This document also provides a lot of information about multiple spanning Tree Hill 82.1 se.
You're not expected to know all the detail in this document, but it provides a nice reference.
If you're interested, I'll cover some of the basics now and then you can read the document if you're
interested in more information.
Multiple spending tree is the new industry standard inspired by Cisco's proprietary, multiple instance
spanning tree protocol.
Cisco developed multiple instance spanning tree protocol to solve some of the issues that you have with
PV ist.
So as the number of VLANs configured in switched networks increases the overhead when running PV ist
also increases.
If you configure a 1000 VLANs with PV ist and rapid PV ist, you end up having a thousand spanning tree
instances, but with multiple spanning tree and the proprietary multiple instance spanning tree that
existed before multiple spanning tree, you can map a number of VLANs to the same spanning tree instance.
It's fairly simple to do this, but the idea is if you had 1000 VLANs, you would allocate 500 to 1
instance and the other 500 to another instance, which means you only have two spanning tree instances
rather than a thousand spanning tree instances.
So multiple spanning tree standardizes, the concept of multiple spanning trees and incorporates the
convergence of rapid spanning tree.
Multiple spanning tree allows you to group VLANs to a shared spanning tree instance.
It also defines a protocol for interconnecting multiple spanning tree regions.
How to interoperate with existing ADA 2.1 DX and ADA 2.1 CU spanning tree implementations and provide
some best practices.
So have a look at this document if you're interested in a lot of detail.
But here's a quick comparison.
Imagine you had 1000 VLANs on switch A which is connected to both switch D one and DH two.
So in this topology switch A has a 1000 vlans d one is going to be the spanning tree route for some
VLANs and DX two is going to be the spanning tree route for other VLANs.
So switch D1 is configured to be the route for VLANs 501 to 1000 DH two is the route for VLANs 1 to
500.
The interface from switch A to switch D one blocks, VLANs 1500 and from switch A to DH two blocks,
VLANs 501 to 1000.
So once again, route for these VLANs traffic will be forwarded out of this port for those VLANs but
blocked for VLANs 1 to 500.
The switch is the route for these VLANs.
This port will forward traffic out of it for VLANs 1 to 500 but will block a VLANs 501 to 1000.
It's very inefficient to maintain a thousand spanning tree instances in this network.
We have 500 spanning three instances with D, one is the root and we have 500 with RD two as the root.
But logically we actually only require two instances.
RD one should be the root, for instance, one that contains these VLANs and RD two should be the root,
for instance, two that contains these VLANs.
You associate these VLANs to instance one and make rd one the route you associate these VLANs to insulins
to.
And make D2 the route.
That means you have to maintain two instances rather than a thousand instances.
So that kind of detail is explained here.
I'll go through it quickly.
In a Cisco PvZ environment, you need one spanning tree instance for every VLAN, which means you have
a 1000 instances for the two different final logical topologies with DX.
One is the route for one topology and DX to the route for the other topology.
This wastes a lot of CPU cycles for all the switches in the network.
In addition to the bandwidth used by sending BPD use, a thousand BPD use are going to be sent out of
every port every 2 seconds because PVS RT sends a BPD queue for every VLAN because we have an individual
instance mapped to every VLAN.
So the idea with multiple spending tree is that you get the best of PVS tree and traditional spending
tree.
You map several VLANs to specific instances.
So in our topology once again.
You'd make switch one the route, for instance, one switch to the root, for instance, to the sport
would forward, for instance one but block, for instance, to the port would forward, for instance,
to but block for instance, one only two spanning trees are maintained rather than a thousand.
So you still get load balancing because half of the VLANs follow a separate instance and you save on
the CPU because you only have two instances of spanning tree.
So from a technical point of view, multiple spanning trees, the best protocol to use in this example,
but multiple spanning tree is more complex to configure than PV ist and interaction with legacy switches
can be challenging at times.
So you would only want to use multiple spanning tree if you have many VLANs.
So in this example we have 1000, so it makes sense to use multiple spanning tree.
If you only had ten or 20 VLANs in your network, you could continue using PV RT or rapid PV IST.
The document continues with a lot of detail on how to configure multiple spanning tree regions, but
that is out of the scope of the Q&A.
So have a look at this document if you're interested.
We'll wait till you get to your CMP certification.