How Mstp Works - HP 4800G Series Configuration Manual

Table of Contents

In MSTP, port states fall into the following three:

Forwarding: the port learns MAC addresses and forwards user traffic;

Learning: the port learns MAC addresses but does not forward user traffic;

Discarding: the port neither learns MAC addresses nor forwards user traffic.

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

A port state is not exclusively associated with a port role.

each port role ("√" indicates that the port supports this state, while "—" indicates that the port does not

support this state).

Table 1-6 Port states supported by different port roles

Port role (right)

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 called an

MSTI. Among these MSTIs, MSTI 0 is the IST, while all the others are MSTIs. Similar to STP, 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, for each spanning tree. For details, refer to

In MSTP, a VLAN packet is forwarded along the following paths:

Root port/master

Designated port

lists the port state(s) supported by

Alternate port

Table of Contents