Cisco Catalyst 3550 series Software Configuration Manual page 330

Understanding Layer 2 Protocol Tunneling
As an example, Customer A in
through the SP network. If the network does not tunnel PDUs, switches on the far ends of the network
cannot properly run STP, CDP, and VTP. For example, STP for a VLAN on a switch in Customer A's
Site 1 will build a spanning tree on the switches at that site without considering convergence parameters
based on Customer A's switch in Site 2. This could result in the topology shown in
Figure 15-4 Layer 2 Protocol Tunneling
Figure 15-5 Layer 2 Network Topology without Proper Convergence
Catalyst 3550 Multilayer Switch Software Configuration Guide
15-8
Figure 15-4
Customer A Site 1
VLANs 1 to 100
VLAN 30
VLAN 30
Switch 1
Switch 1
Switch 2
VLAN 40
Customer B Site 1
VLANs 1 to 200
Customer A
virtual network
VLANs 1 to 100
Chapter 15 Configuring 802.1Q and Layer 2 Protocol Tunneling
has four switches in the same VLAN that are connected
Service
provider
Trunk
Trunk
ports
ports
Trunk
Trunk
ports ports
VLAN 40
Trunk
Asymmetric link
Figure 15-5.
Customer A Site 2
VLANs 1 to 100
VLAN 30
Switch 3
Switch 4
Customer B Site 2
VLANs 1 to 200
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