Chapter 9, Configuring IEEE 802.1Q and Layer 2 Protocol Tunneling

Virtual private networks (VPNs) provide enterprise-scale connectivity on a shared infrastructure, often Ethernet-based, with the same security, prioritization, reliability, and manageability requirements of private networks. Tunneling is a feature designed for service providers who carry traffic of multiple customers across their networks and are required to maintain the VLAN and Layer 2 protocol configurations of each customer without impacting the traffic of other customers. The ML-Series cards support IEEE 802.1Q tunneling and Layer 2 protocol tunneling.

Understanding IEEE 802.1Q Tunneling

Business customers of service providers often have specific requirements for VLAN IDs and the number of VLANs to be supported. The VLAN ranges required by different customers in the same service-provider network might overlap, and traffic of customers through the infrastructure might be mixed. Assigning a unique range of VLAN IDs to each customer would restrict customer configurations and could easily exceed the IEEE 802.1Q specification VLAN limit of 4096.

Using the IEEE 802.1Q tunneling (QinQ) feature, service providers can use a single VLAN to support customers who have multiple VLANs. Customer VLAN IDs are preserved and traffic from different customers is segregated within the service-provider infrastructure even when they appear to be on the same VLAN. The IEEE 802.1Q tunneling expands VLAN space by using a VLAN-in-VLAN hierarchy and tagging the tagged packets. A port configured to support IEEE 802.1Q tunneling is called a tunnel port. When you configure tunneling, you assign a tunnel port to a VLAN that is dedicated to tunneling. Each customer requires a separate VLAN, but that VLAN supports all of the customer's VLANs.

Customer traffic tagged in the normal way with appropriate VLAN IDs comes from an IEEE 802.1Q trunk port on the customer device and into a tunnel port on the ML-Series card. The link between the customer device and the ML-Series card is an asymmetric link because one end is configured as an IEEE 802.1Q trunk port and the other end is configured as a tunnel port. You assign the tunnel port interface to an access VLAN ID unique to each customer ( Figure 9-1).

Packets coming from the customer trunk port into the tunnel port on the ML-Series card are normally IEEE 802.1Q-tagged with an appropriate VLAN ID. The tagged packets remain intact inside the ML-Series card and, when they exit the trunk port into the service provider network, are encapsulated with another layer of an IEEE 802.1Q tag (called the metro tag) that contains the VLAN ID unique to the customer. The original IEEE 802.1Q tag from the customer is preserved in the encapsulated packet. Therefore, packets entering the service-provider infrastructure are double-tagged, with the outer tag containing the customer's access VLAN ID, and the inner VLAN ID being the VLAN of the incoming traffic.

When the double-tagged packet enters another trunk port in a service provider ML-Series card, the outer tag is stripped as the packet is processed inside the switch. When the packet exits another trunk port on the same core switch, the same metro tag is again added to the packet. Figure 9-2 shows the structure of the double-tagged packet.

Figure 9-2 Normal, IEEE 802.1Q, and IEEE 802.1Q-Tunneled Ethernet Packet Formats

When the packet enters the trunk port of the service-provider egress switch, the outer tag is again stripped as the packet is processed internally on the switch. However, the metro tag is not added when it is sent out the tunnel port on the edge switch into the customer network, and the packet is sent as a normal IEEE 802.1Q-tagged frame to preserve the original VLAN numbers in the customer network.

In Figure 9-1, Customer A was assigned VLAN 30, and Customer B was assigned VLAN 40. Packets entering the ML-Series card tunnel ports with IEEE 802.1Q tags are double-tagged when they enter the service-provider network, with the outer tag containing VLAN ID 30 or 40, appropriately, and the inner tag containing the original VLAN number, for example, VLAN 100. Even if both Customers A and B have VLAN 100 in their networks, the traffic remains segregated within the service-provider network because the outer tag is different. With IEEE 802.1Q tunneling, 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.

