SAN JOSE, Calif. - September 16, 1996 - Cisco Systems, Inc. today introduced a new multilayer switching technology for scaling router and switch backbones, including those of the Internet and large corporate intranets.

Tag Switching 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.

"Tag Switching defines a scalable means for routers to support the advanced traffic management capabilities currently offered only by switched networks, while allowing switched networks to scale in the same manner as today's global router backbones," said John Morency of The Registry, a network consultancy based in Newton, Mass.

This new technology is a cornerstone of Cisco's comprehensive solution set for scaling the Internet. (See accompanying release, "Cisco Scales The Internet.") As such, the company first will implement Tag Switching on its high-end, Cisco 7500 series routers and Cisco StrataCom BPX switches.

Cisco also plans to make Tag Switching available as a Cisco IOS software upgrade on other routers and ATM, wide-area and LAN switches.

Extending Cisco's History of Leadership in Scaling Large Networks

"Tag Switching is the latest in a series of Cisco innovations for scaling all aspects of corporate and service provider networks," said Ed Kozel, Cisco's chief technical officer. "The best way to achieve scalable performance is through multilayer switches, which combine the best aspects of routers and switches."

Scalable Platforms

Cisco has delivered a number of increasingly powerful switching mechanisms, such as NetFlow Switching, to scale its multiprotocol routers. NetFlow Switching enables customers to significantly boost the performance of network-layer services providing security, quality of service (QoS) and traffic accounting capabilities.

Scalable Campus Backbones

In 1994, the company introduced its CiscoFusion architecture for scaling campus networks. This occurred as campus networks began the transition from shared-media hubs to wiring closet switching systems interconnected by switched backbones.

CiscoFusion is based on distributed routing protocols that enable routers to cache packet-forwarding information into edge switches. This technique helps campus network managers optimize the operation of virtual LANs, which address pressing administrative issues such as moves, adds and changes.

For example, these distributed routing protocols and the ATM Forum's Multiprotocol Over ATM (MPOA) specification - being developed by Cisco and other vendors - will allow for the cut-through of traffic between virtual LANs and maximize router utilization.

Scalable Internet/Intranet Backbones

Now, with Tag Switching, Cisco is addressing the urgent need to scale router backbones, which are being stressed by the exponential growth of Internet and intranet traffic.

Router networks generally fall into one of two categories: (1) a large-scale network of interconnected routers or (2) a network of switches (LAN, ATM or Frame Relay) connecting routers at its edge. Tag Switching adds significant new capabilities to both types of router backbones.

How Tag Switching Works

Tag Switching is a new technique for high-performance packet forwarding that assigns a label or "tag" to packets traversing a network of routers and switches.

In a conventional router network, each packet must be processed by each router to determine the next hop of the packet toward its final destination.

In a Tag Switching network, tags are assigned to destination networks or hosts. Packets then are switched through the network with each node simply swapping tags rather than processing each packet. A Tag Switching network will consist of a core of tag switches (either conventional routers or switches), which connect to tag edge routers on the network's periphery.

Tag edge routers and tag switches use standard routing protocols such as EIGRP, BGP and OSPF to identify routes through the network. These systems then use the tables generated by the routing protocols to assign and distribute tag information via a Tag Distribution Protocol. Tag switches and tag edge routers receive the Tag Distribution Protocol information and build a forwarding database. The database maps particular destinations to the tags associated with those destinations and the ports through which they are reachable.

When a tag edge router receives a packet for forwarding across the tag network, it analyzes the network-layer header and performs applicable network layer services. It then selects a route for the packet from its routing tables, applies a tag and forwards the packet to the next-hop tag switch.

The tag switch receives the tagged packet and switches the packet based solely on the tag, without re-analyzing the network-layer header. The packet reaches the tag edge router at the egress point of the network, where the tag is stripped off and the packet delivered.

Tag Switching is complementary to Cisco's NetFlow Switching. NetFlow Switching provides high-performance processing of value-added Layer 3 services in tag edge routers at the edge of a network. Tag Switching provides high-performance switching of packets in the core of the network.

