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Multiple Spanning Tree (MST) Notes

Multiple Spanning Tree (MST)

 – 802.1S

 – Called MSTP by Cisco previously

 – Can map VLANs to STP instances

     -> e.g. – 

          -> Instance 1

               -> VLAN 1 – 32

          -> Instance 2

               -> VLAN 33 – 65

          -> Instance 3

               -> VLAN 66 – 100

 – Only supports RSTP for convergence

     -> Activated automatically

 – Problem

     -> Every switch needs to be configured the same

          -> Manually

     -> VTP cannot be used

     -> Use notepad

 – MST creates a set of switches using the same configuration

     -> Called a Region

     -> All have the exact same configuration

     -> Allows up to 16 instances per Region

          -> Instance 0 – 15

          -> Instance 0

               -> Internal Spanning Tree (IST)

               -> By default, all VLANs map to IST

 – Every MST configuration has 3 variables

     -> Region name

          -> Default is NULL

     -> Revision number

          -> Default is 0

     -> Instance to VLAN mapping

          -> Default, all VLANs map to IST

 – Configure one switch, then copy that configuration to all other switches

 – Can configure all of the MST parameters, then activate MST

 – Only one BPDU is sent

     -> Controlled by the IST

     -> Specific information about instances attaches as “M-Records”

SW1(config)# spanning-tree mst configuration

SW1(config-mst)# name CISCO

 revision 10

 instance 1 vlan 1 – 32

 instance 2 vlan 33 – 65

 instance 3 vlan 66 – 100

 show pending

 exit

SW1(config)# spanning-tree mode mst

Scenario -> In instance 0, configure all VLANs except VLANs 1 – 100.

 – If the question asks you to configure all other VLANs to one instance (after first two), only configure two instances.

     -> All other VLANs will be in instance 0

show spanning-tree mst configuration

show spanning-tree mst [id]

MST introduces MAX HOPS feature

 – Allows equal to MAX-AGE time

 – Change MAX-AGE time to configure the maximum hops

MST Interface Costs

 – 10 Mbps —> 2000000

 – 100 Mbps —> 200000

 – 1 Gbps —> 20000

 – 10 Gbps —> 2000

SW1(config)# spanning-tree mst 1 priority 4096

     – or –

SW1(config)# spanning-tree mst 1 root primary

     – or –

SW1(config)# spanning-tree mst 1 root secondary

Can configure who will be the root for IST (instance 0)

Only the IST communicates between Regions

 – Sends a single BPDU

     -> Removes M-Records

          -> This hides the topology of the Region

          -> Similar to the ABR in OSPF

 – One Region appears as a single switch to another Region

CIST Root

 – The best Bridge-ID switch among all Regions

Region Root

 – Instance 0 root bridge in every Region

 – In any Region, only a boundary switch can be a Region root

     -> Exception, the CIST root is also the Region root and does not have to be a boundary switch

     1.  Root bridge

     2.  Cost to reach root bridge (CIST)

          -> Internal cost within the Region are not considered to determine the Region root

          -> Only external cost is considered

     3.  Sender Bridge-ID

     4.  Sender Port-ID

Master Port

 – Root port of any Region

 – On Region root

 – Seen as “master port” by non-IST instances

     -> Going toward the master

show spanning-tree mst 0

 – Shows the root port

show spanning-tree mst 1

 – Show the master port

show run | in mst|name|rev|ins

show spanning-tree mst configuration

 – Should be the same as

     -> show pending

show spanning-tree mst

clear spanning-tree detected-protocols

 – To switch to MST faster

SW1(config)# spanning-tree mode pvst+

no spanning-tree mst configuration

<paste MST configuration from Notepad>

 show pending

 exit

spanning-tree mode mst

SW2# show spanning-tree mst

 E3/0     Root     FWD     Shr Bound (RSTP)

SW3# show spanning-tree mst

 E3/0     Altn     BLK     Shr Bound (RSTP)

 E3/1     Altn     BLK     Shr Bound (RSTP)

SW2# show spanning-tree mst 1

 E3/0     Mstr     FWD     Shr Bound (RSTP)

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