Configuring 802.1Q Tunneling - Cisco Catalyst 3550 series Software Configuration Manual

Configuring 802.1Q Tunneling

Configuring 802.1Q Tunneling
This section includes this information about configuring 802.1Q tunneling:
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Default 802.1Q Tunneling Configuration
By default, 802.1Q tunneling is disabled because the default switch port mode is dynamic desirable.
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 tunnel. You should also be aware of
configuration requirements for native VLANs and maximum transmission units (MTUs). For more
information about MTUs, see the
Native VLANs
When configuring 802.1Q tunneling on an edge switch, you must use 802.1Q trunk ports for sending
packets into the SP network. However, packets going through the core of the SP network can be carried
through 802.1Q trunks, ISL trunks, 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
Customer A at the ingress edge switch in the SP 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 SP network, which belongs to
access VLAN 40. 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 VLAN 30 tag through the SP 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:
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Catalyst 3550 Multilayer Switch Software Configuration Guide
15-4
Default 802.1Q Tunneling Configuration, page 15-4
802.1Q Tunneling Configuration Guidelines, page 15-4
802.1Q Tunneling and Other Features, page 15-5
Configuring an 802.1Q Tunneling Port, page 15-6
Figure 15-3. VLAN 40 is configured as the native VLAN for the 802.1Q trunk port from
Use ISL trunks between core switches in the SP network. Although customer interfaces connected
to edge switches must be 802.1Q trunks, we recommend using ISL trunks for connecting switches
in the core layer.
Use the vlan dot1q tag native global configuration command to configure the edge switch so that
all packets 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.
Chapter 15
"System MTU" section on page
Configuring 802.1Q and Layer 2 Protocol Tunneling
15-5.
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