Extreme Networks ExtremeWare XOS 10.1, Manual

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Rapid Spanning Tree Protocol
ExtremeWare XOS 10.1 Concepts Guide 153
The following sections provide more information about RSTP behavior.
Root Port Rapid Behavior
In Figure 15, the diagram on the left displays the initial network topology with a single bridge having
the following:
• Two ports connected to a shared LAN segment
• One port is the designated port
• One port is the backup port
The diagram on the right displays a new bridge that:
• Is connected to the LAN segment
• Has a superior STP bridge priority
• Becomes the root bridge and sends a BPDU to the LAN that is received by both ports on the old
bridge
Figure 15: Example of root port rapid behavior
If the backup port receives the BPDU first, STP processes this packet and temporarily elects this port as
the new root port while the designated port’s role remains unchanged. If the new root port is
immediately put into the forwarding state, there is a loop between these two ports.
To prevent this type of loop from occurring, the recent backup timer starts. The root port transition rule
does not allow a new root port to be in the forwarding state until the recent backup timer expires.
Another situation may arise if you have more than one bridge, and you lower the port cost for the
alternate port which makes it the new root port. The previous root port is now an alternate port.
Depending on your STP implementation, STP may set the new root port to the forwarding state before
setting the alternate port to the blocking state. This may cause a loop.
To prevent this type of loop from occurring, the recent root timer starts when the port leaves the root
port role. The timer stops if the port enters the blocking state. RSTP requires that the recent root timer
stops on the previous root port before the new root port can enter the forwarding state.
Backup
port
Designated
port
LAN segment
Inital topology
EX_054
Bridge
Backup
port
Designated
port
Superior STP
bridge priority
New topology
Bridge
Root
bridge