Benefits of Tag Switching

Tag Switching brings a number of benefits to Internet service providers and enterprise network administrators:

• It allows Layer 2 switches to participate in Layer 3 routing. This increases network scalability because it reduces the number of routing peers that each edge router must deal with.
• It enables new traffic tuning mechanisms in router-based networks by integrating virtual circuit capabilities available previously only in Layer 2 fabrics. With Tag Switching, packet flows can be directed across the router network along pre-determined paths, similar to virtual circuits, rather than along the hop-by-hop routes of normal routed networks. This enables routers to perform advanced traffic management tasks, such as load balancing, in the same manner as ATM or Frame Relay switches.
• It can be applied not only to the Internet Protocol (IP) but to any other network-layer protocol. "Tag Switching is independent of the routing protocols employed," said Kozel. "While the Internet runs on IP, a lot of campus backbone traffic is transported on protocols such as IPX. A pure IP solution would not be adequate for most customers."

Extensions to Tag Switching will allow tag-switched networks to set up multiple paths across the network and offer differentiated QoS to packet flows. In this way, networks can support real-time packet traffic. Tag Switching also will support the routing hierarchy required for large-scale networks.

According to Morency, the first application of Tag Switching likely will be in the Internet, where scalability issues are most prevalent. "Unlike other approaches, Tag Switching, by associating tags with groups of destinations, obviates the need for cumbersome protocol and packet-processing overhead," Morency said. "In addition to conserving connection space, Tag Switching minimizes the need for slow-flow classification within switches and greatly increases scalability and performance.

"Flows classification - allowed but not mandated by Tag Switching - is important for real-time traffic, but does not scale for general Internet traffic," he added. "Over time, scalability issues likely will arise in large-scale corporate intranets, as well."

Tag Switching offers a particularly significant benefit to ATM switching systems used in wide area networks. Their scalability isn't limited by signaling performance issues. Today, the standards for packet transport over ATM, such as LAN Emulation and the emerging MPOA, are more appropriate for local campus networks than wide area networks because they can be signaling-intensive.

Well-designed ATM switches can cope with the signaling set-up rates required in most campus networks. But setting up connections for each of the packet flows on the Internet is not feasible because it would require signaling set-up rates far greater than anything possible today or in the foreseeable future.

With Tag Switching, however, fast paths can be pre-established through the network to all attached networks. This minimizes the need for exercising ATM signaling protocols and greatly reduces the processing burden on ATM switches.

Unlike other approaches, Tag Switching "still allows ATM switches to perform ATM Forum standard User-to-Network (UNI) signaling and Private Network-to-Network (P-NNI) routing functions in parallel," said Morency. "The switches reserve UNI and P-NNI only for the real-time, native ATM traffic that requires ATM's quality of service on-demand capabilities, while transferring the burden of transporting highly dynamic packet flows to the less processor-intensive Tag Switching mechanism."

Standardization and Licensing

Cisco is submitting the key Tag Switching specifications to the Internet Engineering Task Force (IETF) for standardization through cross-industry deliberation. Drafts of the Tag Switching Architecture Overview and the Tag Distribution Protocol specification are available at:

• ftp://ftp-eng.cisco.com/bdavie/draft-doolan-tdp-spec-00.txt, respectively.

These documents also will be available on the IETF's draft directories.

The company plans to extend Tag Switching to other switching technologies such as Fast Ethernet and Gigabit Ethernet. Cisco will work within the Institute of Electrical and Electronic Engineers (IEEE) to build upon the IEEE 802.1Q group's work on virtual LAN identifiers for Tag Switching on such LAN media.

"Beyond standards, Tag Switching places a premium on the sophistication of routing protocols implemented on tag switches," Morency noted, "because these protocols provide the intelligence for tag switches." Cisco plans to license Cisco IOS Tag Switching technology to its Cisco IOS partners. For example, the company expects to work with partner vendors to implement Tag Switching on network interface cards used for high-speed server interconnection.

"We believe the industry will greatly benefit from Tag Switching's ability to alleviate bottlenecks in Internet and intranet backbones," Kozel said. "And we welcome the industry's help in refining and finalizing our Tag Switching mechanism. Neither customers nor the industry will benefit from the current proliferation of limited and proprietary mechanisms."

Availability

Tag Switching technology is expected to be available for field trials on Cisco 7500 series routers and Cisco LightStream. 1010 campus ATM switches in the first half of 1997. Versions for the Cisco StrataCom BPX wide area ATM switch are expected to be available later in 1997.