4000 Router Series Layer 2 - LAN Switching Configuration Guide

• Forwarding—The port receives and sends BPDUs, learns MAC addresses, and forwards user

traffic.

• Learning—The port receives and sends BPDUs, learns MAC addresses, but does not forward user

traffic. Learning is an intermediate port state.

• Discarding—The port receives and sends BPDUs, but does not learn MAC addresses or forward

user traffic.

NOTE:

hen in different MSTIs, a port can be in different states.

A port state is not exclusively associated with a port role. Table 4 lists the port states that each port role

supports. (A check mark [√] indicates that the port supports this state, while a dash [—] indicates that the

port does not support this state.)

Table 4 Port states that different port roles support

Port role (right)

Port state (below)

Root port/master

port

Designated port

Alternate port Backup port

Forwarding √ √ — —

Learning √ √ — —

Discarding √ √ √ √

How MSTP works

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

CST. Inside an MST region, multiple spanning trees, called MSTIs, are calculated. Among these MSTIs,

MSTI 0 is the IST.

Like STP, MSTP uses configuration BPDUs to calculate spanning trees. An important difference is that an

MSTP BPDU carries the MSTP configuration of the bridge from which the 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. 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. For each spanning tree, MSTP performs a separate calculation process

similar to spanning tree calculation in STP. For more information, see "Calculation process of the STP

algor

ithm."

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.