At the outbound tunnel port, the original VLAN numbers on the customer's network are recovered. If the traffic coming from a customer network is not tagged (native VLAN frames), these packets are bridged or routed as if they were normal packets, and the metro tag is added (as a single-level tag) when they exit toward the service provider network.

If the native VLAN (VLAN 1) is used in the service provider network as a metro tag, this tag must always be added to the customer traffic, even though the native VLAN ID is not normally added to transmitted frames. If the VLAN 1 metro tag is not added on frames entering the service provider network, then the customer VLAN tag appears to be the metro tag, with disastrous results. The global configuration vlan dot1q tag native command must be used to prevent this by forcing a tag to be added to VLAN 1. Avoiding the use of VLAN 1 as a metro tag transporting customer traffic is recommended to reduce the risk of misconfiguration. A best practice is to use VLAN 1 as a private management VLAN in the service provider network.

The IEEE 802.1Q class of service (COS) priority field on the added metro tag is set to zero by default, but can be modified by input or output policy maps.

Configuring IEEE 802.1Q Tunneling

This section includes the following information about configuring IEEE 802.1Q tunneling:

IEEE 802.1Q Tunneling and Compatibility with Other Features

Although IEEE 802.1Q tunneling works well for Layer 2 packet switching, there are incompatibilities with some Layer 2 features and with Layer 3 switching:

•

Layer 3 quality of service (QoS) ACLs and other QoS features related to Layer 3 information are not supported on tunnel ports. MAC-based QoS is supported on tunnel ports.

•

EtherChannel port groups are compatible with tunnel ports as long as the IEEE 802.1Q configuration is consistent within an EtherChannel port group.

•

Port Aggregation Protocol (PAgP) and Unidirectional Link Detection (UDLD) Protocol are not supported on IEEE 802.1Q tunnel ports.

•

Dynamic Trunking Protocol (DTP) is not compatible with IEEE 802.1Q tunneling because you must manually configure asymmetric links with tunnel ports and trunk ports.

•

When a port is configured as an IEEE 802.1Q tunnel port, spanning tree bridge protocol data unit (BPDU) filtering is automatically disabled on the interface.

Configuring an IEEE 802.1Q Tunneling Port

Beginning in privileged EXEC mode, follow these steps to configure a port as an IEEE 802.1Q tunnel port:


	Command	Purpose
Step 1	`Router# configure terminal`	Enters global configuration mode.
Step 2	`Router(config)# bridge bridge-number protocol bridge-protocol`	Creates a bridge number and specifies a protocol.
Step 3	`Router(config)# interface fastEthernet number`	Enters the interface configuration mode and the interface to be configured as a tunnel port. This should be the edge port in the service-provider network that connects to the customer switch. Valid interfaces include physical interfaces and port-channel logical interfaces (port channels 1 to 64).
Step 4	`Router(config-if)# bridge-group number`	Assigns the tunnel port to a bridge-group. All traffic from the port (tagged and untagged) will be switched based on this bridge-group. Other members of the bridge-group should be VLAN subinterfaces on a provider trunk interface.
Step 5	`Router(config-if)# mode dot1q-tunnel`	Sets the interface as an IEEE 802.1Q tunnel port.
Step 6	`Router(config)# end`	Returns to privileged EXEC mode.
Step 7	`Router# show dot1q-tunnel`	Displays the tunnel ports on the switch.
Step 8	`Router# copy running-config startup-config`	(Optional) Saves your entries in the configuration file.

Note The VLAN ID (VID) range of 2 to 4095 is recommended for IEEE 802.1Q tunneling on the ML-Series card.

Note If VID 1 is required to be used as a metro tag, use the following command:
Router (config)# VLAN dot1Q tag native

Use the no mode dot1q-tunnel interface configuration command to remove the IEEE 802.1Q tunnel from the interface.

