Cisco Support Community
Showing results for 
Search instead for 
Did you mean: 
Community Member

Regarding dot1Q.

Can some please elobarate this ?

IEEE 802.1Q defines       a single instance of spanning tree that runs on the native VLAN for all the       VLANs in the network. This is called Mono Spanning Tree (MST). This lacks the       flexibility and load balancing capability of PVST that is available with ISL.       However, PVST+ offers the capability to retain multiple spanning tree       topologies with 802.1Q trunking.



Regards, Chandu
Everyone's tags (2)

Re: Regarding dot1Q.

Hi Chandu,

I'm not familiar with the term "Mono Spanning Tree" - perhaps that was used in early versions of the IEEE documents. In recent versions it is called "Common Spanning Tree"; MST is known as the "Multiple Spanning Tree", originally definded in IEEE 802.1s.

The earlier IEEE Spanning-Tree Standards defined a single Spanning-Tree for a bridged domain, regardless of the existence or number of VLANs. Port-states like Forwarding or Blocking affect the whole link, not an instance (like a VLAN).

However, vendors like Cisco developed and implemented such technologies often before any standards were published, another example for that would be ISL as trunking mechanism, which was available on Cisco devices previous to 801.1Q.

Actually such proprietary implementations often triggered the formulation of a standard.

As you already know, the Cisco PVST implementation built a topology for every VLAN and worked only with ISL on trunk-links, which is not compatible to 802.1Q trunking. When Cisco implemented the 802.1Q trunking option, this proprietary ST implementation was adapted to be .1Q-compatible but still to built an instance for every single VLAN. This was called PVST+; and in addition to the so-called SSTP BPDUs (tagged and untagged) for the per-VLAN instances, untagged BPDUs are also send on trunks with the IEEE destination address for compatibility to non-Cisco (standard-based) switches.

The 802.1s Multiple Spanning Tree Standard (also supported by Cisco [and there was also a Cisco pre-standard]) is something in between: You define instances for the spanning-trees and map VLANs to the instances (normally you'll have much less instances than VLANs).

As .1s is also an IEEE standard, it has to be compatible to the .1Q common spanning-tree definitions, so you'll find this (important) term again in the MST documents.



CreatePlease to create content