Cisco Systems 8600 Series manual Tag Switching Benefits, Tag Switching Overview

Tag Switching Benefits

Tag Switching Benefits

For multi-service networks, tag switching enables the BPX switch to provide ATM, frame relay, and IP Internet service all on a single platform in a highly scalable way. Support of all these services on a common platform provides operational cost savings and simplifies provisioning for multi-service providers.

For internet service providers (ISPs) using ATM switches at the core of their networks, tag switching enables the Cisco BPX 8600 series and the Lightstream 1010 ATM switches to provide a more scalable and manageable networking solution than just overlaying IP over an ATM network. Tag switching avoids the scalability problem of too many router peers and provides support for a hierarchical structure within an ISPs network, improving scalability and manageability.

By integrating the switching and routing functions, tag switching combines the reachability information provided by the router function with the traffic engineering optimizing capabilities of the switches.

When integrated with ATM switches, tag switching takes advantage of switch hardware that is optimized to take advantage of the fixed length of ATM cells, and to switch these cells at wire speeds.

Tag Switching Overview

Tag switching is a high-performance, packet (frame) forwarding technology. It integrates the performance and traffic management capabilities of data link layer 2 with the scalability and flexibility of network layer 3 routing.

Tag switching enables switch networks to perform IP forwarding. It is applicable to networks using any layer 2 switching, but has particular advantages when applied to ATM networks. It integrates IP routing with ATM switching to offer scalable IP-over-ATM networks.

Tag switching is based on the concept of label switching, in which packets or cells are assigned short, fixed length labels. Switching entities perform table lookups based on these simple labels to determine where data should be forwarded.

In conventional layer 3 forwarding, as a packet traverses the network, each router extracts all the information relevant to forwarding from the layer 3 header. This information is then used as an index for a routing table lookup to determine the packet’s next hop. This is repeated at each router across a network.

In the most common case, the only relevant field in the header is the destination field. However, as other fields could be relevant, a complex header analysis must be done at each router through which the packet travels.

In tag switching the complete analysis of the layer 3 header is performed just once, at the tag edge router at each edge of the network. It is here that the layer 3 header is mapped into a fixed length label, called a tag.

At each router across the network, only the tag needs to be examined in the incoming cell or packet in order to send the cell or packet on its way across the network. At the other end of the network, a tag edge router swaps the label out for the appropriate header data linked to that label.

9-2Cisco BPX 8600 Series Reference