Release 15.5SY Supervisor Engine 6T Software Configuration Guide

IEEE 802.1Q Tunneling

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• Prerequisites for 802.1Q Tunneling
• Restrictions for 802.1Q Tunneling
• Information About 802.1Q Tunneling
• Default Settings for 802.1Q Tunneling
• How to Configure 802.1Q 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 802.1Q Tunneling

None.

Restrictions for 802.1Q Tunneling

• Use asymmetrical links to put traffic into a tunnel or to remove traffic from a tunnel.
• Configure tunnel ports only to form an asymmetrical link.
• Dedicate one VLAN for each tunnel.
• Assign only tunnel ports to VLANs used for tunneling.
• Trunks require no special configuration to carry tunnel VLANs.
• Tunnel ports are not trunks. Any commands to configure trunking are inactive while the port is configured as a tunnel port.
• Tunnel ports learn customer MAC addresses.
• We recommend that you use ISL trunks to carry tunnel traffic between devices that do not have tunnel ports. Because of the 802.1Q native VLAN feature, using 802.1Q trunks requires that you be very careful when you configure tunneling: a mistake might direct tunnel traffic to a non-tunnel port.
• By default, the native VLAN traffic of a dot1q trunk is sent untagged, which cannot be double-tagged in the service provider network. Because of this situation, the native VLAN traffic might not be tunneled correctly. Be sure that the native VLAN traffic is always sent tagged in an asymmetrical link. To tag the native VLAN egress traffic and drop all untagged ingress traffic, enter the global vlan dot1q tag native command.
• Configure jumbo frame support on tunnel 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.

• Jumbo frames can be tunneled as long as the jumbo frame length combined with the 802.1Q tag does not exceed the maximum frame size.
• Because tunnel traffic has the added ethertype and length field and retains the 802.1Q tag within the switch, the following restrictions exist:

– The Layer 3 packet within the Layer 2 frame cannot be identified in tunnel traffic.

– Layer 3 and higher parameters cannot be identified in tunnel traffic (for example, Layer 3 destination and source addresses).

– Because the Layer 3 addresses cannot be identified within the packet, tunnel traffic cannot be routed.

– The switch can provide only MAC-layer filtering for tunnel traffic (VLAN IDs and source and destination MAC addresses).

– The switch can provide only MAC-layer access control and QoS for tunnel traffic.

– QoS cannot detect the received CoS value in the 802.1Q 2-byte Tag Control Information field.

• On an asymmetrical link, the Cisco Discovery Protocol (CDP) reports a native VLAN mismatch if the VLAN of the tunnel port does not match the native VLAN of the 802.1Q trunk. The 802.1Q tunnel feature does not require that the VLANs match. Ignore the messages if your configuration requires nonmatching VLANs.
• Asymmetrical links do not support the Dynamic Trunking Protocol (DTP) because only one port on the link is a trunk. Configure the 802.1Q trunk port on an asymmetrical link to trunk unconditionally.
• The 802.1Q tunneling feature cannot be configured on ports configured to support private VLANs.
• The 802.1Q tunneling feature cannot be configured on ports configured to support EVCs.
• The following Layer 2 protocols work between devices connected by an asymmetrical link:

– CDP

– UniDirectional Link Detection (UDLD)

– Port Aggregation Protocol (PAgP)

– Link Aggregation Control Protocol (LACP)

• PortFast BPDU filtering is enabled automatically on tunnel ports.
• CDP is automatically disabled on tunnel ports.
• VLAN Trunk Protocol (VTP) does not work between the following devices:

– Devices connected by an asymmetrical link

– Devices communicating through a tunnel

Note VTP works between tunneled devices if Layer 2 protocol tunneling is enabled. See “Layer 2 Protocol Tunneling,” for configuration details.

• To configure an EtherChannel as an asymmetrical link, all ports in the EtherChannel must have the same tunneling configuration. Because the Layer 3 packet within the Layer 2 frame cannot be identified, you must configure the EtherChannel to use MAC-address-based frame distribution.

The following configuration guidelines are required for your Layer 2 protocol tunneling configuration:

• On all the service provider edge switches, PortFast BPDU filtering must be enabled on the 802.1Q tunnel ports as follows:

Note PortFast BPDU filtering is enabled automatically on tunnel ports.

• At least one VLAN must be available for native VLAN tagging (vlan dot1q tag native option). If you use all the available VLANs and then try to enable the vlan dot1q tag native option, the option will not be enabled.
• On all the service provider core switches, tag native VLAN egress traffic and drop untagged native VLAN ingress traffic by entering the following command:

• On all the customer switches, either enable or disable the global vlan dot1q tag native option.

Note If this option is enabled on one switch and disabled on another switch, all traffic is dropped; all customer switches must have this option configured the same on each switch.