IEEE 802.1Q Example

The following examples show how to configure the example in Figure 9-1. Example 9-1 applies to Router A, and Example 9-2 applies to Router B.

Understanding VLAN-Transparent and VLAN-Specific Services

The ML-Series card supports combining VLAN-transparent services and one or more VLAN-specific services on the same port. All of these VLAN-transparent and VLAN-specific services can be point-to-point or multipoint-to-multipoint.

This allows a service provider to combine a VLAN-transparent service, such as IEEE 802.1Q tunneling (QinQ), with VLAN-specific services, such as bridging specific VLANs, on the same customer port. For example, one customer VLAN can connect to Internet access and the other customer VLANs can be tunneled over a single provider VLAN to another customer site, all over a single port at each site. Table 9-1 outlines the differences between VLAN-transparent and VLAN-specific services.

Table 9-1 VLAN-Transparent Service Versus VLAN-Specific Services
VLAN-Transparent Services	VLAN-Specific Services
Bridging only	Bridging or routing
One service per port	Up to 254 VLAN-specific services per port
Applies indiscriminately to all VLANs on the physical interface	Applies only to specified VLANs

Note VLAN-transparent service is also referred to as Ethernet Wire Service (EWS). VLAN-specific service is also referred to as QinQ tunneling trunk UNI in Metro Ethernet terminology.

A VLAN-specific service on a subinterface coexists with the VLAN-transparent service, often IEEE 802.1Q tunneling, on a physical interface. VLANs configured for a VLAN-transparent service and a VLAN-specific service follow the VLAN-specific service configuration. If you need to configure 802.1Q tunneling, configure this VLAN-transparent service in the normal manner, see the "Configuring IEEE 802.1Q Tunneling" section.

A VLAN-specific service can be any service normally applicable to a VLAN. To configure an ERMS VLAN-specific service, configure the service in the normal manner.

VLAN-Transparent and VLAN-Specific Services Configuration Example

In this example, the Gigabit Ethernet interface 0 on both the ML-Series card A and ML-Series card C are the trunk ports in an IEEE 802.1Q tunnel, a VLAN-transparent service. VLAN 10 is used for the VLAN-transparent service, which would normally transport all customer VLANs on the ML-Series card A's Gigabit Ethernet interface 0. All unspecified VLANs and VLAN 1 would also be tunneled across VLAN 10.

VLAN 30 is prevented from entering the VLAN-transparent service and is instead forwarded on a specific-VLAN service, bridging Gigabit Ethernet interface 0 on ML-Series card A and Gigabit Ethernet interface 0 on ML-Series card B. Figure 9-3 is used as an example to performing configuration examples 9-3, 9-4, and 9-5.

Understanding Layer 2 Protocol Tunneling

Customers at different sites connected across a service-provider network need to run various Layer 2 protocols to scale their topology to include all remote sites, as well as the local sites. Spanning Tree Protocol (STP) must run properly, and every VLAN should build a proper spanning tree that includes the local site and all remote sites across the service-provider infrastructure. Cisco Discovery Protocol (CDP) must discover neighboring Cisco devices from local and remote sites. VLAN Trunking Protocol (VTP) must provide consistent VLAN configuration throughout all sites in the customer network.

When protocol tunneling is enabled, edge switches on the inbound side of the service-provider infrastructure encapsulate Layer 2 protocol packets with a special MAC address and send them across the service-provider network. Core switches in the network do not process these packets, but forward them as normal packets. CDP, STP, or VTP Layer 2 protocol data units (PDUs) cross the service-provider infrastructure and are delivered to customer switches on the outbound side of the service-provider network. Identical packets are received by all customer ports on the same VLANs with the following results:

•

Users on each of a customer's sites are able to properly run STP and every VLAN can build a correct spanning tree based on parameters from all sites and not just from the local site.

•

CDP discovers and shows information about the other Cisco devices connected through the service-provider network.

•

VTP provides consistent VLAN configuration throughout the customer network, propagating through the service provider to all switches.

