How Mstp Works - HP A6600 Configuration Manual

• Boundary port: Connects an MST region to another MST region or to an STP/RSTP-running device.
In MSTP calculation, a boundary port's role on an MSTI is consistent with its role on the CIST. But
that is not true with master ports. A master port on MSTIs is a root port on the CIST.
Port states
In MSTP, a port may be in one of the following three states:
• Forwarding: the port receives and sends BPDUs, learns MAC addresses, and forwards user traffic.
Learning: the port receives and sends BPDUs, 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 forwards user
•
traffic.
When in different MSTIs, a port can be in different states.
A port state is not exclusively associated with a port role.
port role ("√" indicates that the port supports this state, while "—" indicates that the port does not support
this state).
Table 12 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 called 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 "
Root port/master
Designated port
port
√ √
√ √
√ √
Table 12 lists the port states supported by each
Alternate port
—
—
√
53
Backup port
—
—
√