8-6
Catalyst 6500 Series Switch Software Configuration Guide—Release 8.7
OL-8978-04
Chapter 8 Configuring IEEE 802.1Q Tunneling and Layer 2 Protocol Tunneling
Understanding How Layer 2 Protocol Tunneling Works
Understanding How Layer 2 Protocol Tunneling Works
Layer 2 protocol tunneling allows the protocol data units (PDUs) (CDP, STP, and VTP) to be tunneled
through a network. Some terminology that is used in this section is defined as follows:
•
Edge switch
—
The switch that is connected to the customer switch and placed on the boundary of
the service provider network (see
Figure 8-1
).
•
Layer 2 protocol tunnel port—A port on the edge switch on which a specific tunneled protocol can
be encapsulated or deencapsulated. The tunnel port is configured through CLI commands.
•
Tunneled PDU
—
A CDP, STP, or VTP PDU.
In the current implementation of 802.1Q tunneling, spanning-tree BPDUs are flooded only on the special
802.1Q tunnel ports that belong to the same edge switch. This implementation prevents loops between
the edge switch and the customer switch at each site. The BPDUs are not flooded on the ports that are
connected to other service provider switches inside the service provider network. This handling of the
BPDUs creates different spanning-tree domains (different spanning-tree roots) for the customer
network. For example, STP for a VLAN on switch 1 (see
Figure 8-1
) builds a spanning-tree topology on
Switches 1, 2, and 3 without considering the convergence parameters that are based on Switches 4 and
5. To provide a single spanning-tree domain for the customer, a generic scheme to tunnel BPDUs was
created for control protocol PDUs (CDP, STP, and VTP). This process is referred to as Layer 2 protocol
tunneling.
Figure 8-1
Layer 2 Protocol Tunneling Network Configuration
Layer 2 protocol tunneling provides a scalable approach to PDU tunneling by software encapsulating the
PDUs in the ingress edge switches and then multicasting them in hardware. All switches inside the
service provider network treat these encapsulated frames as data packets and forward them to the other
end. The egress edge switch listens for these special encapsulated frames and deencapsulates them; they
are then forwarded out of the tunnel.
The encapsulation rewrites the destination Media Access Control (MAC) address in the PDU. An ingress
edge switch rewrites the destination MAC address of the PDUs that are received on a tunneled port with
the Cisco proprietary multicast address (01-00-0c-cd-cd-d0). The PDU is then flooded to the native
VLAN of the tunneled port. If you enable Layer 2 protocol tunneling on a port, the PDUs of an enabled
protocol are not sent out. If you disable Layer 2 protocol tunneling on a port, the disabled protocols
behave the same way that they behaved before Layer 2 protocol tunneling was disabled on the port.
Service provider
network
Customer switches
Customer switches
Edge
switches
Switch 1
Switch 3
Switch 4
Switch 5
Switch 2
Switch A
Switch B
77066
