Quick BPDU question

Unanswered Question
Feb 18th, 2010
User Badges:

Hey,


This all refers to PVST+.


Does the vlan-ID actually get sent in the BPDU?


Let's say two switches connect and are configured as access ports, but the vlans differ (so A (accessport 20) connects to B (accessport 30)). How will the BPDU's behave in this situation? I know we'll get the CDP native vlan mismatch, but will switch B think the BPDU it received from switch A belongs to vlan 30 or will it do something else (ignore/err-disable/etc...)?


I guess the same question goes for a 802.1q trunk. Will the bpdu itself say which vlan it belongs to, or will the 802.1q header determine what vlan that BPDU is interpreted for?


I know mac-address reduction adds the vlan number as part of the priority, but I don't know if the vlan number is extrapolated from this or not.


Thanks so much for any and all help.

  • 1
  • 2
  • 3
  • 4
  • 5
Overall Rating: 0 (0 ratings)
Loading.
Jon Marshall Thu, 02/18/2010 - 14:37
User Badges:
  • Super Blue, 32500 points or more
  • Hall of Fame,

    Founding Member

  • Cisco Designated VIP,

    2017 LAN, WAN

tdistlists wrote:


Hey,


This all refers to PVST+.


Does the vlan-ID actually get sent in the BPDU?


Let's say two switches connect and are configured as access ports, but the vlans differ (so A (accessport 20) connects to B (accessport 30)). How will the BPDU's behave in this situation? I know we'll get the CDP native vlan mismatch, but will switch B think the BPDU it received from switch A belongs to vlan 30 or will it do something else (ignore/err-disable/etc...)?


I guess the same question goes for a 802.1q trunk. Will the bpdu itself say which vlan it belongs to, or will the 802.1q header determine what vlan that BPDU is interpreted for?


I know mac-address reduction adds the vlan number as part of the priority, but I don't know if the vlan number is extrapolated from this or not.


Thanks so much for any and all help.


You are right, with mac-address reduction the vlan ID will actually be part of the bridge ID. However i don't believe that the vlan ID is extrapolated from this.


On an 802.1q trunk the 802.1q tag is used to verify the vlan. On an access port the vlan is determined by which vlan the port has been allocated to ie. "switchport access vlan "


If you have access ports misconfigured on either end of the link then you have simply joined 2 vlans together. The BPDU will not be used to determine they are misconfigured, simply STP will think it is calculating a loop free topology for one vlan ie. it won't know you have accidentally joined together 2 vlans.


Jon

Giuseppe Larosa Thu, 02/18/2010 - 23:23
User Badges:
  • Super Silver, 17500 points or more
  • Hall of Fame,

    Founding Member

Hello,

in my understanding PVST+ proprietary BPDU used on 802.1Q trunk contains the vlan-id in some internal field.


the receiving switch compares the external 802.1Q vlan tag with this value.

if the two values don't match an error message is displayed and the port is placed in errordisable.


I've seen this some times.


Edit:

the message is something like "inconsistent BPDU and vlan tag received on port"


Hope to help

Giuseppe

milan.kulik Thu, 02/18/2010 - 23:54
User Badges:
  • Red, 2250 points or more

Hi Giuseppe,


you are right.

There will be no problem detected if switches are connected via access ports belonging to different VLANs.

But on a trunk, error message "SPANTREE-2-BLOCK_PVID_PEER: Blocking [char] on [char]. Inconsistent peer vlan."

or "SPANTREE-2-BLOCK_PVID_LOCAL: Blocking [char] on [char]. Inconsistent local vlan." appears.

Here are the links with details:

http://www.cisco.com/en/US/tech/tk389/tk621/technologies_tech_note09186a00801d11a0.shtml

http://www.cisco.com/en/US/docs/ios/12_1t/12_1t3/feature/guide/dtbridge.html#1020611

http://www.cisco.com/en/US/docs/switches/lan/catalyst4500/12.1/19ew/system/message/emsg.html#wp1186226


BR,

Milan


Jon Marshall Fri, 02/19/2010 - 04:19
User Badges:
  • Super Blue, 32500 points or more
  • Hall of Fame,

    Founding Member

  • Cisco Designated VIP,

    2017 LAN, WAN

giuslar wrote:


Hello,

in my understanding PVST+ proprietary BPDU used on 802.1Q trunk contains the vlan-id in some internal field.


the receiving switch compares the external 802.1Q vlan tag with this value.

if the two values don't match an error message is displayed and the port is placed in errordisable.


I've seen this some times.


Edit:

the message is something like "inconsistent BPDU and vlan tag received on port"


Hope to help

Giuseppe


Giuseppe


Thanks for this information.


Jon

Actions

This Discussion