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Catalyst 3750 SwitchSoftware Configuration Guide
OL-8550-09
Chapter17 Configuring IEEE 802.1Q and Layer 2 Protocol Tunneling
Configuring Layer 2 Protocol Tunneling
See Figure 17-4, with Customer X and Customer Y in access VLANs 30 and 40, respectively.
Asymmetric links connect the customers in Site 1 to edge switches in the service-provider network. The
Layer 2 PDUs (for example, BPDUs) coming into Switch 2 from Customer Y in Site 1 are forwarded to
the infrastructure as double-tagged packets with the well-known MAC address as the destination MAC
address. These double-tagged packets have the metro VLAN tag of 40, as well as an inner VLAN tag
(for example, VLAN 100). When the double-tagged packets enter Switch D, the outer VLAN tag 40 is
removed, the well-known MAC address is replaced with the respective Layer 2 protocol MAC address,
and the packet is sent to Customer Y on Site 2 as a single-tagged frame in VLAN 100.
You can also enable Layer 2 protocol tunneling on access ports on the edge switch connected to access
or trunk ports on the customer switch. In this case, the encapsulation and decapsulation process is the
same as described in the previous paragraph, except that the packets are not double-tagged in the
service-provider network. The single tag is the customer-specific access VLAN tag.
Layer 2 protocol tunneling configuration is distributed among all stack members. Each stack member
that receives an ingress packet on a local port encapsulates or decapsulates the packet and forwards it to
the appropriate destination port. On a single switch, ingress Layer 2 protocol-tunneled traffic is sent
across all local ports in the same VLAN on which Layer 2 protocol tunneling is enabled. In a stack,
packets received by a Layer 2 protocol-tunneled port are distributed to all ports in the stack that are
configured for Layer 2 protocol tunneling and are in the same VLAN. All Layer 2 protocol tunneling
configuration is handled by the stack master and distributed to all stack members.
These sections contain this configuration information:
•Default Layer 2 Protocol Tunneling Configuration, page 17-11
•Layer 2 Protocol Tunneling Configuration Guidelines, page17-12
•Configuring Layer 2 Protocol Tunneling, page17-13
•Configuring Layer 2 Tunneling for EtherChannels, page 17-14
Default Layer 2 Protocol Tunneling Configuration
Table17-1 shows the default Layer 2 protocol tunneling configuration.
Table17-1 Default Layer 2 Ethernet Interface VLAN Configuration
Feature Default Setting
Layer 2 protocol tunneling Disabled.
Shutdown threshold None set.
Drop threshold None set.
CoS value If a CoS value is configured on the interface, that value is used to
set the BPDU CoS value for Layer 2 protocol tunneling. If no CoS
value is configured at the interface level, the default value for CoS
marking of L2 protocol tunneling BPDUs is 5. This does not apply
to data traffic.