spanning-tree port cost is of local significance

When you are tuning spanning tree topology, to force a port into blocking or forwarding mode.

Increasing the port cost on a switch makes it less desirable for that switch and if its cost is higher than the current route, this port will move from forwarding to blocking.

p19s3#sh spanning-tree vlan 4 | i Int
Interface Fa0/3 (port 15) in Spanning tree 4 is FORWARDING
Interface Fa0/5 (port 17) in Spanning tree 4 is BLOCKING
Interface Fa0/6 (port 18) in Spanning tree 4 is BLOCKING

p19s3(config-if)#int f0/3
p19s3(config-if)#spanning-tree vlan 4 cost 57

p19s3#sh spanning-tree vlan 4 | i Int
Interface Fa0/3 (port 15) in Spanning tree 4 is BLOCKING
Interface Fa0/5 (port 17) in Spanning tree 4 is LISTENING
Interface Fa0/6 (port 18) in Spanning tree 4 is BLOCKING

By looking at the network topology you can count the hops to the root through the path you prefer, and set the cost of the current port forwarding traffic to a higher value.

Each hop cost is different depending on the link speed.

MST configured switches represent one giant virtual switch so you do not need to count hops inside MST.

MST - IST - CST - PVST all the confusing terms.

PVST - Per VLAN Spanning Tree

• ·         One spanning tree instance per vlan.
• ·         Can achieve load balancing.
• ·         CPU resource intensive, multiple vlans and few logical topologies.

CST – Common Spanning Tree - 802.1q

• ·         One spanning tree for all VLANs.
• ·         No load balancing.
• ·         CPU is spared.

MST - Multiple Spanning Tree – 802.1s

• ·         Combines best of PVST and CST.
• ·         Multiple VLANs mapped to a spanning tree instance.
• ·         Can achieve load balancing and Spare the cpu.
• ·         Mandates that an MST bridge be able to handle IST and one or more MSTIs.

MST Region

·         Maintains the mapping of VLAN to STP instance.

·         Each MST region switches share same config
(MST Region name, revision number, VLAN-to-STP mapping).

·         A digest of the VLAN-to-STP is sent in the BPU if the received digest
matches the local digest the switch port is in the region,
if it differs the switch knows it’s a boundary port.

IST – Internal Spanning Tree

·         Allows for interoperation between MST and CST.

·         IST represents the entire MST region as a virtual switch running CST.

·         Sends BPDU outside of the MST region.

·         IST is MSTI 0

MSTI - MST Instances

·         MSTIs are RSTP instances that run only inside the MST region.

·        Does not send BPDUs outside of the region.


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The relation between them and how trees are formed.

This is intended to be a summary.
for more details refer to Understanding MST