3Com Switch 8800 Configuration Guide Chapter 38 MSTP Region-configuration
38-4
z Master port is the port connecting the entire region to the Common Root Bridge
and located on the shortest path between them.
z An alternate port is a backup of the mater port, and also a backup port of a root
port in the region. As a backup of the mater port, an alternate port will become a
new master port after a master port is blocked.
z If two ports of a switch are connected, there must be a loop. In this case, the switch
blocks one of them. The blocked one is called a backup port.
A port can play different roles in different spanning tree instances.
The following figure illustrates the earlier-mentioned concepts for your better
understanding. In this figure, the switch A, B, C, and D make up a MST region. Port 1
and 2 on switch A connects to the common root bridge; port 5 and 6 on switch C forms
a loop; port 3 and 4 on switch D connects to other MST regions in the downstream
direction.
C
A
B
D
Port 1 Port 2
Master port Alternate port
Port 3 Port 4
Port 5 Port 6
Backup port
Edge port
Designated
port
Connect to the root
MST area
Figure 38-2 Port roles
XI. TC packet
Topology change (TC) means the structure of the MSTP spanning tree changes due to
some bridge change or some port change on the network. In versatile routing platform
(VRP) implementation, when a port state changes from discarding to forwarding, it
means TC occurs.
The following section describes two kinds of STP packets:
1) MSTP BPDU packet
MSTP modules communicate with each other among bridges by MSTP BPDU packets.
The following figure shows the MSTP BPDU packet format: