H3C Technologies H3C WX6000 Series Access Controllers - Manual (Page 172)

20-14
Table 20-6 Ports states supported by different port roles
Role
State
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 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.
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.
In addition to basic MSTP functions, many management-facilitating special functions are provided, as
follows:
z
Root bridge hold
z
Root bridge backup
z
Root guard
z
BPDU guard
z
Loop guard
z
TC-BPDU guard
Protocols and Standards
MSTP is documented in:
z
IEEE 802.1d: Spanning Tree Protocol