Brocade Multi-Service IronWare Switching Configuration Guide (Supporting R05.6.00) - Manual (Page 367)

Multi-Service IronWare Switching Configuration Guide 337
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SuperSpan™
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BPDU forwarding
When the Brocade device receives a customer’s BPDU on a boundary interface, the Brocade device
changes the destination MAC address of the BPDU from the bridge group address
(00-00-00-00-00-00) as follows:
• The first byte (locally administered bit) is changed from 01 to 03, to indicate that the BPDU
needs to be tunneled.
• The fourth and fifth bytes are changed to the customer STP ID specified on the boundary
interface.
For example, if the customer's STP ID is 1, the destination MAC address of the customer's BPDUs is
changed to the following: 00-00-00-00-01-00.
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
(00-00-00-00-00-00).
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 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 58 shows an example of how the Preforwarding state is used.
FIGURE 58 SuperSpan Preforwarding state