Cisco Systems 4500 Configuring 802.1Q Tunneling

25-3
Software Configuration Guide—Release 15.0(2)SG
OL-23818-01
Chapter 25 Configuring 802.1Q Tunneling, VLAN Mapping, and Layer 2 Protocol Tunneling Configuring 802.1Q Tunneling
Figure 25-2 Original (Normal), 802.1Q, and Double-Tagged Ethernet Packet Formats
When the packet enters the trunk port of the service provider egress switch, the metro tag is again
stripped as the switch processes the packet. However, the metro tag is not added when the packet is sent
out the tunnel port on the edge switch into the customer network. The packet is sent as a normal
802.1Q-tagged frame to preserve the original VLAN numbers in the customer network.
All packets entering the service provider network through a tunnel port on an edge switch are treated as
untagged packets, whether they are untagged or already tagged with 802.1Q headers. The packets are
encapsulated with the metro tag VLAN ID (set to the access VLAN of the tunnel port) when they are
sent through the service provider network on an 802.1Q trunk port. The priority field on the metro tag
is set to the interface class of service (CoS) priority configured on the tunnel port. (The default is zero
if none is configured.)
In Figure 25-1, Customer A was assigned VLAN 30, and Customer B was assigned VLAN 40. Packets
entering the edge-switch tunnel ports with 802.1Q tags are double-tagged when they enter the service
provider network, with the metro tag containing VLAN ID 30 or 40, appropriately, and the inner tag
containing the original customer VLAN number, for example, VLAN 100. Even if Customers A and B
both have VLAN 100 in their networks, the traffic remains segregated within the service provider
network because the metro tag is different. Each customer controls its own VLAN numbering space,
which is independent of the VLAN numbering space used by other customers and the VLAN numbering
space used by the service provider network.
Configuring 802.1Q Tunneling
These sections describe 802.1Q tunneling configuration:
• 802.1Q Tunneling Configuration Guidelines, page 25-3
• 802.1Q Tunneling and Other Features, page 25-5
• Configuring an 802.1Q Tunneling Port, page 25-6
Note By default, 802.1Q tunneling is disabled because the default switch port mode is dynamic 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 tunnel. You should also be aware of
configuration requirements for native VLANs and maximum transmission units (MTUs). For more
information about MTUs, see the “System MTU” section on page 25-5.
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