BPDU, access ports and PVST+

[zafar_118]

  Hello,

If we have spanning tree protocol configured on switch (see attachement), will any of switch ports will be blocking?

How about if we have PVST+ vs MST?

I think none of the ports will be blocked if we are running PVST+ but two ports (one port on each side of switch) will be blocked if running MST.

Thanks

[JohnTylerPearce]

Zafar,

If spanning tree protocol is running, and you have "any" redundant links in your switched L2 network, you are going to have some ports blocked, there is no way around that.

In a stable Spanning Tree Protocol scenario the following happens on a regular basis.

1. There is one Root Switch selected for all vlans, or specific VLANs.

2. This Root Switch will send Configuration BPDUs out of its Designated Port every 2 seconds

3. Every non-root switch will receive this Configuration BPDU on its Root Port  and update the following information within the Configuration BPDU [BID, Root Path Cost, and Port Priority], and then forward the updates Configuration BPDU downstream

4. If this non-root switch has a link to another switch, then it will update the Configuration BPDU like above, and sends it down to that switch's Root Port.

This happens over and over and over, by default every 2 seconds.

[zafar_118]

Hi John,

Thanks for your prompt response.

I wonder then what is purpose of PVST if all ports will be treated as if they are member of same vlan.

When bpdus are sent out, aren't they tagged?

[JohnTylerPearce]

Zafar,

By default, all ports on Cisco switches,  are a member of VLAN1. As of right now, most if not all Cisco switches, are configured with PVST+. BPDUs are not tagged as they are sent out, tagging of frames with the 802.1q tag, only occur on trunk ports. This tag includes a VLAN-ID field as well.

The purpose of PVST+, is that each VLAN can have it's own spanning tree instance, and in theory, you can load balance traffic if you configure it that way.