H3C S7500E Series Ethernet Switches Operation Manual

Operation Manual – MSTP
H3C S7500E Series Ethernet Switches Chapter 1 MSTP Configuration
1-17
A port state is not exclusively associated with a port role. Table 1-6 lists the port state(s)
supported by each port role (“√” indicates that the port supports this state, while
“—“ indicates that the port does not support this state).
Table 1-6 Ports states supported by different port roles
Role
State
Root
port/Master
port
Designated
port
Alternate port Backup port
Forwarding √ √ — —
Learning √ √ — —
Discarding √ √ √ √
III. 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
generated through calculation, each spanning tree called an MST instance. Among
these MST instances, instance 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.
1) CIST calculation
By comparison of configuration BPDUs, 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.
2) MSTI calculation
Within an MST region, MSTP generates different spanning tree instances 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
How STP works.
In MSTP, a VLAN packet is forwarded along the following paths:
z Within an MST region, the packet is forwarded along the corresponding MSTI.
z Between two MST regions, the packet is forwarded along the CST.
IV. Implementation of MSTP on devices
MSTP is compatible with STP and RSTP. STP and RSTP protocol packets can be
recognized by devices running MSTP and used for spanning tree calculation.