pvst+ and dot1q

[sarahr202]

Hi everybody.

dot1q requires one stp instance for all the vlans.

Cisco's PVST+ runs one instance per vlan and yet still can use dot1q.

My questions how did Cisco overcome the requirement of one instance for all vlans as required by dot1q?

Does cisco make some change to dot1q to allow one vlan per instance?

thanks and have a great weekend.

[Tom Watts]

Hi Sarah, 802.1q does not require one instance for all vlans. MSTP was introduced in 1998 as 802.1s and later ammended in to 802.1q in 2005. Original PVST operated over ISL trunk only. PVST+ rectified this limitation by becoming compliant to the IEEE MST while maintaining the ability to work over ISL trunk as well. It is all in the BPDU.

-Tom

[sarahr202]

Hi Thomas.

First of all thanks for your response.

Please look at the following link.

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

I quoted below pertinent part to our discussion from the link.

standard 802.1q Case

The original IEEE 802.1q  standard defines much more than simply trunking. This standard defines a  Common Spanning Tree (CST) that only assumes one spanning tree instance  for the entire bridged network, regardless of the number of VLANs. If  the CST is applied to the topology of this diagram, the result resembles the diagram shown here:

In a network running the CST, these statements are true:

• No load balancing is possible; one Uplink needs to block for all VLANs.

• The CPU is spared; only one instance needs to be computed.

Note: The Cisco implementation enhances the 802.1q in order to  support one PVST. This feature behaves exactly as the PVST in this  example. The Cisco per-VLAN BPDUs are tunneled by pure 802.1q bridge

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1) So we pick up third party switches say 3 com and want to use dot1q as trunk encapsulation between the switches, then we can have only one stp instance for all the vlans.

2) Cisco made some modifications in dot1q which allow one instance per vlan .My question is did other vendors also make modification in dot1q so they can run one instance per vlan?

3) In nut shell if i see a cisco switched network connected by dot1q trunks, I could say here  dot1q can support one instance per vlan. However if a see third party switches using dot1q, i could say here the switched network is using one stp instance for all vlans.

Thanks and have a great weekend.

[Tom Watts · Accepted solution]

Hi Sarah,

What you're referencing is true - 15 years ago. 802.1q changed to include 802.1s. Cisco made PVST+ to be compliant to 802.1q.

Simply defining encapsulation 802.1q does not define all attribute of how your trunk links works. That is why there is classic STP, RSTP and MSTP. While using PVST+ will send 802.1q compliant BPDU if there is a MSTP / 802.1q link present.

If a 3rd party vendor switch chooses not to implement MSTP as a spanning tree feature set (which is a part of 802.1q) it is their limitation. The issue is, although there is an IEEE standard, it is up to the interpretation of the vendor to implement it correctly and fully.

Here is the IEEE information;

http://standards.ieee.org/getieee802/download/802.1Q-2011.pdf

-Tom

[Reza Sharifi · Accepted solution]

Hi Sarah,

This may answer your questions.  Since Cisco owns most of the switching market, most venders modify thier software to work with Cisco devices.  Here is some info from HP documentation:

PVST+

HP devices and Cisco devices support separate spanning trees on an individual port-based VLAN basis. However, until the IEEE standard for multiple spanning trees is finalized, vendors are using different methods to support multiple spanning trees within their own products. PVST+ is an extension to PVST that enables a Cisco device to interoperate with other devices that are running a single spanning tree (IEEE 802.1Q) while still running a separate spanning tree in each VLAN.

PVST+ uses 802.1Q single STP BPDUs on VLAN 1 and PVST BPDUs (which have a proprietary format) for other VLANs. In this case, the Cisco device uses devices running 802.1Q as tunnels for PVST (non-802.1Q) traffic. The 802.1Q single STP BPDUs are addressed to the well-known STP MAC address 01-80-C2-00-00-00. The PVST BPDUs for the other VLANs are addressed to multicast address 01-00-0C-CC-CC-CD.

The PVST+ method can require manual configuration of STP parameters on the 802.1Q devices to ensure that traffic for the PVST VLANs is not blocked. In addition, the opportunities to adjust STP parameters to load balance traffic on a VLAN basis are limited when using PVST+.

Using HP Single STP with Cisco PVST+€

Since HP’s single STP feature complies with IEEE 802.1Q (the single STP specification), you also can use an HP device running single STP to interoperate with a Cisco device running PVST+. When you enable single STP on an HP device, the PVST compatibility feature is not enabled, even if a port receives a PVST BPDU.

More infor here:

http://www.hp.com/rnd/support/manuals/pdf/release_06628_07110/Bk2_Ch5_STP.pdf

HTH

Have a nice weekend

Reza