VLAN1

Answered Question
Aug 25th, 2008
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I want to know that Cisco always start every thing with “ZERO” but Cisco make default VLAN 1, why is not VLAN0 “ZERO”?

Is there any logic or standard? Please let me know.


Correct Answer by dominic.caron about 8 years 11 months ago

In 802.1q, a vlan value of 0 means that the frame does not belong to any vlan. Vlan 1 is the first usable vlan. That's why Cisco is using it.


Cisco IPphone learn the voice vlan configured on the switch by cdp, it's not using vlan 0

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sreekanth sarma Mon, 08/25/2008 - 08:32
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vlan 0 is also there in Cisco where it is used for QoS purposes when VoIp Solutions are implemented


when "switchport voice vlan 802.1p" sub interface command is used the voice traffic is tagged as of vlan 0 and sent over the trunk link

Correct Answer
dominic.caron Mon, 08/25/2008 - 10:42
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In 802.1q, a vlan value of 0 means that the frame does not belong to any vlan. Vlan 1 is the first usable vlan. That's why Cisco is using it.


Cisco IPphone learn the voice vlan configured on the switch by cdp, it's not using vlan 0

sreekanth sarma Mon, 08/25/2008 - 19:50
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I AM talking about 802.1p where vlan 0 is a auxillary vlan used for tagging voice traffic and i am sure about it as i read it in BCMSN official cert guide there are three options of configuring voice vlan


switchport voice vlan 20 - specifies that vlan 20 is an auxillary vlan for voice traffic


switchport voice valn dot1p - specifies the vlan 0 as an auxillary vlan and send the voice traffic tagged with vlan 0


switchport voic vlan none - specifies the voice traffic is untagged


please correct me if i am wrong

Sreekanth,


1) Correct - voice traffic will be tagged with 20, data will be tagged with whatever else the switch port is configured to be in.


2) 802.1p is QoS - nothing to do with 802.1q


3) Correct - but if the switch port is not assigned to a vlan - voice & data are tagged with that VLAN ID.


HTH>

steffen1967 Wed, 08/27/2008 - 04:50
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Question:

In which VLAN will a switch forward its Spanning-Tree BPDU's?


CDP? VTP?


If in VLAN 1, what if someone took off VLAN 1 from a trunk? No Spanning-Tree?


Please clearify...


steffen1967 Wed, 08/27/2008 - 07:13
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Hi Andrew,


sorry but these docs are well-known to me, and they do NOT answer my question!


Maybe I didn't find it, but that's why I ask here, explicitly.


Any help?

My guess is, that, I know it sounds strange, the Cisco switches do not send CDP and VTP frames (Layer 2!) in ANY Vlan! But somehow directly from the CPU, bypassing the according Interface Vlan which one considers the "management vlan".


And what about BPDU's? Same or really sent "within" the according vlan?





Steffen,


Sorry for the late reply - been busy.


OK -CDP & VTP do not user spanningtree at all, in the slighest. For want of a better description - they are seperate services that the switch runs for different reasons. However although they have differenet functions, - they use the same mechanism. By default any router and switch has cdp enabled by default. Any switch has VTP enabled by default - so they are all listening for multicast layer 2 frames with a destination mac of 01-00-0c-cc-cc-cc.


Spanngintree is also enabled by default - and again uses multicast - the address is different 01-80-c2-00-00-00.


So as you can see - STP different process. CDP & VTP differnet process, but uses the same mutilcast address.


HTH>

steffen1967 Thu, 08/28/2008 - 00:58
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Hi Andrew,


no prob with delay, I am busy too!


so these control protocols, vtp, cdp, pagp, dtp are produced by the switches themselves, by their cpu, right?


and they get always through a trunk, even when vlan 1 and/or the native vlan would have been restricted from that trunk, right?


BECAUSE they are not "in" the untagged vlan or "in" vlan 1?

or BECAUSE the switches make exceptions through such trunks for them, explicitly?


remember, my question was "are vtp, cdp, etc. sent in Vlan 1?".


given this one question answered, do the bpdu's behave the same or different from the other control protocols?





Steffen,


so these control protocols, vtp, cdp, pagp, dtp are produced by the switches themselves, by their cpu, right? Yes


and they get always through a trunk, even when vlan 1 and/or the native vlan would have been restricted from that trunk, right? Yes


BECAUSE they are not "in" the untagged vlan or "in" vlan 1?

or BECAUSE the switches make exceptions through such trunks for them, explicitly? whebn you configure a switch port to be in a specific vlan - the CDP hello/response contain the "native vlan" information - so if the port is a switch port in vlan 10 say - then the native vlan would be 10. if the port is a trunk - then the native vlan would be set to 1. this is becuase you cannot restrict vlan 1 or 1002-1005 on a trunk, by default that cannot be changed you cannot restrict those vlans from a trunk port.


They behave the differently as they have different functions:-


CDP - Discovers next hop devices.

VTP - Administers additions, changes and deletions of VLAN's in a multiple switch network.

PAGP - Is the method used for forming etherchannels. There is also LACP.

DTP - Is the process to determine if a switch port can become a trunk port - automaticall or even negotiate a method of trunking.



NONE - of the above use BPDU's


BPDU's are only used in spanningtree and depend on the version running - 802.1d or 802.1s


802.1d - There is no vlan field, just a Bridge Priority & MAC Address feilds - as it's loop detection/avoidance for ALL VLANS.


802.1s - There is a field called the System ID extension - this is the vlan number, sio you can have spanningtree PER VLAN.


HTH>

Jon Marshall Thu, 08/28/2008 - 01:44
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Steffen


CDP/VTP/PagP are always sent on vlan 1 regardless of whether you restrict this vlan on the trunk links or not. If vlan 1 is the native vlan then these packets will be sent untagged. If vlan 1 is not the native vlan then the packets will be tagged with a vlan ID.


DTP is sent on the native vlan.


STP is normally PVST+ or Rapid-PVST+ these days and so each vlan sends it's own BPDU's. If you clear a vlan from the trunk then no STP bpdu's will be sent for that vlan.


Jon

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