Cisco Catalyst 3750 Software Configuration Manual page 269

Chapter 13 Configuring IEEE 802.1Q and 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
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
Figure 13-5 Layer 2 Protocol Tunneling
78-15870-01
Figure 13-5, Customer X has four switches in the same VLAN that are connected
13-6.
Customer X Site 1
VLANs 1 to 100
VLAN 30
VLAN 30
Switch A
Switch B
VLAN 40
Customer Y Site 1
VLANs 1 to 200
Understanding Layer 2 Protocol Tunneling
Service
provider
Trunk
Trunk
ports
ports
Trunk
Trunk
ports
ports
VLAN 40
Trunk
Asymmetric link
Catalyst 3750 Metro Switch Software Configuration Guide
Customer X Site 2
VLANs 1 to 100
VLAN 30
Switch C
Switch D
Customer Y Site 2
VLANs 1 to 200
13-11