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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 re-calculation 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.
PVST
In an STP- or RSTP-enabled LAN, all bridges share one spanning tree. Traffic from all VLANs is
forwarded along the spanning tree, and ports cannot be blocked on a per-VLAN basis to prune loops.
PVST allows every VLAN to have its own spanning tree, which increases usage of links and
bandwidth. Because each VLAN runs STP or RSTP independently, a spanning tree only serves its
VLAN.
A PVST-enabled H3C device can communicate with a third-party device that is running Rapid PVST
or PVST. The PVST-enabled H3C device supports fast network convergence like RSTP when
connected to PVST-enabled H3C devices or third-party devices enabled with Rapid PVST.
Summary of Contents for H3C S7500E-X
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