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Software Configuration Guide—Release 15.0(2)SG
OL-23818-01
Chapter 25 Configuring 802.1Q Tunneling, VLAN Mapping, and Layer 2 Protocol Tunneling
About Layer 2 Protocol Tunneling
• VTP provides consistent VLAN configuration throughout the customer network, propagating to all
switches through the service provider.
Layer 2 protocol tunneling can enabled on trunk, access and tunnel ports. If protocol tunneling is not
enabled, remote switches at the receiving end of the service provider network do not receive the PDUs
and cannot properly run STP, CDP, and VTP. When protocol tunneling is enabled, Layer 2 protocols
within each customer's network are totally separate from those running within the service provider
network.
As an example, Customer A in Figure 25-6 has four switches in the same VLAN that are connected
through the service provider 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 builds a spanning tree on the switches at that site without considering convergence
parameters based on Customer A’s switch in Site 2. Figure 25-6 shows one possible spanning tree
topology.
Figure 25-6 Layer 2 Protocol Tunneling
Customer A Site 2
VLANs 1 to 100
Customer B Site 2
VLANs 1 to 200
Customer B Site 1
VLANs 1 to 200
Customer A Site 1
VLANs 1 to 100
VLAN 30
Trunk
ports
Switch 1
Switch 1
Trunk
ports
VLAN 30
VLAN 40
Service
provider
74073
Trunk
Asymmetric link
VLAN 30
VLAN 40
Trunk
ports
Switch 2
Switch 3
Switch 4
Trunk
ports