Catalyst 4500 Series Switch Cisco IOS Software Configuration Guide, 12.2(25)EW

Configuring 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 who are required to maintain the VLAN and Layer 2 protocol configurations of each customer without impacting the traffic of other customers. The Catalyst 4500 series switch supports IEEE 802.1Q tunneling and Layer 2 protocol tunneling.

Note 802.1Q requires Supervisor Engine V; Layer 2 protocol tunneling is supported on all supervisor engines.

Note For complete syntax and usage information for the switch commands used in this chapter, refer to the Catalyst 4500 Series Switch Cisco IOS Command Reference and related publications at
http://www.cisco.com/univercd/cc/td/doc/product/software/ios122/122cgcr/index.htm.

This chapter contains these sections:

•Understanding 802.1Q Tunneling

•Configuring 802.1Q Tunneling

•Understanding Layer 2 Protocol Tunneling

•Configuring Layer 2 Protocol Tunneling

•Monitoring and Maintaining Tunneling Status

Understanding 802.1Q Tunneling

The VLAN ranges required by different customers in the same Service Provider network might overlap, and customer traffic 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 VLAN limit (4096) of the 802.1Q specification.

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 ID that is dedicated to tunneling. Each customer requires a separate Service Provider VLAN ID, but that Service Provider VLAN ID supports VLANs of all the customers.

Customer traffic tagged in the normal way with appropriate VLAN IDs comes from an 802.1Q trunk port on the customer device and into a tunnel port on the Service Provider edge switch. The link between the customer device and the edge switch is asymmetric 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 interface to an access VLAN ID that is unique to each customer. See Figure 19-1.

Figure 19-1 802.1Q Tunnel Ports in a Service Provider Network

Packets coming from the customer trunk port into the tunnel port on the Service Provider edge switch are normally 802.1Q-tagged with the appropriate VLAN ID. When the tagged packets exit the trunk port into the Service Provider network, they are encapsulated with another layer of an 802.1Q tag (called the metro tag) that contains the VLAN ID that is unique to the customer. The original customer 802.1Q tag is preserved in the encapsulated packet. Therefore, packets entering the Service Provider network are double-tagged, with the metro tag containing the customer's access VLAN ID, and the inner VLAN ID being that of the incoming traffic.

When the double-tagged packet enters another trunk port in a Service Provider core switch, the metro tag is stripped as the switch processes the packet. When the packet exits another trunk port on the same core switch, the same metro tag is again added to the packet. Figure 19-2 shows the tag structures of the Ethernet packets starting with the original, or normal, frame.

Figure 19-2 Original (Normal), 802.1Q, and Double-Tagged Ethernet Packet Formats

When the packet enters the trunk port of the Service Provider egress switch, the metro tag is again stripped as the switch processes the packet. However, the metro tag is not added when the packet is sent out the tunnel port on the edge switch into the customer network. The packet is sent as a normal 802.1Q-tagged frame to preserve the original VLAN numbers in the customer network.

All packets entering the Service Provider network through a tunnel port on an edge switch are treated as untagged packets, whether they are untagged or already tagged with 802.1Q headers. The packets are encapsulated with the metro tag VLAN ID (set to the access VLAN of the tunnel port) when they are sent through the Service Provider network on an 802.1Q trunk port. The priority field on the metro tag is set to the interface class of service (CoS) priority configured on the tunnel port. (The default is zero if none is configured.)

In Figure 19-1, Customer A was assigned VLAN 30, and Customer B was assigned VLAN 40. Packets entering the edge-switch tunnel ports with 802.1Q tags are double-tagged when they enter the Service Provider network, with the metro tag containing VLAN ID 30 or 40, appropriately, and the inner tag containing the original customer VLAN number, for example, VLAN 100. Even if Customers A and B both have VLAN 100 in their networks, the traffic remains segregated within the Service Provider network because the metro tag is different. 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.

