3Com Switch 8800 Configuration Guide
Chapter 38 MSTP Region-configuration
38-6
7 6 5 4 3 2 1 0
Tc
Proposal
Learning
Forwarding
Agreem ent
TcAck
7 6 5 4 3 2 1 0
Tc
Proposal
Learning
Forwarding
Agreem ent
TcAck
Figure 38-5
Meaning of 1-byte Flags in BPDU packets
The second and third bits together indicate MSTP port role.
2) TC
packet
A TC packet is also an MSTP BPDU packet, but the lowest bit of its flags field is set to 1,
which endows the TC packet with special meaning. So the TC packet has its special
meaning. After receiving or detecting TC packets, a port will broadcast TC packets to
tell the whole network the changed topology information at the fastest speed.
38.1.2 MSTP Principles
MSTP divides the entire Layer 2 network into several MST regions and calculates and
generates CST for them. Multiple spanning trees are generated in a region and each of
them is called an MSTI. The instance 0 is called IST, and others are called MSTI.
Similar to RSTP, MSTP also use configuration messages to calculate and generate
spanning trees, the difference is that it is the MSTP configuration information on the
switches that is carried in the configuration messages.
I. CIST calculation
The CIST root is the highest-priority switch elected from the switches on the entire
network through comparing their configuration BPDUs. MSTP calculates and
generates IST in each MST region; at the same time it regards each MST region as a
single "switch" and then calculates and generates the CST between the regions. The
CST and ISTs together make up the CIST which connects all the switches in the whole
switching network.
II. MSTI calculation
Inside an MST region, MSTP generates different MSTIs for different VLANs according
to the association between VLAN and the spanning tree. The calculation process of
MSTI is like that of RSTP.
The following introduces the calculation process of one MSTI.
The fundamental of STP is that the switches exchange a special kind of protocol packet
(which is called configuration Bridge Protocol Data Units, or BPDU, in IEEE 802.1D) to
decide the topology of the network. The configuration BPDU contains the information
enough to ensure the switches to compute the spanning tree.
Figure 38-6 shows the Designated bridge and designated port.
