R2511-HP MSR Router Series Layer 2 - LAN Switching Configuration Guide(V5)

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However, the newly calculated configuration BPDU cannot be propagated throughout the network
immediately, so the old root ports and designated ports that have not yet detected the topology change
continue to forward 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 as follows:
• 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
right away, a temporary loop will likely occur.
For this reason, as a mechanism for state transition in STP, the newly elected root ports or
designated ports require twice the forward delay time before they transit to the forwarding state to
make sure that the new configuration BPDU has propagated throughout the network.
• Hello time—The device sends hello packets at the hello time interval to neighboring devices to make
sure that the paths are valid.
• 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 if the old root port on the device has
stopped forwarding data and 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 (a port that
directly connects to a user terminal rather than to another network device or a shared LAN segment) or
it connects to a point-to-point link. Edge ports directly enter the forwarding state. Connecting to a
point-to-point link, a designated port enters the forwarding state immediately after the device receives a
handshake response from the directly connected device.
MSTP
STP and RSTP limitations
STP does not support rapid state transition of ports. A newly elected port must wait twice the forward
delay time before it transits to the forwarding state, even if it connects to a point-to -point link or is an edge
port.
Although RSTP supports rapid network convergence, it has the same drawback as STP. All bridges within
a LAN share the same spanning tree, and packets from all VLANs are forwarded along the same
spanning tree, so redundant links cannot be blocked based on VLAN, and load sharing among VLANs
cannot be implemented.