Dell PowerConnect B-RX Configuration Manual page 425

Each Brocade device that is configured for SuperSpan forwards the BPDU using the changed
destination MAC address. At the other end of the tunnel, the Brocade device connected to the
customer's network changes the destination MAC address back to the bridge group address
(01-80-c2-00-00-00).
Preforwarding state
To ensure that the customer's network has time to converge at Layer 2 and prevent loops, the
Brocade devices configured for SuperSpan use a special forwarding state, Preforwarding. The
Preforwarding state occurs between the Learning and Forwarding states and by default lasts for
five seconds. During the Preforwarding state, the Brocade device forwards tunneled BPDUs from
customers only and does not forward data traffic. This ensures that the customer's network will
detect the Layer 2 loop and block a port. The SP network remains unblocked. After the
Preforwarding state, the Brocade ports change to the Forwarding state and forward data traffic as
well as BPDUs.
The default length of the Preforwarding state is five seconds. You can change the length of the
Preforwarding state to a value from 3 – 30 seconds.
Figure 35
FIGURE 35
In this example, a customer has two links to the SP. Since the SP is running SuperSpan, the SP
ports enter the Preforwarding state briefly to allow the customer ports connected to the SP to
detect the Layer 2 loop and block one of the ports.
NOTE
If you add a new device to a network that is already running SuperSpan, you must enable SuperSpan
on the new device, at least on the VLANs that will be tunneling the customer traffic. Otherwise, the
new device does not use the Preforwarding state. This can cause temporary loops in the network.
BigIron RX Series Configuration Guide
53-1002253-01
shows an example of how the Preforwarding state is used.
SuperSpan preforwarding state
Cust 1
During Preforwarding state, SP
forwards all tunneled customer
BPDUs, allowing customer time
to detect the loop and block a port.
SuperSpan
root bridge
FWD
Preforwarding
Preforwarding
BLK
SuperSpan™
SP 1
Tunneled BPDU
SP 2
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