Configuring 802.1Q Tunneling

These sections describe 802.1Q tunneling configuration:

•802.1Q Tunneling Configuration Guidelines

•802.1Q Tunneling and Other Features

•Configuring an 802.1Q Tunneling Port

Note By default, 802.1Q tunneling is disabled because the default switch port mode is dynamic auto. Tagging of 802.1Q native VLAN packets on all 802.1Q trunk ports is also disabled.

802.1Q Tunneling Configuration Guidelines

When you configure 802.1Q tunneling, you should always use asymmetrical links for traffic going through a tunnel and should dedicate one VLAN for each tunnel. You should also be aware of configuration requirements for native VLANs and maximum transmission units (MTUs). For more information about MTUs, see the "System MTU" section.

Native VLANs

When configuring 802.1Q tunneling on an edge switch, you must use 802.1Q trunk ports for sending packets into the Service Provider network. However, packets going through the core of the Service Provider network can be carried through 802.1Q trunks, ISL trunks, or nontrunking links. When 802.1Q trunks are used in these core switches, the native VLANs of the 802.1Q trunks must not match any native VLAN of the nontrunking (tunneling) port on the same switch because traffic on the native VLAN would not be tagged on the 802.1Q sending trunk port.

See Figure 19-3. VLAN 40 is configured as the native VLAN for the 802.1Q trunk port from Customer A at the ingress edge switch in the Service Provider network (Switch 2). Switch 1 of Customer A sends a tagged packet on VLAN 30 to the ingress tunnel port of Switch 2 in the Service Provider network, which belongs to access VLAN 40. Because the access VLAN of the tunnel port (VLAN 40) is the same as the native VLAN of the edge-switch trunk port (VLAN 40), the metro tag is not added to tagged packets received from the tunnel port. The packet carries only the VLAN 30 tag through the Service Provider network to the trunk port of the egress-edge switch (Switch 3) and is misdirected through the egress switch tunnel port to Customer B.

These are some ways to solve this problem:

•Use ISL trunks between core switches in the Service Provider network. Although customer interfaces connected to edge switches must be 802.1Q trunks, we recommend using ISL trunks for connecting switches in the core layer.

•Use the switchport trunk native vlan tag per-port command and the vlan dot1q tag native global configuration command to configure the edge switch so that all packets going out an 802.1Q trunk, including the native VLAN, are tagged. If the switch is configured to tag native VLAN packets on all 802.1Q trunks, the switch accepts untagged packets, but sends only tagged packets.

•Ensure that the native VLAN ID on the edge-switch trunk port is not within the customer VLAN range. For example, if the trunk port carries traffic of VLANs 100 to 200, assign the native VLAN a number outside that range.

Figure 19-3 Potential Problem with 802.1Q Tunneling and Native VLANs

System MTU

The default system MTU for traffic on the Catalyst 4500 series switch is 1500 bytes. You can configure the switch to support larger frames by using the system mtu global configuration command. Because the 802.1Q tunneling feature increases the frame size by 4 bytes when the metro tag is added, you must configure all switches in the Service Provider network to be able to process larger frames by increasing the switch system MTU size to at least 1504 bytes. The maximum allowable system MTU for Catalyst 4500 Gigabit Ethernet switches is 9198 bytes; the maximum system MTU for Fast Ethernet switches is 1552 bytes.

802.1Q Tunneling and Other Features

Although 802.1Q tunneling works well for Layer 2 packet switching, there are incompatibilities between some Layer 2 features and Layer 3 switching.

•A tunnel port cannot be a routed port.

•IP routing is not supported on a VLAN that includes 802.1Q ports. Packets received from a tunnel port are forwarded based only on Layer 2 information. If routing is enabled on a switch virtual interface (SVI) that includes tunnel ports, untagged IP packets received from the tunnel port are recognized and routed by the switch. Customers can access the Internet through the native VLAN. If this access is not needed, you should not configure SVIs on VLANs that include tunnel ports.

•Tunnel ports do not support IP access control lists (ACLs).

•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 802.1Q configuration is consistent within an EtherChannel port group.

•Port Aggregation Protocol (PAgP), Link Aggregation Control Protocol (LACP), and UniDirectional Link Detection (UDLD) are supported on 802.1Q tunnel ports.

