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Software Configuration Guide—Release 12.2(25)SG
OL-7659-03
Chapter18 Configuring 802.1Q and Layer 2 Protocol Tunneling
Configuring 802.1Q Tunneling
Configuring 802.1Q Tunneling
These sections describe 802.1Q tunneling configuration:
•802.1Q Tunneling Configuration Guidelines, page 18-4
•802.1Q Tunneling and Other Features, page 18-5
•Configuring an 802.1Q Tunneling Port, page 18-6
Note By default, 802.1Q tunneling is disabled because the default switch port mode is dynam ic auto. Tagging
of 802.1Q native VLAN packets on all 802.1Q trunk ports is also disabled.

802.1Q Tunneling Configuration Guidelines

When you configure 802.1Q tunneling, you should always use asymmetrical links for traffic going
through a tunnel and should dedicate one VLAN for each tun nel. You should also be aware of
configuration requirements for native VLANs and maximum transmission uni ts (MTUs). For more
information about MTUs, see the “System MTU” section on page 18-5.

Native VLANs

When configuring 802.1Q tunneling on an edge switch, you must use 802.1Q trunk ports for sending
packets into the Service Provider network. However, packets going through the core of the Service
Provider network can be carried through 802.1Q trunks, ISL trun ks, or nontrunking links. When 802.1Q
trunks are used in these core switches, the native VLANs of the 802.1Q trunks must not match any native
VLAN of the nontrunking (tunneling) port on the same switch because traffic on the native VLAN would
not be tagged on the 802.1Q sending trunk port.
See Figure18-3. VLAN 4 0 is configured as the native VLAN for the 802.1Q trunk port from Cust omer A
at the ingress edge switch in the Service Provider network (Switch 2). Switch 1 of Customer A sends a
tagged packet on VLAN 30 to the ingress tunnel port of Switch 2 in the Service Provider network, whi ch
belongs to access VLAN40. Because the access VLAN of the tunnel port (VLAN 40) is the same as the
native VLAN of the edge-switch trunk port (VLAN 40), the metro tag is not added to tagged packets
received from the tunnel port. The packet carries only the VLA N 30 tag through the Service Provider
network to the trunk port of the egress-edge switch (Switch 3) and is misdirected through the egress
switch tunnel port to Customer B.
These are some ways to solve this problem:
•Use ISL trunks between core switches in the Service Provider network. Although customer
interfaces connected to edge switches must be 802.1Q trunks, we recommend using ISL trunks fo r
connecting switches in the core layer.
•Use the switchport trunk native vlan tag per-port command and the vlan dot1q tag native global
configuration command to configure the edge switch so that all packet s going out an 802.1Q trunk,
including the native VLAN, are tagged. If the switch is configured to tag native VLAN packets on
all 802.1Q trunks, the switch accepts untagged packets, but sends only tagged packets.
•Ensure that the native VLAN ID on the edge-switch trunk port is not within the customer VLAN
range. For example, if the trunk port carries traffic of VLANs 100 to 200, assign the native VLAN
a number outside that range.