Lessons Learned:
IEEE (802.1s) – is
the open standard response to PVST and PVST+
It supports Rapid STP (802.1w)
Note: With RVST – uplinkfast and backbone fast are enabled
by default.
Instances are
separate from VLANs
-
PVST+ uses one instance per vlan
-
MST use definable instances
MST is highly
scalable
-switches with same instances, config revision number, and
name form a “region”
-different regions see each other as virtual bridges.
Note: Common STP
features are integrated into Rapid STP (uplink fast . backboneFast, portfast,
etc)
The Major difference between MST and PVST – is that with
PVST There is one instance of STP per every vlan you have.
The issue with PVST is that if there are a number of vlans
that are sharing the same physical forwarding path. You still have
Still need to do a separate root bridge and root port
election on a per instance basis.
MST – is generally less overhead than PVST – because you can
define which particular vlans map to the MST instances.
With MST – you have a concept known as Intra-region vs
inter-region scalability
With Intra-region
operability, devices share the same instances of MST. This is the vlan to
instance mapping – they share the config rev # and the name. Any switches that
share the cfg revision and name will be in the same region.
Any switches that do not share that information will be in
different regions. There will a different path selection pre region.
The goal of MST is that between regions – we hide not only
the reachability information but the failure information.
Basically if a link goes down on one region it will not
affect the other regions.
MST Path Selection:
Same election process
as CST/PVST
-Root bridge
Lowest
bridge ID
(made up of priority / system ID ext / and MAC Add)
-
Priority will by 32768 by default
-
Root Port
-lowest cost
-lowest upstream BID
-lowest port ID
Note: with the lowest port ID – this will only be used if
there’s multiple connections to the same upstream bridge.
Changing MST root Bridge election:
Changing MST root Bridge election:
Manually change BID:
Spanning-tree mst (instance ) priority (lower is better)
Use root bridge macro
-spanning-tree mst (instance) root (primary | secondary)
-sets local priority based on currently root bridge
Verification
Sh spaning-tree mst (instance)
Sh spanning-tree root
Note: you
basically are changing the Vlan key word with the MST keyword then vlan # with
MST instance number
Before you change anything – you need to verify:
What are the instance numbers
What are the vlans that belong to those instance numbers
What are the cfg numbers and names of the region.
Note: the default
instance is always going to be zero.
Configuring MST:
From global mode:
Switch01(config)#spanning-tree
mst configuration
(define the instance mappings / name of the region / and cfg
revision number – These ALL have to match for he switches to be in the same
region.
----------------------------------------------
Note: to find out what features are supported on a specific
platform go to:
MST Features – comparison
----------------------------------------------
Set the required parameters:
Switch01(config-mst)#name
Switch01(config-mst)#name MST1
Switch01(config-mst)#rev
Switch01(config-mst)#revision 1
Next we need to configure the instance mappings. What are
the vlan the SW and what vlans to map the instance:
Ex:
Switch01(config-mst)#instance 1 vlan 10,20,40,50
Ex:
Switch01(config-mst)#instance 2 vlan 60,70,80
Note: any vlan
not explicitly listed will fall back to Instance zero.
Result:
spanning-tree mst configuration
name MST1
revision 1
instance 1 vlan 10, 20, 40, 50
instance 2 vlan 60, 70, 80
to keep the other switches in the same region – basically
copy and paste the config into your other switches.
This will stage the configs until you enable MST.
Enable MST:
Once all switches agree on the configuration then you can
enable the configuration by enabling the mode.
From Global mode.
Switch01(config)#spanning-tree
mode mst
This must be enabled on all switches in the MST domain.
Note: If you have not configured the MST perquisites- you’ll
most likely get an error like this:
%SPANTREE-3-PRESTD_NEIGH:
pre-standard MST interaction not configured (FastEthernet0/24). Please,
configure: 'spanning-tree mst pre-standard' on ports connected to MST
pre-standard switches.
MST is backwards compatible with CST and PVST.
