Qtech QSW-2900-24T-AC [153/209] Protocols and standards

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c) VII. How MSTP works

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

CST. Inside an MST region, multiple spanning trees are generated through computing, each spanning tree called an

MST instance. Among these MST instances, instance 0 is the IST, while all the others are MSTIs. Similar to STP,

MSTP uses configuration BPDUs to compute spanning trees. The only difference between the two protocols being in

that what is carried in an MSTP BPDU is the MSTP configuration on the device from which this BPDU is sent.

1) CIST computing

By comparison of “configuration BPDUs”, the device with the highest priority is elected as the root bridge of

the CIST. MSTP generates an IST within each MST region through computing, and, at the same time, MSTP regards

each MST region as a single device and generates a CST among these MST regions through computing. The CST

and ISTs constitute the CIST of the entire network.

2) MSTI computing

Within an MST region, MSTP generates different MSTIs for different VLANs based on the VLAN-to-instance

mappings.

MSTP performs a separate computing process, which is similar to spanning tree computing in STP, for each

spanning tree.

In MSTP, 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.

d) VIII. Implementation of MSTP 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 computing.

In addition to basic MSTP functions, many management-facilitating special functions are provided, as

follows:

· Root bridge hold

· Root bridge backup

· Root guard

· BPDU guard

· Loop guard

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

10.1.3 Protocols and Standards

MSTP is documented in:

· IEEE 802.1D: Spanning Tree Protocol

· IEEE 802.1w: Rapid Spanning Tree Protocol

· IEEE 802.1s: Multiple Spanning Tree Protocol