How mstp works, Cist calculation, Msti calculation – H3C Technologies H3C S12500 Series Switches User Manual
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•
Forwarding—The port receives and sends BPDUs, learns MAC addresses, and forwards user
traffic.
•
Learning—The port receives and sends BPDUs, and learns MAC addresses, but does not forward
user traffic. Learning is an intermediate port state.
•
Discarding—The port receives and sends BPDUs, but does not learn MAC addresses or forward
user traffic.
When in different MSTIs, a port can be in different states. A port state is not exclusively associated with
a port role.
lists the port states supported by each port role ("√" indicates that the port supports
the state, and "—" indicates that the port does not support the state).
Table 10 Port states supported by different port roles
Port role (right)
Root port/master
port
Designated port
Alternate port
Backup port
Port state
(below)
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, called MSTIs, are calculated. Among
these MSTIs, MSTI 0 is the IST.
Similar to STP, MSTP uses configuration BPDUs to calculate spanning trees. However, an important
difference is that an MSTP BPDU carries the MSTP configuration of the device from which the 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. For each spanning tree, MSTP performs a separate calculation process,
which is similar to spanning tree calculation in STP. For more information, see "
In MSTP, a VLAN packet is forwarded along the following paths:
•
Within an MST region, the packet is forwarded along the corresponding MSTI.
•
Between two MST regions, the packet is forwarded along the CST.