The following configuration guidelines are optional for your Layer 2 protocol tunneling configuration:

• Because all the BPDUs are being dropped, spanning tree PortFast can be enabled on Layer 2 protocol tunnel ports as follows:

• If the service provider does not want the customer to see its switches, CDP should be disabled on the 802.1Q tunnel port as follows:

Information About 802.1Q Tunneling

802.1Q tunneling enables service providers to use a single VLAN to support customers who have multiple VLANs, while preserving customer VLAN IDs and keeping traffic in different customer VLANs segregated.

A port configured to support 802.1Q tunneling is called a tunnel port. When you configure tunneling, you assign a tunnel port to a VLAN that you dedicate to tunneling, which then becomes a tunnel VLAN. To keep customer traffic segregated, each customer requires a separate tunnel VLAN, but that one tunnel VLAN supports all of the customer’s VLANs.

802.1Q tunneling is not restricted to point-to-point tunnel configurations. Any tunnel port in a tunnel VLAN is a tunnel entry and exit point. An 802.1Q tunnel can have as many tunnel ports as are needed to connect customer switches.

The customer switches are trunk connected, but with 802.1Q tunneling, the service provider switches only use one service provider VLAN to carry all the customer VLANs, instead of directly carrying all the customer VLANs.

With 802.1Q tunneling, tagged customer traffic comes from an 802.1Q trunk port on a customer device and enters the service-provider edge switch through a tunnel port. The link between the 802.1Q trunk port on a customer device and the tunnel port is called an asymmetrical link because one end is configured as an 802.1Q trunk port and the other end is configured as a tunnel port. You assign the tunnel port to an access VLAN ID unique to each customer. See Figure 29-1 and Figure 29-2.

Figure 29-1 IEEE 802.1Q Tunnel Ports in a Service-Provider Network

Figure 29-2 Untagged, 802.1Q-Tagged, and Double-Tagged Ethernet Frames

When a tunnel port receives tagged customer traffic from an 802.1Q trunk port, it does not strip the received 802.1Q tag from the frame header; instead, the tunnel port leaves the 802.1Q tag intact, adds a 2-byte Ethertype field (0x8100) followed by a 2-byte field containing the priority (CoS) and the VLAN. The received customer traffic is then put into the VLAN to which the tunnel port is assigned. This Ethertype 0x8100 traffic, with the received 802.1Q tag intact, is called tunnel traffic.

A VLAN carrying tunnel traffic is an 802.1Q tunnel. The tunnel ports in the VLAN are the tunnel’s ingress and egress points.

The tunnel ports do not have to be on the same network device. The tunnel can cross other network links and other network devices before reaching the egress tunnel port. A tunnel can have as many tunnel ports as required to support the customer devices that need to communicate through the tunnel.

An egress tunnel port strips the 2-byte Ethertype field (0x8100) and the 2-byte length field and transmits the traffic with the 802.1Q tag still intact to an 802.1Q trunk port on a customer device. The 802.1Q trunk port on the customer device strips the 802.1Q tag and puts the traffic into the appropriate customer VLAN.

Note Tunnel traffic carries a second 802.1Q tag only when it is on a trunk link between service-provider network devices, with the outer tag containing the service-provider-assigned VLAN ID and the inner tag containing the customer-assigned VLAN IDs.

Default Settings for 802.1Q Tunneling

None.

How to Configure 802.1Q Tunneling

• Configuring 802.1Q Tunnel Ports
• Configuring the Switch to Tag Native VLAN Traffic

Caution Ensure that only the appropriate tunnel ports are in any VLAN used for tunneling and that one VLAN is used for each tunnel. Incorrect assignment of tunnel ports to VLANs can forward traffic inappropriately.

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Configuring Ports Not to Tag Native VLAN Traffic

To configure a port not to tag traffic in the native VLAN, perform this task:

 

	Command	Purpose
Step 1	Router(config)# interface type slot/port	Selects the LAN port to configure.
Step 2	Router(config-if)# switchport	Configures the LAN port for Layer 2 switching. You must enter the switchport command once without any keywords to configure the LAN port as a Layer 2 interface before you can enter additional switchport commands with keywords. Required only if you have not entered the switchport command already for the port.
Step 3	Router(config-if)# no switchport trunk native vlan tag	When the switch is configured to tag native VLAN traffic globally, configures the Layer 2 port not to tag native VLAN traffic.
Step 4	Router(config-if)# end	Exits configuration mode.

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Note The switchport trunk native vlan tag interface command does not enable native VLAN tagging unless the switch is configured to tag native VLAN traffic globally.

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This example shows how to configure Gigabit Ethernet port 1/4 to tag traffic in the native VLAN and verify the configuration:

Router# configure terminal

Router(config)# interface gigabitethernet 1/4

Router(config-if)# switchport trunk native vlan tag

Router(config-if)# end

Router# show interface gigabitethernet 1/4 switchport | include tagging

Administrative Native VLAN tagging: enabled

Operational Native VLAN tagging: disabled

Router#

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Tip For additional information about Cisco Catalyst 6500 Series Switches (including configuration examples and troubleshooting information), see the documents listed on this page:

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

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