BCMSN Buffs - Help me understand something here..

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?

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Comments

networker050184

4096 VLANs 0-4095. 0 and 4095 are reserved for system use which leaves 1 - 4094 available for use.

cisco_trooper

networker050184 wrote:

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.

kryolla

what ever vlans are turned up and allowed will traverse the trunk links

cisco_trooper

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...

kryolla

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

cisco_trooper

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....

kryolla

check the RFC for 802.1Q
ISL it is 1024 vlans

dtlokee

cisco_trooper 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.

cisco_trooper

dtlokee wrote:

cisco_trooper 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_trooper

cisco_trooper wrote:

dtlokee wrote:

cisco_trooper 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....

EdTheLad

cisco_trooper 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?

cisco_trooper

EdTheLad wrote:

cisco_trooper 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...

CCIE_2011

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.