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Catalyst 3750 SwitchSoftware Configuration Guide
OL-8550-09
Chapter17 Configuring IEEE 802.1Q and Layer 2 Protocol Tunneling
Understanding Layer 2 Protocol Tunneling
When protocol tunneling is enabled, edge switches on the inbound side of the service-provider network
encapsulate Layer 2 protocol packets with a special MAC address and send them across the
service-provider network. Core switches in the network do not process these packets but forward them
as normal packets. Layer 2 protocol data units (PDUs) for CDP, STP, or VTP cross the service-provider
network and are delivered to customer switches on the outbound side of the service-provider network.
Identical packets are received by all customer ports on the same VLANs with these results:
•Users on each of a customer’s sites can properly run STP, and every VLAN can build a correct
spanning tree based on parameters from all sites and not just from the local site.
•CDP discovers and shows information about the other Cisco devices connected through the
service-provider network.
•VTP provides consistent VLAN configuration throughout the customer network, propagating to all
switches through the service provider.
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. When Layer 2 protocol tunneling is enabled on the trunk port, the encapsulated
tunnel MAC address is removed and the protocol packets have their normal MAC address.
Layer 2 protocol tunneling can be used independently or can enhance IEEE 802.1Q tunneling. If protocol
tunneling is not enabled on IEEE 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 IEEE 802.1Q tunneling achieve complete knowledge
of the customer’s VLAN. If IEEE 802.1Q tunneling is not used, you can still enable Layer 2 protocol
tunneling by connecting to the customer switch through access ports and by enabling tunneling on the
service-provider access port.
For example, in Figure17-4, 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 17-5.