In this topic, we
gonna take a look at MSTP, that’s “Multiple
Spanning Trees Protocol”, some of the Literature might see this
written as “Multiple Spanning Tree Protocol” or
MST, and it might be abbreviated MST, but either way we talking
about same thing and also on the Literature, you might read that, MSTP is defined in IEEE 802.1s Standard and that’s true but here is an update to that, sends
the IEEE 802.1s Standard was released that has been rolled into the update to the IEEE
802.1q Standard, but let’s think about exactly what is
going on here.
Notice on Picture,
we got Switches A, B and C, what we can do with
Multiple Spanning Trees Protocol is, instead of having a Spanning Tree Protocol Instance for every Single Vlan,
which could start to put the Processor impact on
our Switches, if we had a lot’s of Vlan’s, instead of doing that.
What
we can do is defined these instances.
And we can say that “maybe Switch A is the Root for instance 1, and Switch B is the Root for instance 2”, and then
we can assign different Vlan’s to different
instances, now instead of having,
a 8 different Spanning Tree Protocols instances,
we got these 2 instances running but to be (Technically accurate, there is a Third instance running,
their always an instances 0, that would include all other Vlan’s but that’s
less than the number of Spanning Tree Instances), if we will be
running something like PVST+, where every single
Vlan got its own instances of Spanning Tree.
Now where you might need to use MSTP is, where you have lots of Vlan’s,
maybe you have Enterprise with lots of building’s,
and your Trunking between different buildings, an Vlan’s appearing than 1
Building, they go to all the other building’s, that would be an
end-to-end Vlan design, that’s where you might need MSTP.
You might not need MSTP
is much, if you had more local Vlan design,
where you got a few Vlan’s in each building,
but in order to get from 1 building to another
building your Routing and case like that you probably don’t have any one Switch,
that’s got a huge number of Vlan’s and if don’t
have a huge number of Vlan’s, you might not want to do MSTP, but
if you do, it’s a great solution and let’s think for movement about how this works.
We going
to have an each of our Switches a Single MSTP
Configuration and this Configuration is gonna contain
Ø A name for the
Configuration
Ø There gonna be Revision
Number for the Configuration
Ø And Mapping Table that
say’s which Vlan’s are Mapped up to which instance.
And if we have a group of Switches that are
interconnected and they agree on these different
Attributes of their MSTP Configuration, their said to be a Region, a few things we
need to know about this though.
Unlike VTP, where we make a change on a one Switch and that change
is Propagated to another Switch, here if we make a change and
say “this Vlan is part of this instance, that
change has to be entered on each Switch, we can do that via CLI
or we can do that through some sort for a network managed platform, but it’s not going to happened automatically.
Also, the Revision
Number, remember with VTP, we had
that Configuration Revision number that
automatically got incremented to say “here is the latest version of the
information. Well here MSTP, The
Revision Number says “ here is the latest version of this configuration, but it
doesn’t incremented automatically that’s something we manually need to do,
And also, when Switches within a Region are exchanging MSTP information,
they do not exchange the entire MSTP Mapping Table that might be a lot of data
to be exchanging,
What they do instead is,
they create a Hash, they run a Hash Algorithm’s on
the Table and they send a Hash result to another Switch in that Region and that Switch is
going to run the Hashing Algorithm on their table, and if the Hash is match then the Switch concludes that they have
matching Mapping Tables.
However,
if the Switch receives a digest and it doesn’t
match its own local computed digest. It’s going to assume that
this is a boundary between one Region and another
Region, and also a good that, MSTP
is compatible with other versions of Spanning Tree Protocol, it
can notice that a neighboring Switch is not
running MSTP, may be its running PVST+, that’s ok, it can exchange appropriate messages with that Switch is running
PVST+ or may be that Switches is Rapid PVST+, that’s ok as well,
and now that, we reviewed of bit of the theory
surrounding MSTP, and in our next topic we wanna go out and take a look, how to
configured it
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