H3C Technologies H3C S5560 Series Switches - Manual (Page 102)

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• If a designated port receives a configuration BPDU with a lower priority than its configuration BPDU,
the port immediately responds with its configuration BPDU.
• If a path fails, the root port on this path no longer receives new configuration BPDUs and the old
configuration BPDUs will be discarded due to timeout. The device generates a configuration BPDU
with itself as the root and sends the BPDUs and TCN BPDUs. This triggers a new spanning tree
calculation process to establish a new path to restore the network connectivity.
However, the newly calculated configuration BPDU cannot be propagated throughout the network
immediately. As a result, the old root ports and designated ports that have not detected the topology
change continue forwarding data along the old path. If the new root ports and designated ports begin
to forward data as soon as they are elected, a temporary loop might occur.
STP timers
The most important timing parameters in STP calculation are forward delay, hello time, and max age.
• Forward delay
Forward delay is the delay time for port state transition.
A path failure can cause spanning tree recalculation to adapt the spanning tree structure to the
change. However, the resulting new configuration BPDU cannot propagate throughout the
network immediately. If the newly elected root ports and designated ports start to forward data
immediately, a temporary loop will likely occur.
The newly elected root ports or designated ports require twice the forward delay time before they
transit to the forwarding state. This allows the new configuration BPDU to propagate throughout
the network.
• Hello time
The device sends hello packets at the hello time interval to the neighboring devices to make sure
the paths are fault-free.
• 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.
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.
A newly elected RSTP root port rapidly enters the forwarding state when the following conditions exist:
• The old root port on the device has stopped forwarding data.
• The upstream designated port has started forwarding data.
A newly elected RSTP designated port rapidly enters the forwarding state if it is an edge port or it
connects to a point-to-point link.
• A port that directly connects to a user terminal can be configured as an edge port. Edge ports
directly enter the forwarding state.
• When a designated port connects to a point-to-point link, it enters the forwarding state immediately
after the device receives a handshake response from the directly connected device.