Extreme Networks ExtremeWare 7.2e, Installation And User Manual

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Rapid Spanning Tree Protocol
ExtremeWare 7.2e Installation and User Guide 207
•
Is a designated port and attaches to another bridge by a point-to-point link and receives an “agree”
message from the other bridge port.
• Is an edge port.
An edge port is a port connected to a non-STP device and is in the forwarding state.
The preceding sections provide more information about RSTP behavior.
Root Port Rapid Behavior
In Figure 34, 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 34: 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 old root port to the blocking state. This may cause a loop.
EW_102a
S4
S5
S3
EAPS1
Partner
S9
S11
S10
S1
S2
EAPS2
EAPS3
Controller
S6
S8
S7
Master
Master
Master
P1
P2
P3
P4
P5
P6
P7
P8
Common link