To verify the MST config:
Switch01#sh
spanning-tree mst 1
##### MST1 vlans
mapped: 10,20,40,50
Bridge address
000c.303b.ba80 priority 32769 (32768 sysid 1)---- local bridge
·
Root
this switch for MST1 -------Root bridge
Interface Role
Sts Cost Prio.Nbr Type
---------------- ---- --- --------- --------
--------------------------------
Fa0/24 Desg
FWD 200000 128.24 P2p Pre-STD-Rx
Notice the cost value – is more granular to accommodate for higher
speed interfaces, ex: Gig anf ten gigE.
Note: Rapid STP
is automatically enabled when you enable MST. This means ports will transition to
either DESg or BLK / ALT. must faster
than CST.
Now – let’s modify the Root bridge. Currently I have Switch
one as the root for my MST instance 1. So instead of doing this on a perVLAN –
we’re now going to do it on a per instance basis.
=----------------------------
Switch-01:
MST1
Spanning tree
enabled protocol mstp
Root ID Priority
32769
Address 000c.303b.ba80
This
bridge is the root
Hello
Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority
32769 (priority 32768 sys-id-ext
1)
Address 000c.303b.ba80
Hello
Time 2 sec Max Age 20 sec Forward Delay 15 sec
Interface
Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- --------
--------------------------------
Fa0/24
Desg FWD 200000 128.24 P2p Pre-STD-Rx
This can be accomplished two way:
The legacy way of: Spanning-tree root primary or on the
other switch we can say
#spanning-tree MST 1 priority (lower #)
Switch-02:
MST01
Spanning tree
enabled protocol mstp
Root ID Priority
4097 - I
changed the priority to 4096.
Address 0013.60bb.4980
This
bridge is the root
Hello
Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority
4097 (priority 4096 sys-id-ext
1)
Address 0013.60bb.4980
Hello
Time 2 sec Max Age 20 sec Forward Delay 15 sec
Interface Role
Sts Cost Prio.Nbr Type
---------------- ---- --- --------- --------
--------------------------------
Fa0/24 Desg
FWD 200000 128.24 P2p
There’s no root port – because SW2 is not the root.
Note: All VLAN’s that are not configured in an MST instance –
will be put into MST instance 0.
Switch-02#sh spanning-tree root
Root
Hello Max Fwd
MST Instance
Root ID Cost Time Age Dly
Root Port
---------------- -------------------- ------ ----- ---
--- ----------------
MST00 32768
0003.e347.b900 200000 2 20
15 Fa0/24
MST01 4097
0013.60bb.4980 0 2
20 15
MST02 32770 000c.303b.ba80 200000 2
20 15 Fa0/24
------------------------------
Note: VTP still
plays a part in MST. VTP will still advertise the instance of the MST between
the neighbors.
VTP Version 3 will
advertise the MST instance information if you add and remove a VLAN from the instance.
This is in available in release 12.33
Note: The following
command shows the instance mappings.
Switch01#sh
spanning-tree mst configuration
Name [MST1]
Revision 1 Instances configured 3
Instance Vlans mapped
--------
---------------------------------------------------------------------
0
1-9,11-19,21-39,41-49,51-59,61-69,71-79,81-4094
1 10,20,40,50
2 60,70,80
-------------------------------------------------------------------------------
Switch01#
Note: The vlan brief command – shows all vlans. There could
a case where the MST instance does not define all vlans are not defines in an
MST config. The vlans not in an instance will all back to the default MST instance
0.
Sh vlan brief –
Switch01#sh vlan brief
VLAN Name Status Ports
---- -------------------------------- ---------
-------------------------------
1 default active Fa0/1, Fa0/2, Fa0/3, Fa0/4
Fa0/5, Fa0/6, Fa0/7, Fa0/8
Fa0/9, Fa0/10, Fa0/11, Fa0/12
Fa0/13, Fa0/14, Fa0/15, Fa0/16
Fa0/17, Fa0/18, Fa0/19, Fa0/20
Fa0/21, Fa0/22, Fa0/23, Gi0/1
Gi0/2
10 VLAN0010 active
20 VLAN0020 active
40 VLAN0040 active
50 VLAN0050 active
60 VLAN0060 active
70 VLAN0070 active
80 VLAN0080 active
90 VLAN0090 active
1002 fddi-default act/unsup
1003 token-ring-default act/unsup
1004 fddinet-default act/unsup
1005 trnet-default act/unsup
No comments:
Post a Comment