Let’s take a look at how to configure
MSTP, Multiple Spanning Trees Protocol and in this example.
Let’s say that, we want to
create a couple of Multiple Spanning Trees Instances,
and we want to assign Vlan’s 100 and 300 to
instance number 1, and let’s assign
Vlan 200 to instance number 2, then we can say that Switch Sw1 is going to be the Root, The Primary Root for
Instance 1 and the Secondary Root for Instance 2, then we can go
to Switch Sw3 and say “it’s gonna be the Primary
Root Instance 2 and Secondary Root for instance 1, to begin with
and we need to visit each Switch, this information
is not automatically propagated, but on each Switch we need to
defined our instances.
Ø Sw1(config)#spanning-tree
mst configuration
Ø Sw1(config-mst)#instance 1
vlan 100, 300
ü
1:- instance Number
ü
In instance 1, we want to assign Vlan’s 100 and 300
Ø Sw1(config-mst)#instance 1
vlan 200
ü
In instance 2, we want to assign Vlan 200
Let’s do that same on Sw2 and Sw3
Ø Sw2(config)#spanning-tree
mst configuration
Ø Sw2(config-mst)#instance 1
vlan 100, 300
Ø Sw2(config-mst)#instance 1
vlan 200
Ø Sw3(config)#spanning-tree
mst configuration
Ø Sw3(config-mst)#instance 1
vlan 100, 300
Ø Sw3(config-mst)#instance 1
vlan 200
That’s the first step, we have now
defined identical instances and Vlan’s Mappings to those Instances each of our
Switches, now let’s go to Sw1 and say that, it is going to be
the Root specifically the Primary Root for Instance 1 and we will say, it can
be Secondary Root for Instance 2, to do that we need go to Global Configuration
Mode.
Ø Sw1(config)#spanning-tree
mst 1 root primary
ü
1: - instance number 1
Ø Sw1(config)#spanning-tree
mst 2 root secondary
Ø
2: - instance number 2
Let’s go to Switch Sw3 and say that, it’s gonna be Primary for Vlan 200, another word, Primary
Root for instance 2 and it’s gonna be Secondary for instance number 1.
Ø Sw3(config)#spanning-tree
mst 2 root primary
Ø Sw3(config)#spanning-tree
mst 1 root secondary
We have, now defined our MSTP
or MST Configuration on all over our Switches, their identical
configuration, that we want to be Primary and
Secondary Roots for the instances.
Now we need to go in each Switches and Turned on or
enable MST
Ø Sw1(config)#spanning-tree
mode mst
Ø Sw2(config)#spanning-tree
mode mst
Ø Sw3(config)#spanning-tree
mode mst
ü we have now enabled MSTP on all our
switches.
Let’s see, it’s behaving as
like we suspect
Ø Sw1#show spanning-tree
summary
And we can see that, the Switch
is in MSTP Mode, and we can see
our 3 instances, remember there was MST0, that not something that we create,
it’s instance 0, and we can see that we created Two instances 1 and 2.
Another Verification Command, that’s
really handy is
Ø Sw1#spanning-tree mst
configuration
And this tells us that, we have
three instances, the 0 instance
which is there by default. Notice it, includes all Vlan’s that we did not assigned to a different
instances and Two Instances, that
we can configured, instance 1 contains Vlan 100
and 300, instance 2 contains Vlan
200, and we were talking earlier about the parameter that define
Multiple Spanning Trees Protocol Configuration on
a Switch, and we said, we can set a
Ø Name
Ø Revision Number
We
can see here that, we didn’t set Name and
Revision Number, it’s still works but it’s a good practice to
set those, especially when we start changes, we
would want to start incrementing that Revision Number and if we did have
Multiple Region, we would want to them have unique names.
Now go to Global Configuration Mode, and Specify Name
and a Revision Number.
Ø
Sw1(config)#spanning-tree mst configuration
Ø
Sw1(config-mst)#name SWITCH
Ø
Sw1(config-mst)#revision 1 (1:-
Revision Number)
Ø
Sw1(config-mst)#end
Now
make that match on other Switches
Ø
Sw2(config)#spanning-tree mst configuration
Ø
Sw2(config-mst)#name SWITCH
Ø
Sw2(config-mst)#revision 1
Ø
Sw2(config-mst)#end
Ø
Sw3(config)#spanning-tree mst configuration
Ø
Sw3(config-mst)#name SWITCH
Ø
Sw3(config-mst)#revision 1
Ø
Sw3(config-mst)#end
Now all that information matches,
let’s reissue the command we gave earlier
Ø
Sw1#show spanning-tree mst configuration
ü Now we can see, here is our name,
here is our Revision number
Verification & Troubleshooting
Commands
Ø
Sw1#show spanning-tree vlan 100 or 300
Ø
Sw1#show spanning-tree vlan 200
Ø
Sw3#show spanning-tree vlan 200
And
one more Verification Command is
Ø
Sw3#show spanning-tree mst configuration digest
Remember,
we said that when we are exchanging information
with other switches in our Region, we are not sending the entire Mapping Table,
we were sending a digest a Hash digest of that Table.
Here we get to see that digest, if we tell a neighboring switch that the name of my Multiple
Spanning Trees Protocol configuration is SWITCH, our Revision Number is 1, and
here is my digest if that information matching on those neighboring switches,
then those switches would know their part of the same Multiple Spanning Trees Protocol Region, and
by the way, we can influence our Path Selection,
similar to how we did it with PVST+.
We can directly
manipulate a Port Cost, notice the Cost are
different then, we had with PVST+, the valid range of Cost value is anywhere
from 1 to 200 million, here based on Interface Speed of 100mpbs, we got a Cost of 200000,
however the Port identifiers that we talked about
previously, they are the same, and if we need to we can manipulate those Port
ID’s to influence Path Selection.
And we wrap up this
demonstration of MSTP with a design best practice
and that is Cisco recommendation that, we using MSTP on our switches, that
those switches be interconnected with Trunks, and here is big difference.
Cisco
Strongly recommends that, we do not PRUN any of Vlan’s of those Trunks, even
though it’s fine to do that with PVST+, if we start Pruning Vlan’s of the
Trunks interconnected our Switches, that could leave to connectivity issues
with MSTP.
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