84
•
Max age
The device uses the max age to determine whether a stored configuration BPDU has expired
and discards it if the max age is exceeded. By default, the max age is 20 seconds. In the CIST
of an MSTP network, the device uses the max age timer to determine whether a configuration
BPDU received by a port has expired. If it is expired, a new spanning tree calculation process
starts. The max age timer does not take effect on MSTIs.
If a port does not receive any configuration BPDUs within the timeout period, the port transits to the
listening state. The device will recalculate the spanning tree. It takes the port 50 seconds to transit
back to the forwarding state. This period includes 20 seconds for the max age, 15 seconds for the
listening state, and 15 seconds for the learning state.
To ensure a fast topology convergence, make sure the timer settings meet the following formulas:
•
2 × (forward delay – 1 second)
≥
max age
•
Max age
≥
2 × (hello time + 1 second)
RSTP
RSTP achieves rapid network convergence by allowing a newly elected root port or designated port
to enter the forwarding state much faster than STP.
RSTP protocol frames
An RSTP BPDU uses the same format as an STP BPDU except that a Version1 length field is added
to the payload of RSTP BPDUs. The differences between an RSTP BPDU and an STP BPDU are as
follows:
•
Protocol version ID
—The value is 0x02 for RSTP.
•
BPDU
type
—The value is 0x02 for RSTP BPDUs.
•
Flags
—All 8 bits are used.
•
Version1
length
—The value is 0x00, which means no version 1 protocol information is
present.
RSTP does not use TCN BPDUs to advertise topology changes. RSTP floods BPDUs with the TC
flag set in the network to advertise topology changes.
Basic concepts in RSTP
Port roles
In addition to root port and designated port, RSTP also uses the following port roles:
•
Alternate
port
—Acts as the backup port for a root port. When the root port is blocked, the
alternate port takes over.
•
Backup
port
—Acts as the backup port of a designated port. When the designated port is
invalid, the backup port becomes the new designated port. A loop occurs when two ports of the
same spanning tree device are connected, so the device blocks one of the ports. The blocked
port is the backup port.
•
Edge
port
—Directly connects to a user host rather than a network device or network segment.
Port states
RSTP uses the discarding state to replace the disabled, blocking, and listening states in STP.
shows the differences between the port states in RSTP and STP.