HP 9304m Installation And Getting Started Manual page 185

Installation and Getting Started Guide
Figure 6.1 SuperSpan example
In this example, the SP network contains two devices that are running SuperSpan. The SP is connected to two
customer networks. Each customer network is running its own instance of STP. SuperSpan prevents Layer 2
loops in the traffic flow with each customer while at the same time isolating each customer's traffic and spanning
tree from the traffic and spanning trees of other customers. For example, the SP devices provide loop prevention
for Customer 1 while ensuring that Customer 1's traffic is never forwarded to Customer 2. In this example,
customer 1 has two interfaces to the SP network, ports 1/1 and 1/2 connected to SP 1. The SP network behaves
like a non-blocking hub. BPDUs are tunneled through the network. To prevent a Layer 2 loop, customer 1's port
1/2 enters the blocking state.
Customer ID
SuperSpan uses a SuperSpan customer ID to uniquely identify and forward traffic for each customer. You assign
the customer ID as part of the SuperSpan configuration of the HP devices in the SP. In Figure 6.1, the spanning
trees of customer 1 and customer 2 do not interfere with one another because the SP network isolates each
customer's spanning tree based on the SuperSpan customer IDs in the traffic.
BPDU Forwarding
When an HP device receives a customer's BPDU on a boundary interface, the device changes the destination
MAC address of the BPDU from the bridge group address (01-80-c0-00-00-00) 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: 03-80-c0-00-01-00.
Each HP device that is configured for SuperSpan forwards the BPDU using the changed destination MAC
address. At the other end of the tunnel, the HP device connected to the customer's network changes the
destination MAC address back to the bridge group address (01-80-c0-00-00-00).
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
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Port 1/1
Port 1/1
Cust 1
FWD
Port 1/2
BLK
Port 1/2
Port 2/1
Port 1/1
FWD
BLK
Cust 2 Port 1/2
Port 2/2
SuperSpan
root bridge
SP 1
SP 2