802.1q and stp instance

[sarahr202]

Hi everybody

I just want to confirm that originally 802.1q allows one stp instance. Does subsequent 802.1q standard allow multiple stp instance?

thanks and have a great weekend.

[Reza Sharifi · Accepted solution]

Hi Sarah,

With MST, you don't need one instance per vlan.  You can combine a bunch vlans in one MST instance.  The other spanning protocols like PVST and RSTP utilize one instance per vlan.  Usually MST scales much better than the other spanning tree protocols.

http://www.cisco.com/en/US/tech/tk389/tk621/technologies_white_paper09186a0080094cfc.shtml

Have a nice weekend

HTH

[Aileron88 · Accepted solution]

Technically you are correct. Only the native untagged VLAN BPDU is recognized and sent over the 802.1q trunk. To allow for other VLANs BPDU's to be sent accross the 802.1q trunk they are tunneled within a untagged VLAN BPDU using a special multicast address. The tunneled BPDU contain the VLAN values of VLANs other than the native.

Thanks,

Adam

[sarahr202]

Thanks Adam.

I still have some misunderstanding,below I will try to explain that using some examples.

In above set-up, there are only two vlans, vlan1 as native and vlan 2 ;dot.1q trunk is used between switches and pvst+ is also used.   For vlan 1, with default setting, the network is converged with sw1 elected as root switch (mac=1) as shown below:

We want to use redundant link between sw2 and sw3  for load sharing

In order to achieve that we want sw2 as root switch for vlan2. We change the sw2's priority so that it will be elected as root switch for vlan 2.

The problem is 802.1q will support one instance of stp for all vlans. In my mind I visualize stp instance as a map where one path exist between root switch and non root switch.  Since 802.1q only carry stp stance that resulted for native vlan, therefore problem arise if we wish to to use redundant link.

For example  we want sw2 to act as root switch for vlan2  so that blocked link on f1/6 could be used. The question is how can we do that with 802.1q? You said sw3 will send vlan2 bpdu within untagged bpdu for native vlan and before sending it over 802.1q trunk. But the f1/6 on sw3 is blocked which means no bpdu can be sent . Keeping this fact in mind then how can we use redundant link f1/6 for vlan 2?

thanks and have a great weekend.

[Aileron88 · Accepted solution]

F1/6 is blocked for VLAN 1, and VLAN 1 only. So no VLAN 1 BPDU's are forwarded from F1/6, but others can be. I should explain my previous comment a little.

The VLAN 1 (Native) BPDU's are sent to the IEEE Multicast MAC Address wheras for VLANs other than the native the BPDU's are sent to a special Shared Spanning-Tree MAC Address (SSTP). So basically to overcome the restriction of 802.1q only supporting a single spanning-tree Cisco changed the destination multicast MAC Address for the BPDU's. PVST+ basically splits the single spanning-tree instance of an 802.1q trunk into multiple logical trunks.

Hopefully that explains it a little better?

Thanks,

Adam

[sarahr202]

Thanks Adam.

In cisco PVST+ environment, 802.1q ,cisco uses the destination stp mac (01:80:c2:00:00:00)

The bpdu for each vlan is sent at this above address in Cisco's PVST+ implementation

So will it be correct  "in Cisco's pvst+ switched network using 802.1q trunk, bpdus for each vlan are sent at particular multicast address.; they are are not tunneled inside native vlan bpdu.That is why redundant links can be utilized for load sharing."?

But we have a non-cisco  switched network say 3com using 802.1q trunk, bpdu for a vlan say vlan 2 is tunneled inside native vlan bpdu. The result is redundant link cannot be used for load sharing. Is that correct?

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My last question is based on your previous response which i quoted below:

"Technically you are correct. Only the native untagged VLAN BPDU is  recognized and sent over the 802.1q trunk. To allow for other VLANs  BPDU's to be sent across the 802.1q trunk they are tunneled within a  untagged VLAN BPDU using a special multicast address. "

My understanding is bpdu are not tagged with 802.1q. Only user' frames are tagged with 802.1q header.  Could you please clarify that ?

thanks for your help and patience.