Layer 2 protocol tunneling can be used independently or to enhance IEEE 802.1Q tunneling. If protocol tunneling is not enabled on IEEE 802.1Q tunneling ports or on specific VLANs, remote switches at the receiving end of the service-provider network do not receive the PDUs and cannot properly run STP, CDP, and VTP. When protocol tunneling is enabled, Layer 2 protocols within each customer's network are totally separate from those running within the service-provider network. Customer switches on different sites that send traffic through the service-provider network with IEEE 802.1Q tunneling achieve complete knowledge of the customer's VLAN. If IEEE 802.1Q tunneling is not used, you can still enable Layer 2 protocol tunneling by connecting to the customer switch through access ports and enabling tunneling on the service-provider access port.

Configuring Layer 2 Protocol Tunneling

Layer 2 protocol tunneling (by protocol) is enabled on the tunnel ports or on specific tunnel VLANs that are connected to the customer by the edge switches of the service-provider network. ML-Series card tunnel ports are connected to customer IEEE 802.1Q trunk ports. The ML-Series card supports Layer 2 protocol tunneling for CDP, STP, and VTP at the interface and subinterface level. Multiple STP (MSTP) Tunneling support is achieved through subinterface protocol tunneling. The ML-Series cards connected to the customer switch perform the tunneling process.

When the Layer 2 PDUs that entered the inbound ML-Series switch through the tunnel port exit the switch through the trunk port into the service-provider network, the switch overwrites the customer PDU-destination MAC address with a well-known Cisco proprietary multicast address (01-00-0c-cd-cd-d0). If IEEE 802.1Q tunneling is enabled, packets are also double-tagged; the outer tag is the customer metro tag and the inner tag is the customer VLAN tag. The core switches ignore the inner tags and forward the packet to all trunk ports in the same metro VLAN. The ML-Series switches on the outbound side restore the proper Layer 2 protocol and MAC address information and forward the packets. Therefore, the Layer 2 PDUs are kept intact and delivered across the service-provider infrastructure to the other side of the customer network.

This section contains the following information about configuring Layer 2 protocol tunneling:

Default Layer 2 Protocol Tunneling Configuration

Table 9-2 Default Layer 2 Protocol Tunneling Configuration
Feature	Default Setting
Layer 2 protocol tunneling	Disabled for CDP, STP, and VTP.
Class of service (CoS) value	If a CoS value is configured on the interface for data packets, that value is the default used for Layer 2 PDUs. If none is configured, there is no default. This allows existing CoS values to be maintained, unless the user configures otherwise.

Layer 2 Protocol Tunneling Configuration Guidelines

These are some configuration guidelines and operating characteristics of Layer 2 protocol tunneling:

•

The ML-Series card supports Per-VLAN Protocol Tunneling (PVPT), which allows protocol tunneling to be configured and run on a specific subinterface (VLAN). PVPT configuration is done at the subinterface level.

•

PVPT should be configured on VLANs that carry multi-session transport (MST) BPDUs on the connected devices.

•

The ML-Series card supports tunneling of CDP, STP (including MSTP and VTP protocols). Protocol tunneling is disabled by default but can be enabled for the individual protocols on IEEE 802.1Q tunnel ports or on specific VLANs.

•

Tunneling is not supported on trunk ports. If you enter the l2protocol-tunnel interface configuration command on a trunk port, the command is accepted, but Layer 2 tunneling does not take affect unless you change the port to a tunnel port.

•

EtherChannel port groups are compatible with tunnel ports as long as the IEEE 802.1Q configuration is configured within an EtherChannel port group.

•

If an encapsulated PDU (with the proprietary destination MAC address) is received from a tunnel port or access port with Layer 2 tunneling enabled, the tunnel port is shut down to prevent loops.

•

Only decapsulated PDUs are forwarded to the customer network. The spanning tree instance running on the service-provider network does not forward BPDUs to tunnel ports. No CDP packets are forwarded from tunnel ports.

