrect or is your Interpretation incorrect.Does anyone really care?

    You obviously do... icon_rolleyes.gif
  • CCIE_2011CCIE_2011 Member Posts: 134
    vlan 0 is used for voice sometime!!
    switchport voice vlan dot1p <
    will use vlan 0 for voice.
    you didn't create it & can use it as other vlans.
    . : | : . : | : .
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