Network, Security and OS: 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.