How Mstp Works; Implementation Of Mstp On Devices - HP 5820X series Configuration Manual

A port state is not exclusively associated with a port role.
port role ("√" indicates that the port supports this state, and "—" indicates that the port does not support
this state).
Table 14 Port states supported by different port roles
Port role (right)
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 are calculated, each being an MSTI.
Among these MSTIs, MSTI 0 is the IST. 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.
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. MSTP performs a separate calculation process, which is similar to spanning tree
calculation in STP, for each spanning tree. 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.
•

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, the following functions are provided for ease of management:
Root bridge hold
•
Root bridge backup
•
Root guard
•
BPDU guard
•
Root port/master
Designated port
port
√ √
√ √
√ √
Table 14 lists the port states supported by each
Alternate port
—
—
√
"How STP
69
Backup port
—
—
√
works."