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Catalyst 3550 Multilayer Switch Software Configuration Guide
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Chapter 15 Configuring 802.1Q and Layer 2 Protocol Tunneling
Understanding Layer 2 Protocol Tunneling
This example shows how to configure an interface as a tunnel port, enable tagging of native VLAN
packets, and verify the configuration. In this configuration, the VLAN ID for the customer connected to
Gigabit Ethernet interface 7 is VLAN 22.
Switch(config)# interface gigabitethernet0/7
Switch(config-if)# switchport access vlan 22
% Access VLAN does not exist. Creating vlan 22
Switch(config-if)# switchport mode dot1q-tunnel
Switch(config-if)# exit
Switch(config)# vlan dot1q tag native
Switch(config)# end
Switch# show dot1q-tunnel interface gigabitethernet0/7
Port
-----
Gi0/1Port
-----
Switch# show vlan dot1q tag native
dot1q native vlan tagging is enabled
Understanding Layer 2 Protocol Tunneling
Customers at different sites connected across a SP network need to use various Layer 2 protocols to scale
their topologies to include all remote sites, as well as the local sites. STP must run properly, and every
VLAN should build a proper spanning tree that includes the local site and all remote sites across the SP
network. Cisco Discovery Protocol (CDP) must discover neighboring Cisco devices from local and
remote sites. VLAN Trunking Protocol (VTP) must provide consistent VLAN configuration throughout
all sites in the customer network.
When protocol tunneling is enabled, edge switches on the inbound side of the SP network encapsulate
Layer 2 protocol packets with a special MAC address and send them across the SP 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 SP network and are delivered to customer
switches on the outbound side of the SP 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 SP
network.
•
VTP provides consistent VLAN configuration throughout the customer network, propagating to all
switches through the SP.
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 SP
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 SP network. Customer switches on different sites that send traffic through the SP 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 and by enabling tunneling on the SP access port.