HP VPN Firewall Appliances Network Management Configuration Guide

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• 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 can be in one of the following 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 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. Table 11 lists port states that each port ro
le supports. A check mark [√] indicates that the port
supports this state, while a dash [—] indicates that the port does not support this state.
Table 11 Port states that different port roles support
Port role (right)
Port state (below)
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 connected by a calculated
CST. Inside an MST region, multiple spanning trees are calculated. Each spanning tree is called an
"MSTI." Among these MSTIs, MSTI 0 is the IST.
Like STP, MSTP uses configuration BPDUs to calculate spanning trees. An important difference is that an
MSTP BPDU carries the MSTP configuration of the bridge 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. 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 VLAN-to-instance
mappings. For each spanning tree, MSTP performs a separate calculation process similar to spanning
tree calculation in STP. For more information, see "STP algorithm."
In M
STP, a VLAN packet is f
orwarded 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.