CANOGA PERKINS 9151 Network Interface Device - Manual (Page 30)

Ethernet bridges or switches use the Spanning Tree Protocol (STP), an algorithm that creates a logical
topology that connects all network segments and ensures only one path between any two stations.
When STP is enabled, the 9151 monitors the incoming data packets and periodically sends Bridge
Protocol Data Units (BPDU). STP monitors the incoming BPDUs to detect any loops. If the same
BPDU arrives on two ports, STP blocks one port to remove the loop. Because this tree-like structure
spans all nodes in the network, it is called Spanning Tree. Figure 5 shows a typical STP application.
Note: For details on the Spanning Tree Algorithm, see the IEEE 802.1 specifications.
A
C
E
B
D
1
2
3 4
5 6
A small test network
A
C
E
B
D
1
2
3 4
5 6
Spanning Tree from source (C)
Figure 5. Typical Spanning Tree Application.
In a service provider environment, you can group VLANs that carry individual tags from different
customers within a single LAG that carries a unique additional (double) tag. The features that enable
tagging and double tagging add the tag on a packet as it exits a port; or when disabled, strip the tag as
it exits a port. The customer's tag for a VLAN is assigned through the VLAN; the service provider's
(double) tag is derived from the PVID and assigned before the packet exits the 9151. For example in
Figure 6, a packet arrives at customer port 1 with VLAN ID 3. The service provider ID tag, 5, is
added as the packet exits the 9151 through service provider port 1 as part of the LAG. When the
packet arrives at the remote 9151, service provider ID tag 5 is stripped from the packet, but the packet
retains VLAN ID 3 as it exits through customer port 1.
Customer port 1 Customer port 1
Customer port x
Customer port x
Service Provider port 1 Service Provider port 1
Service Provider port 2 Service Provider port 2
Packet
Packet
Packet
Service
Provider ID
Customer
VLAN ID
Customer
VLAN ID
Customer
VLAN ID 3
3
3
5
Figure 6. Double Tagging Example
9151 Network Interface Device 3-16