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

•Loopback detection is supported on 802.1Q tunnel ports.

•When a port is configured as an 802.1Q tunnel port, spanning-tree bridge protocol data unit (BPDU) filtering is automatically enabled on the interface. Cisco Discovery Protocol (CDP) is automatically disabled on the interface.

Configuring an 802.1Q Tunneling Port

To configure a port as an 802.1Q tunnel port, perform this task:

Use the no vlan dot1q tag native global command and the no switchport mode dot1q-tunnel interface configuration command to return the port to the default state of dynamic auto. Use the no vlan dot1q tag native global configuration command to disable tagging of native VLAN packets.

This example shows how to configure an interface as a tunnel port, enable tagging of native VLAN packets, and verify the configuration. In this configuration, the VLAN ID for the customer connected to Gigabit Ethernet interface 2/7 is VLAN 22.

Switch(config)# interface gigabitethernet2/7

Switch(config-if)# switchport access vlan 22

% Access VLAN does not exist. Creating vlan 22

Switch(config-if)# switchport mode dot1q-tunnel

Switch(config-if)# exit

Switch(config)# vlan dot1q tag native

Switch(config)# end

Switch# show dot1q-tunnel interface gigabitethernet2/7

Port

-----

LAN Port(s)

-----	

Gi2/7

Switch# show vlan dot1q tag native

dot1q native vlan tagging is enabled globally

Understanding Layer 2 Protocol Tunneling

Customers at different sites connected across a Service Provider network need to use various Layer 2 protocols to scale their topologies to include all remote and local sites. 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 network. 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 network 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. Layer 2 protocol data units (PDUs) for CDP, STP, or VTP cross the Service Provider network 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 these results:

•Users on each of a customer's sites can 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 to all switches through the Service Provider.

Layer 2 protocol tunneling can be used independently or can enhance 802.1Q tunneling. If protocol tunneling is not enabled on 802.1Q tunneling ports, 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 802.1Q tunneling achieve complete knowledge of the customer's VLAN. If 802.1Q tunneling is not used, you can still enable Layer 2 protocol tunneling by connecting to the customer switch through access ports and by enabling tunneling on the Service Provider access port.

As an example, Customer A in Figure 19-4 has four switches in the same VLAN that are connected through the Service Provider network. If the network does not tunnel PDUs, switches on the far ends of the network cannot properly run STP, CDP, and VTP. For example, STP for a VLAN on a switch in Customer A's Site 1 will build a spanning tree on the switches at that site without considering convergence parameters based on Customer A's switch in Site 2. Figure 19-5 shows one possible spanning tree topology.

Figure 19-4 Layer 2 Protocol Tunneling

Figure 19-5 Layer 2 Network Topology without Proper Convergence

Configuring Layer 2 Protocol Tunneling

You can enable Layer 2 protocol tunneling (by protocol) on the access ports or tunnel ports that are connected to the customer in the edge switches of the Service Provider network. The Service Provider edge switches connected to the customer switch perform the tunneling process. Edge-switch tunnel ports are connected to customer 802.1Q trunk ports. Edge-switch access ports are connected to customer access ports.

When the Layer 2 PDUs that entered the Service Provider inbound edge switch through the tunnel port or the access port exit through its 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 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's VLAN tag. The core switches ignore the inner tags and forward the packet to all trunk ports in the same metro VLAN. The edge switches on the outbound side restore the proper Layer 2 protocol and MAC address information and forward the packets to all tunnel or access ports in the same metro VLAN. Therefore, the Layer 2 PDUs remain intact and are delivered across the Service Provider network to the other side of the customer network.

See Figure 19-4, with Customer A and Customer B 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 B 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 4, the metro 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 B 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 ports on the customer switch. In this case, the encapsulation and de-encapsulation 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.

This section contains the following subsections:

•Default Layer 2 Protocol Tunneling Configuration

•Layer 2 Protocol Tunneling Configuration Guidelines

•Configuring Layer 2 Tunneling

Default Layer 2 Protocol Tunneling Configuration

Table 19-1 shows the default configuration for Layer 2 protocol tunneling.

