Tag Switching

Tag switching provides additional benefits in the areas of functionality, scalability, traffic management, and flexibility for service providers. For example, tag switching offers a flexible and scalable method to provide virtual private network (VPN) services with QoS. The traffic engineering features of tag switching are useful for managing traffic and link utilization in a routed network. Finally, tag switching's ability to integrate ATM and IP technology is of interest to those who want to use an ATM backbone to build a multiservice network.

The Internet Engineering Task Force (IETF) is developing a standard for tag switching based on Cisco's technology. The IETF term for its tag switching standard is Multiprotocol Label Switching (MPLS).

Tag switches receive TDP information from the tag edge routers and build their own forwarding database. Tag switches then switch the packets based on the tags only (VPI/VCI in the case of ATM), without looking at the Layer 3 header.

A TIB can exist for a whole switch, an interface, or a combination of the two.

In a tag switching network, the Layer 3 header is analyzed just once. It is then mapped into a short fixed-length tag. At each hop, the forwarding decision is made by looking only at the value of the tag. There is no need to reanalyze the Layer 3 header. Because the tag is a fixed-length, unstructured value, lookup is fast and simple.

When a tag edge router at the entry point of a tag switching network receives a packet for forwarding, the following process occurs:

1. Tag edge routers and tag switches use standard routing protocols to identify routes through the network. This routing information is summarized in the FIB.

2. Tag switches use the tables generated by the standard routing protocols to assign and distribute tag information using TDP. Tag switches receive TDP information and build the TFIB that makes use of the tags.

3. When a tag edge router receives a packet for forwarding across the network, it does the following:

(a) Analyzes the network-layer header

(b) Performs applicable network-layer services

(c) Selects a route for the packet from its routing tables

(d) Applies a tag and forwards the packet to the next-hop tag switch

4. The tag switch receives the tagged packet and switches the packet based solely on the tag, without reanalyzing the network-layer header.

5. The packet reaches the tag edge router at the egress point of the network, where the tag is stripped off and the packet delivered.

Advantages of implementing tag switching in an ATM network include the following:

• Combines the performance and traffic management capabilities of Layer 2 (data link layer) switching with the scalability and flexibility of Layer 3 (network layer) routing. You can direct packet flows across a router-based network along predetermined paths, such as virtual connections, rather than hop-by-hop as in a typical router-based network. This allows routers to perform advanced traffic management tasks, such as load balancing, in ATM switch routers.

• Uses standard routing protocols and TDP to distribute tag values with other tag switches and with tag edge routers. Unlike standard ATM, there is no call setup procedure.

• There is no need to use switched virtual connections (SVCs) for dynamic IP packet flows. This frees CPU processing power for PNNI and UNI flows, which can be reserved for transferring real-time voice and video and other time-sensitive traffic.

• Implementing tag switching on an ATM switch does not preclude the ability to support an ATM control plane, such as PNNI, on the same switch. On physical interfaces on the ATM switch router, tag switching and the ATM control plane operate in a ships in the night (SIN) mode and are unaware of each other.

• There are no high call setup rates. Standard ATM uses a connection setup procedure to allocate VCIs and program the ATM switching hardware, but tag switching uses standard routing protocols and TDP.

• ATM switches appear as routers to adjacent routers. This provides a scalable alternative to the overlay model (ATM switches in the core network and routers on the periphery) and removes the necessity for ATM addressing, routing, and signaling schemes.

• ATM switches can participate fully in hierarchical routing protocols and act as peers to tag edge routers. Thus the tag edge routers see far fewer peers than if the edge routers were interconnected via virtual connections over the ATM network. Therefore, in terms of the number of network devices, the network can scale to much larger sizes.

Limitations of tag switching include the following:

• Only the Open Shortest Path First (OSPF) routing protocol is supported in the tag switching implementation on the ATM switch router.

• Understanding tag switching at the troubleshooting level requires a knowledge of OSPF, the Border Gateway Protocol (BGP), and Cisco Express Forwarding (CEF).

Tag switching on the ATM switch router has the following software restrictions:

• OSPF is the only routing protocol currently supported

• IP is the only network layer protocol supported.

• Hierarchical VP tunnels and tag switching cannot co-exist on a physical interface.

• No tag switching virtual connections (or any other virtual connections) can be set up on a physical interface with a hierarchical VP tunnel.

Step 1 Configure a loopback interface.

Step 2 Enable tag switching on the ATM interface.

Step 3 Configure OSPF.

Step 4 Configure a VPI range (optional).

Step 5 Configure TDP control channel (optional).

Step 6 Configure tag switching on VP tunnels.

Step 7 Configure VC merge (optional).

Step 8 Configure CoS.

Configuring the loopback interface requires the following steps:

Step 1 Enter interface configuration mode and assign a number to the loopback interface.

Step 2 Assign an IP address and subnet mask to the loopback interface.

Step 1 Select the ATM interface to configure and enter interface configuration mode.

(a) Enable IP unnumbered on the ATM interface and assign the unnumbered interface to an interface that has an IP address. This is the recommended method. It allows you to conserve IP addresses and reduces the number of tag virtual channels (TVCs) terminating on the switch.

(b) Assign an IP address and subnet mask to the ATM interface.

All parallel interfaces between ATM switch routers should be configured with the same method.

Step 3 Enable tag switching of IPv4 packets on the interface.

Configuring OSPF requires the following steps:

Step 1 Enable OSPF and assign it a process number.

Step 2 Define the network prefix, a wildcard subnet mask, and the associated area number on which to run OSPF.

Repeat this step for each additional area you want to add to the OSPF process.

• It is an administrative policy to use a VPI value other than 1, the default VPI.

• There is a large number of TVCs on an interface.

Changing the TDP control channel requires the following steps:

Step 1 Select the interface to configure and enter interface configuration mode.

Step 2 Specify the new VPI and VCI values for the new TDP control channel configuration.

Configuring tag switching on a VP tunnel requires the following steps:

Step 1 Select the interface to configure and enter interface configuration mode.

Step 2 Create a PVP with a VPI value.

Step 3 Select the interface and subinterface you specified in the previous steps.

Configuring the cross-connect in the intermediate switch requires the following steps:

Step 1 Select one of the interfaces and enter interface configuration mode.

Step 2 Connect the PVP from the source switch to the destination switch by specifying the PVP on this interface, the number of the opposite interface, and the PVP to use on that end.

VC merge is enabled by default on the ATM switch router. No manual configuration is required for it to work.

Each service class is assigned a relative weight. These weights are configurable, and range from 1 to 15. Configuring the service class and relative weight requires the following steps:

Step 1 Select the interface to configure and enter interface configuration mode.

Step 2 Enter the service class and relative weight for the interface.

The number of threshold groups available on the ATM switch router is platform dependent. For details, refer to the ATM Switch Router Software Configuration Guide.

Each threshold group has a set of eight regions, and each region has a set of thresholds. When these thresholds are exceeded, cells are dropped to maintain the integrity of the shared memory resource.

Each ATM Forum service category is mapped into a distinct threshold group. All the connections in a particular service category map into one threshold group. Similarly, all the TBR classes have best effort traffic; the service differentiation comes mainly by assigning different weights. Each of the TBR classes map into four different threshold groups whose parameters are the same as the UBR threshold group.