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Catalyst 3750 MetroSwitch Software Configuration Guide
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Chapter 13 Configuring IEEE 802.1Q and Layer 2 Protocol Tunneling
Understanding Layer 2 Protocol Tunneling
• VTP provides consistent VLAN configuration throughout the customer network, propagating
through the service provider to all switches.
Note To provide interoperability with third-party vendors, you can use the Layer 2 protocol-tunnel bypass
feature. Bypass mode transparently forwards control PDUs to vendor switches that have different ways
of controlling protocol tunneling. You implement bypass mode by enabling Layer 2 protocol tunneling
on the egress trunk port.
Layer 2 protocol tunneling can be used independently or can enhance 802.1Q tunneling. If protocol
tunneling is not enabled on 802.1Q tunneling ports, 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. Customer switches on different sites that send
traffic through the service-provider network with 802.1Q tunneling achieve complete knowledge of the
customer’s VLAN. If 802.1Q tunneling is not used, you can still enable Layer 2 protocol tunneling by
connecting to the customer switch through access ports or trunk ports and enabling tunneling on the
service-provider access or trunk port.
For example, in Figure 13-5, Customer X 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 X, Site 1 will build a spanning tree on the switches at that site without considering
convergence parameters based on Customer X’s switch in Site 2. This could result in the topology shown
in Figure 13-6.
Figure 13-5 Layer 2 Protocol Tunneling
Customer X Site 2
VLANs 1 to 100
Customer Y Site 2
VLANs 1 to 200
Customer Y Site 1
VLANs 1 to 200
Customer X Site 1
VLANs 1 to 100
VLAN 30
Trunk
ports
Switch A
Trunk
ports
VLAN 30
VLAN 40
Service
provider
101822
Trunk
Asymmetric link
VLAN 30
VLAN 40
Trunk
ports
Switch B
Switch C
Switch D
Trunk
ports
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