Layer 2 Protocol Tunneling Configuration Guidelines

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

•The switch supports tunneling of CDP, STP, including multiple STP (MSTP), and VTP. Protocol tunneling is disabled by default but can be enabled for the individual protocols on 802.1Q tunnel ports or on access ports.

•Dynamic Trunking Protocol (DTP) is not compatible with Layer 2 protocol tunneling because you must manually configure asymmetric links with tunnel ports and trunk ports.

•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 or an access port.

•EtherChannel port groups are compatible with tunnel ports when the 802.1Q configuration is consistent within an EtherChannel port group.

•If an encapsulated PDU (with the proprietary destination MAC address) is received from a tunnel port or an access port with Layer 2 tunneling enabled, the tunnel port is shut down to prevent loops. The port also shuts down when a configured shutdown threshold for the protocol is reached. You can manually re-enable the port (by entering a shutdown and a no shutdown command sequence). If errdisable recovery is enabled, the operation is retried after a specified time interval.

•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. CDP packets are not forwarded from tunnel ports.

•When protocol tunneling is enabled on an interface, you can set a per-protocol, per-port, shutdown threshold for the PDUs generated by the customer network. If the limit is exceeded, the port shuts down. You can also limit the BPDU rate by using QoS ACLs and policy maps on a tunnel port.

•When protocol tunneling is enabled on an interface, you can set a per-protocol, per-port, drop threshold for the PDUs generated by the customer network. If the limit is exceeded, the port drops PDUs until the rate at which it receives them is below the drop threshold.

•Because tunneled PDUs (especially STP BPDUs) must be delivered to all remote sites so that the customer virtual network operates 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.

Configuring Layer 2 Tunneling

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

Use the no l2protocol-tunnel [cdp | stp | vtp] interface configuration command to disable protocol tunneling for one of the Layer 2 protocols or for all three. Use the no l2protocol-tunnel shutdown-threshold [cdp | stp | vtp] and the no l2protocol-tunnel drop-threshold [cdp | stp | vtp] commands to return the shutdown and drop thresholds to the default settings.

This example shows how to configure Layer 2 protocol tunneling for CDP, STP, and VTP and how to verify the configuration.

Switch(config)# interface FastEthernet2/1

Switch(config-if)# l2protocol-tunnel cdp

Switch(config-if)# l2protocol-tunnel stp

Switch(config-if)# l2protocol-tunnel vtp

Switch(config-if)# l2protocol-tunnel shutdown-threshold 1500

Switch(config-if)# l2protocol-tunnel drop-threshold 1000

Switch(config-if)# exit

Switch(config)# l2protocol-tunnel cos 7

Switch(config)# end

Switch# show l2protocol

COS for Encapsulated Packets: 7

Port    Protocol Shutdown  Drop      Encapsulation Decapsulation Drop

                 Threshold Threshold Counter       Counter       Counter

------- -------- --------- --------- ------------- ------------- -------------

Fa2/11  cdp           1500      1000 2288          2282          0

        stp           1500      1000 116           13            0

        vtp           1500      1000 3             67            0

Monitoring and Maintaining Tunneling Status

Table 19-2 shows the commands for monitoring and maintaining 802.1Q and Layer 2 protocol tunneling.

Note With Release 12.2(20)EW, the BPDU filtering configuration for both dot1q and Layer 2 protocol tunneling is no longer visible in the running configuration as "spanning-tree bpdufilter enable." Instead, it is visible in the output of the show spanning tree int detail command as shown below.

Switch# show spann int f6/1 detail

 Port 321 (FastEthernet6/1) of VLAN0001 is listening

   Port path cost 19, Port priority 128, Port Identifier 128.321.

   Designated root has priority 32768, address 0008.e341.4600

   Designated bridge has priority 32768, address 0008.e341.4600

   Designated port id is 128.321, designated path cost 0

   Timers: message age 0, forward delay 2, hold 0

   Number of transitions to forwarding state: 0

   Link type is point-to-point by default

   ** Bpdu filter is enabled internally **

   BPDU: sent 0, received 0