HP ProCurve 9304M Installation And Configuration Manual page 272

Installation and Basic Configuration Guide
Preforwarding State
To ensure that the customer's network has time to converge at Layer 2 and prevent loops, the HP 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 HP 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 HP 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 8.27 shows an example of how the Preforwarding state is used.
Figure 8.27 SuperSpan Preforwarding state
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 the wrong ports to be blocked.
Mixing Single STP and Multiple Spanning Trees
You can use SuperSpan in any of the following combinations:
• Customer and SP networks both use multiple spanning trees (a separate spanning tree in each VLAN).
• Customer uses multiple spanning trees but SP uses Single STP (all STP-enabled VLANs are in the same
spanning tree).
• Customer uses Single STP but SP uses multiple spanning trees.
• Customer and SP networks both use Single STP.
The following sections provide an example of each combination.
8 - 66
FWD
Cust 1
BLK
During Preforwarding state, SP
forwards all tunneled customer
BPDUs, allowing customer time
to detect the loop and block a port.
SuperSpan
root bridge
SP 1
FWD
FWD
SP 2