How Mstp Works; Mstp Implementation On Devices - HP FlexNetwork NJ5000 User Manual

Table 63 Ports states supported by different port roles
Port state
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, called MSTIs, are calculated. Among
these MSTIs, MSTI 0 is the CIST.
Similar to RSTP, 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 the
VLAN-to-instance mappings. For each spanning tree, MSTP performs a separate calculation
process, which is similar to spanning tree calculation in STP/RSTP. For more information, see
"Calculation process of the STP
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.

MSTP implementation 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 device provides the following functions for ease of
management:
•
Root bridge hold
•
Root bridge backup
•
Root guard
•
BPDU guard
•
Loop guard
•
TC-BPDU (a message that notifies the device of topology changes) guard
•
Support for the hot swapping of interface boards and switchover of the active and standby main
boards.
Port role
Root Designate
port/master port d port
√ √
√ √
√ √
algorithm"
Boundary
port
√
√
√
158
Alternate
Backup port
port
— —
— —
√ √