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Software Configuration Guide—Release 12.2(25)SG
OL-7659-03
Chapter18 Configuring 802.1 Q 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 2/7 is VLAN 22.
Switch(config)# interface gigabitethernet2/7
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 gigabitethernet2/7
Port
-----
LAN Port(s)
-----
Gi2/7
Switch# show vlan dot1q tag native
dot1q native vlan tagging is enabled globally
Understanding Layer 2 Protocol Tunneling
Customers at different sites connected across a Service Provider network need to use various Layer 2
protocols to scale their topologies to include all remote and 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
Service Provider 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 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 ju st 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, pr opagating to all
switches through the Service Provider.
Layer 2 protocol tunneling can be used independently or can enhance 802.1Q tunneling. If prot ocol
tunneling is not enabled on 802.1Q tunneling ports, remote switches at t he 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 tota lly 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 and by enabling tunneling on the Service
Provider access port.
As an example, Customer A in Figure18-4 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