R2511-HP MSR Router Series Layer 2 - LAN Switching Configuration Guide(V5)

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. 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 VLAN-to-instance

mappings. For each spanning tree, MSTP performs a separate calculation process in the same way the

spanning tree is calculated in STP. For more information, see "STP algorithm."

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. Devices that run MSTP can recognize STP and RSTP protocol

packets used for spanning tree calculation.

In addition to basic MSTP functions, the following functions are provided 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 on MSR50 routers

Protocols and standards

• IEEE 802.1d, Media Access Control (MAC) Bridges

• IEEE 802.1w, Part 3: Media Access Control (MAC) Bridges—Amendment 2: Rapid Reconfiguration

• IEEE 802.1s, Virtual Bridged Local Area Networks—Amendment 3: Multiple Spanning Trees

Spanning tree configuration task lists

Configuration prerequisites

Before configuring a spanning tree, you must determine the spanning tree protocol to be used (STP, RSTP,

or MSTP) and plan device roles (the root bridge or leaf node).

Configuration restrictions and guidelines

• If GVRP and a spanning tree protocol are enabled on a device at the same time, GVRP packets are

forwarded along the CIST. To advertise a certain VLAN within the network through GVRP, make