What hebavior of rstp/per-vlan when the link's vlan port assocation are different?

[zlinuxboy]

hi,

I am trying to figure out the behavior of rstp/per-vlan when two link's vlan port association(VPA) are different: one end is untagged, the other end is tagged.

original topo:

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        (untag)       (untag)     (untag)     (untag)
        2/1/18       1/1/18      1/1/3       1/1/4
os6900-202 +----------+ os6900-203 +----------+ os6860-204

Three switches are running RSTP/per-vlan, os6900-203 is the root bridge. so far by now, every thing work as expected.

When I change the os6900-202's 2/1/18 from untagged to tagged. According to the aos 8 network configuration guide, os6900-203's single spantree instance will untag bpdu through 1/1/18, but os6900-202 could not received it because 2/1/18 is a tagged port.

So os6900-202 should take the root bridge role and 2/1/18 should be desginated port. but the real is os6900-202 is designated bridge, and 2/1/18 is root port. why?

result:

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os6900-202 > show spantree vlan 21
Spanning Tree Parameters for Vlan 21
  Spanning Tree Status :                   ON,
  Protocol             :       IEEE Rapid STP,
  mode                 : Per VLAN (1 STP per Vlan),
  Priority             :       32768 (0x8000),
  Bridge ID            :   8000-e8:e7:32:4a:58:99,
  Designated Root      :   1000-e8:e7:32:4a:5b:a9,
  Cost to Root Bridge  :                    2,
  Root Port            :               2/1/18,
  TxHoldCount          :                    3,
  Topology Changes     :                   16,
  Topology age         :            00:31:40,
  Last TC Rcvd Port    :               2/1/18,
  Last TC Rcvd Bridge  :   1000-e8:e7:32:4a:5b:a9,
    Current Parameters (seconds)
      Max Age              =    20,
      Forward Delay        =    15,
      Hello Time           =     2
    Parameters system uses when attempting to become root
      System Max Age       =    20,
      System Forward Delay =    15,
      System Hello Time    =     2


os6900-202 > show spantree vlan 21 ports
Spanning Tree Port Summary for Vlan 21
        Oper  Path   Desig        Prim.   Op  Op  Loop
Port     St   Cost    Cost   Role Port    Cnx Edg Guard  Desig Bridge ID        Note
-------+----+-------+-------+----+-------+---+---+------+----------------------+------
 2/1/18 FORW       2       0 ROOT  2/1/18 PTP  NO  DIS   1000-e8:e7:32:4a:5b:a9

os6900-203 > show spantree vlan 21
Spanning Tree Parameters for Vlan 21
  Spanning Tree Status :                   ON,
  Protocol             :       IEEE Rapid STP,
  mode                 : Per VLAN (1 STP per Vlan),
  Priority             :        4096 (0x1000),
  Bridge ID            :   1000-e8:e7:32:4a:5b:a9,
  Designated Root      :   1000-e8:e7:32:4a:5b:a9,
  Cost to Root Bridge  :                    0,
  Root Port            :                 None,
  TxHoldCount          :                    3,
  Topology Changes     :                   15,
  Topology age         :             01:51:52,
  Last TC Rcvd Port    :                1/1/3,
  Last TC Rcvd Bridge  :   8000-e8:e7:32:fa:94:db,
    Current Parameters (seconds)
      Max Age              =    20,
      Forward Delay        =    15,
      Hello Time           =     2
    Parameters system uses when attempting to become root
      System Max Age       =    20,
      System Forward Delay =    15,
      System Hello Time    =     2

os6900-203 > show spantree vlan 21 ports
Spanning Tree Port Summary for Vlan 21
        Oper  Path   Desig        Prim.   Op  Op  Loop
Port     St   Cost    Cost   Role Port    Cnx Edg Guard  Desig Bridge ID        Note
-------+----+-------+-------+----+-------+---+---+------+----------------------+------
  1/1/3 FORW       4       0 DESG   1/1/3 PTP  NO  DIS   1000-e8:e7:32:4a:5b:a9
 1/1/18 FORW       2       0 DESG  1/1/18 PTP EDG  DIS   1000-e8:e7:32:4a:5b:a9

[Cristek]

You'll need to provide additional information on what you are trying to achieve.
On the one hand, you're saying that you're using per-vlan variant, but then you are saying that the 6900-203 has a single instance.
And also, what do you mean when you are saying that all ports are untagged except on 202's port 2/1/18 being tagged?

[zlinuxboy]

You'll need to provide additional information on what you are trying to achieve.

sorry, I update the original post by adding some new detail explanation about what I want to achieved.

On the one hand, you're saying that you're using per-vlan variant, but then you are saying that the 6900-203 has a single instance.

under rstp/per-vlan, every vlan should run a spantree instance, os6900-203 only enable rstp on vlan 21, so I think it should be only one rstp instance, is that mean a single instance?

And also, what do you mean when you are saying that all ports are untagged except on 202's port 2/1/18 being tagged?

check the original post.

[silvio]

Both switches are in perVlan in my explanation: If you have at one end of a link vlan 5 untagged and at the other end vlan 5 tagged than traffic in this vlan is not possible. The traffic from untagged vlan 5 will be forwarded to the untagged/default vlan at the other end (f.e. vlan 1). Every port has one untagged vlan.
This is not a good configuration. But if you need this scenario for some reason (vlan 5 to vlan 1), than also the untagged BPDU between vlan 5 and vlan 1 are exchanged. So you mix the bpdu between different vlan (big problem if vlan 1 from the second switch has another way to vlan 1 of the first switch).
So it is very important that at all links between the switches the same vlans are tagged/untagged. And if you need (hopefull not necessary) this scenario you should disable stp for this vlans at this ports and have other features to avoid loops.
BR Silvio

[Cristek]

Each vlan always has its own instance when in per-vlan. But you need to remember that your native vlan doesn't match when you change from untagged to tagged, and when you do, traffic from vlan 5 on one switch is being delivered to vlan 1 on the other switch. You'll have a slow and erratic network at best...
Honestly, use flat mode instead. I always use it myself! Simple to configure and maintain, and if you do have dozens or hundreds of vlans on one of your sites, then what you want to use is MSTP instead.

[zlinuxboy]

Thanks both of you, sir.