Example:Configuring an IEEE802.1Q TunnelingPort, on page 143
IEEE 802.1Q Tunneling Configuration Guidelines
Whenyou configure IEEE 802.1Q tunneling, you should always use an asymmetrical link between the customer
deviceand the edge switch, with the customer device port configured as an IEEE 802.1Q trunk port and the
edgeswitch port configuredas a tunnel port.
Assigntunnel ports only to VLANs that are used for tunneling.
Configurationrequirements for native VLANs and for and maximum transmission units (MTUs) are explained
inthese next sections.

Native VLANs

Whenconfiguring IEEE 802.1Q tunnelingon an edge switch, you must use IEEE 802.1Q trunk ports for
sendingpackets into theservice-provider network. However, packets going through the core of the
service-providernetwork can be carried through IEEE 802.1Q trunks, ISL trunks, or nontrunking links. When
IEEE802.1Q trunks are used in these core switches, the native VLANs of the IEEE 802.1Q trunks must not
matchany native VLAN of the nontrunking (tunneling) port on the same switch because traffic on the native
VLANwould not be tagged on the IEEE 802.1Q sending trunk port.
Inthe following network figure, VLAN 40 is configured as the native VLAN for the IEEE 802.1Q trunk port
fromCustomer X at the ingress edge switch in the service-provider network (Switch B). Switch A of Customer
Xsends a tagged packet on VLAN 30 to the ingress tunnel port of Switch B in the service-provider network,
whichbelongs to access VLAN 40. Because the access VLAN of the tunnel port (VLAN 40) is the same as
thenative VLAN of the edge-switch trunk port (VLAN 40), the metro tag is not added to tagged packets
receivedfrom the tunnel port. The packet carries only the VLAN 30 tag through the service-provider network
tothe trunk port of the egress-edge switch (Switch C) and is misdirected through the egress switch tunnel
portto CustomerY.
Theseare some ways to solve this problem:
•Usethe vlan dot1q tag native global configuration commandto configurethe edgeswitch so that all
packetsgoing out an IEEE 802.1Q trunk, includingthe native VLAN, are tagged. If the switch is
configuredto tag native VLAN packets on all IEEE 802.1Q trunks, the switch accepts untagged packets,
butsends only tagged packets.
Catalyst 2960-XR Switch VLAN Configuration Guide, Cisco IOS Release 15.0(2)EX1
OL-29440-01 127
Configuring IEEE 802.1Q and Layer 2 Protocol Tunneling
IEEE 802.1Q Tunneling Configuration Guidelines