Release 15.4SY Supervisor Engine 6T Software Configuration Guide

Layer 2 Protocol Tunneling

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• Prerequisites for Layer 2 Protocol Tunneling
• Restrictions for Layer 2 Protocol Tunneling
• Information About Layer 2 Protocol Tunneling
• Default Settings for Layer 2 Protocol Tunneling
• How to Configure Layer 2 Protocol Tunneling

Note ● For complete syntax and usage information for the commands used in this chapter, see these publications:

http://www.cisco.com/en/US/products/ps11846/prod_command_reference_list.html

• Cisco IOS Release 15.4SY supports only Ethernet interfaces. Cisco IOS Release 15.4SY does not support any WAN features or commands.

http://www.cisco.com/en/US/products/hw/switches/ps708/tsd_products_support_series_home.html

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Prerequisites for Layer 2 Protocol Tunneling

None.

Restrictions for Layer 2 Protocol Tunneling

None.

Information About Layer 2 Protocol Tunneling

Layer 2 protocol tunneling allows Layer 2 protocol data units (PDUs) (CDP, STP, and VTP) to be tunneled through a network. This section uses the following terminology:

• Edge switch—The switch connected to the customer switch and placed on the boundary of the service provider network (see Figure 30-1).
• Layer 2 protocol tunnel port—A port on the edge switch on which a specific tunneled protocol can be encapsulated or deencapsulated. The Layer 2 protocol tunnel port is configured through CLI commands.
• Tunneled PDU—A CDP, STP, or VTP PDU.

Without Layer 2 protocol tunneling, tunnel ports drop STP and VTP packets and process CDP packets. This handling of the PDUs creates different spanning tree domains (different spanning tree roots) for the customer switches. For example, STP for a VLAN on switch 1 (see Figure 30-1) builds a spanning tree topology on switches 1, 2, and 3 without considering convergence parameters 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 Generic Bridge PDU Tunneling (GBPT).

Figure 30-1 Layer 2 Protocol Tunneling Network Configuration

GBPT 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 involves rewriting the destination media access control (MAC) address in the PDU. An ingress edge switch rewrites the destination MAC address of the PDUs received on a Layer 2 tunnel port with the Cisco proprietary multicast address (01-00-0c-cd-cd-d0). The PDU is then flooded to the native VLAN of the Layer 2 tunnel port. If you enable Layer 2 protocol tunneling on a port, PDUs of an enabled protocol are not sent out. If you disable Layer 2 protocol tunneling on a port, the disabled protocols function the same way they were functioning before Layer 2 protocol tunneling was enabled on the port.

Default Settings for Layer 2 Protocol Tunneling

None.

How to Configure Layer 2 Protocol Tunneling

Note ● Encapsulated PDUs received by an 802.1Q tunnel port are transmitted from other tunnel ports in the same VLAN on the switch.

• Configure jumbo frame support on Layer 2 protocol tunneling ports:

– See the “Configuring Jumbo Frame Support” section.

– Take note of the modules listed in the “Configuring Jumbo Frame Support” section that do not support jumbo frames.

To configure Layer 2 protocol tunneling on a port, perform this task:

When you configure a Layer 2 port as a Layer 2 protocol tunnel port, note the following information:

• Optionally, you may specify a drop threshold for the port. The drop threshold value, from 1 to 4096, determines the number of packets to be processed for that protocol on that interface in one second. When the drop threshold is exceeded, PDUs for the specified protocol are dropped for the remainder of the one-second period. If a drop threshold is not specified, the value is 0 (drop threshold disabled).
• Optionally, you may specify a shutdown threshold for the port. The shutdown threshold value, from 1 to 4096, determines the number of packets to be processed for that protocol on that interface in one second. When the shutdown threshold is exceeded, the port is put in errdisable state. If a shutdown threshold is not specified, the value is 0 (shutdown threshold disabled).
• If you specify both a drop threshold and a shutdown threshold for the port, packets exceeding the drop threshold will not be forwarded but will be counted toward the shutdown threshold.

Note The commands support the l2ptguard keyword:

• errdisable detect cause
• errdisable recovery

This example shows how to configure Layer 2 protocol tunneling and drop and shu tdown thresholds on port 5/1 for CDP, STP, and VTP, and verify the configuration:

This example shows how to display counter information for port 5/1:

This example shows how to clear the Layer 2 protocol tunneling configuration from port 5/1:

This example shows how to clear Layer 2 protocol tunneling port counters:

http://www.cisco.com/en/US/products/hw/switches/ps708/tsd_products_support_series_home.html

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