•

Because tunneled PDUs (especially STP BPDUs) must be delivered to all remote sites for the customer virtual network to operate properly, you can give PDUs higher priority within the service-provider network than data packets received from the same tunnel port. By default, the PDUs use the same CoS value as data packets.

•

Protocol tunneling has to be configured symmetrically at both the ingress and egress point. For example, if you configure the entry point to tunnel STP, CDP, VTP, then you must configure the egress point in the same way.

Configuring Layer 2 Tunneling on a Port

Beginning in privileged EXEC mode, follow these steps to configure a port as a Layer 2 tunnel port:


	Command	Purpose
Step 1	`Router# configuration terminal`	Enters global configuration mode.
Step 2	`Router(config)# bridge bridge-group-number protocol type`	Creates a bridge group number and specifies a protocol.
Step 3	`Router(config)# l2protocol-tunnel cos cos-value`	Associates a CoS value with the Layer 2 tunneling port. Valid numbers for a cos-value range from 0 to 7.
Step 4	`Router(config)# interface type number`	Enters interface configuration mode for the interface to be configured as a tunnel port.
Step 5	`Router(config-if)# bridge-group bridge-group-number`	Assigns a bridge group to the interface.
Step 6	`Router(config-if)# mode dot1q tunnel`	Sets the interface as an IEEE 802.1Q tunnel VLAN.
Step 7	`Router(config-if)# l2protocol-tunnel {all \| cdp \| stp \| vtp]}`	Sets the interface as a Layer 2 protocol tunnel port and enables all three protocols or specifically enables CDP, STP, or VTP. These protocols are off by default.
Step 8	`Router(config-if)# end`	Returns to privileged EXEC mode.
Step 9	`Router# show dot1q-tunnel`	Displays the tunnel ports on the switch.
Step 10	`Router# copy running-config startup-config`	(Optional) Saves your entries in the configuration file.

Configuring Layer 2 Tunneling Per-VLAN

Beginning in privileged EXEC mode, follow these steps to configure a VLAN as a Layer 2 tunnel VLAN:


	Command	Purpose
Step 1	`Router# configuration terminal`	Enters global configuration mode.
Step 2	`Router(config)# bridge bridge-group-number protocol type`	Creates a bridge group number and specifies a protocol.
Step 3	`Router(config)# l2protocol-tunnel cos cos-value`	Associates a CoS value with the Layer 2 tunneling VLAN. Valid numbers for a cos-value range from 0 to 7.
Step 4	`Router(config)# interface type number.subinterface-number`	Enters subinterface configuration mode and the subinterface to be configured as a tunnel VLAN.
Step 5	`Router(config-subif)# encapsulation dot1q bridge-group-number`	Sets the subinterface as an IEEE 802.1Q tunnel VLAN.
Step 6	`Router(config-subif)# bridge-group bridge-group-number`	Assigns a bridge group to the interface.
Step 7	`Router(config-subif)# end`	Returns to privileged EXEC mode.
Step 8	`Router# copy running-config startup-config`	(Optional) Saves your entries in the configuration file.

Monitoring and Verifying Tunneling Status

Table 9-3 shows the privileged EXEC commands for monitoring and maintaining IEEE 802.1Q and Layer 2 protocol tunneling.

Table 9-3 Commands for Monitoring and Maintaining Tunneling
Command	Purpose
`show dot1q-tunnel`	Displays IEEE 802.1Q tunnel ports on the switch.
`show dot1q-tunnel interface interface-id`	Verifies if a specific interface is a tunnel port.
`show l2protocol-tunnel`	Displays information about Layer 2 protocol tunneling ports.
`show vlan dot1q tag native`	Displays IEEE 802.1Q tunnel information.

Table Of Contents

Configuring IEEE 802.1Q Tunneling and Layer 2 Protocol Tunneling