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How MSTP works

MSTP divides an entire Layer 2 network into multiple MST regions, which are interconnected by a

calculated CST. Inside an MST region, multiple spanning trees are calculated, each being an MSTI

(Among these MSTIs, MSTI 0 is called the CIST). Similar to RSTP, MSTP uses configuration BPDUs to

calculate spanning trees. The only difference between the two protocols is that an MSTP BPDU carries the

MSTP configuration on the device from which this BPDU is sent.

CIST calculation

The calculation of a CIST tree is also the process of configuration BPDU comparison. During this process,

the device with the highest priority is elected as the root bridge of the CIST. MSTP generates an IST within

each MST region through calculation, and, at the same time, MSTP regards each MST region as a single

device and generates a CST among these MST regions through calculation. The CST and ISTs constitute

the CIST of the entire network.

MSTI calculation

Within an MST region, MSTP generates different MSTIs for different VLANs based on the

VLAN-to-instance mappings. MSTP performs a separate calculation process, which is similar to spanning

tree calculation in STP/RSTP, for each spanning tree. For more information, see "How STP works."

In M

STP, a VLAN packet is forwarded along the following paths:

• Within an MST region, the packet is forwarded along the corresponding MSTI.

• Between two MST regions, the packet is forwarded along the CST.

MSTP implementation on devices

MSTP is compatible with STP and RSTP. STP and RSTP protocol packets can be recognized by devices

running MSTP and used for spanning tree calculation.

In addition to basic MSTP functions, the device provides the following functions for ease of management:

• Root bridge hold

• Root bridge backup

• Root guard

• BPDU guard

• Loop guard

• TC-BPDU guard

• Support for hot swapping of interface cards and active/standby changeover

Protocols and standards

• IEEE 802.1d, Spanning Tree Protocol

• IEEE 802.1w, Rapid Spanning Tree Protocol

• IEEE 802.1s, Multiple Spanning Tree Protocol

Configuration restrictions and guidelines

When you configure MSTP, follow these restrictions and guidelines:

Other manuals for HP MSR SERIES

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