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Cisco Tag Switching 18513
Cisco Tag Switching 18513

Cisco Tag Switching 18513

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  • Pages: 2 (978 words)
  • Published: October 8, 2018
  • Type: Case Study
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Abstract

The ever-increasing growth of the Internet and corporate Intranets are presenting a serious challenge to service providers and equipment vendors. This paradigm shift has transformed the way the world does business and defines new requirements for any business that wants to gain a competitive edge. Service providers, especially, are faced with the challenge of creating differentiated IP services and getting these new value-added services to market quickly. Cisco' s Tag Switching technology is a key component in Cisco' s plans for meeting these challenges. The two main components of Tag Switching are forwarding and control. This Tag Switching technology can retain the scaling properties of IP, and can help improve the scalability of IP networks. Tag Switching marries the performance of Layer 2 switching with the intelligence of Layer 3 routing to meet future growth demands. 4

Introduction

The continuous expansion of the Internet demands higher bandwidth within the Infrastructures of all corporations, not just Internet Service Providers (ISPs). Nevertheless, growth of the Internet is not the only driving factor for higher bandwidth; demand for higher bandwidth also comes from emerging multimedia applications. This demand for higher bandwidth requires higher throughput performance (packets per second) by routers, for both unicast and multicast traffic.

The expansion of the Internet also demands improved scaling properties of the Internet routing system. The ability to contain the magnitude of routing information maintained by individual routers and the ability to build a hierarchy of routing knowledge are essential to support a quality, scalable routing systems. 2

The requirements are growing for the need to:

  • X Improve scalability
  • X Improve
    ...

    forwarding performance

  • X Add routing functionality to support multicasting
  • X Allow for more flexible control over routing the traffic
  • X Provide the ability to build a hierarchy of routing knowledge 1

Cisco' s multilayer switching technology, known as Tag Switching, provides an effective solution for meeting the aforementioned requirements. Tag switching in essence provides the functionality of a Network Layer router with the performance of an ATM switch. This technology is simple and based on the concept of "label swapping," which uses a label (called a tag) for layer 3 packet forwarding. The simplicity of the tag switching forwarding component (label swapping) enables improved forwarding performance, while maintaining a competitive price performance ratio. To enhance the flexibility of the system, Cisco chose to support multiple granularities within the definition of a tag. This enables a tag switch to forward data based on a wide variety of routing functions, such as multicast, destination-based routing, hierarchy of routing knowledge, and flexible routing control. Ultimately, a combination of all of these routing functions provided in tag switching enables a routing system that will be able to accommodate the demand made by emerging requirements in the corporate backbones as well as the Internet backbone. 7

In this article Cisco' s Tag Switching will be described and defined by it' s components, routing capabilities, ATM switching capabilities, and QoS implementation.

Tag Switching Elements and Components

Again, Tag Switching is an innovative technique for high performance packet forwarding that assigns "tags" to multi-protocol frames for transport across packet, or cell based networks. This technology is based on the concept of "labe

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swapping," where a packet or a cell, carry a short, fixed-length label that tells switching nodes how to process the data sent. 7

There are some observations we need to make from the aforementioned description of the forwarding component. First, the forwarding decision is based on the exact match algorithm using a fixed length, fairly short tag as an index. This, in turn, enables a simplified forwarding procedure, relative to the longest match forwarding traditionally used at the network layer. This allows higher forwarding performance (faster throughput = greater number of packets per second). This forwarding mechanism is simple enough to allow a straightforward hardware implementation. 1

Secondly, note that the forwarding decisions made are independent of the tag' s forwarding granularity. For example, the same forwarding algorithm applies to both unicast and multicast traffic: a unicast entry would have a 1 to 1 relationship, in that it would have a single [outgoing tag, outgoing interface, outgoing link level information,] subentry, while a multicast entry would have a one or more subentries. This demonstrates how the same tag-forwarding criterion can be used in tag switching to support different routing functions. (e.g. unicast, multicast, etc.)

The simple forwarding procedure is thus in essence decoupled from the control component of tag switching. New routing (control) functions can easily be deployed without disturbing the forwarding criterion. Essentially, it does not become necessary to re-optimize the forwarding performance, by modifying either hardware or software, when new routing functionality is added.

Control Component

Binding between a tag and network layer routing (routes) is an essential part of tag switching. The control component is responsible for generating and maintaining a consistent set of tags among a set of TSR devices. Generating a tag involves allocating a tag and then binding that tag to a particular destination. The destination can be a host address, address prefix, multicast group address, or just about any network layer information. The particular destination is usually a TSR. 6

The control component is organized as an aggregation of modules, each designed to support a particular routing function. Adding new modules supports new routing functions. The following sections describe some of those modules. 4

Destination-Based Routing

Destination-based routing for unicast traffic is probably the most straightforward application of tag switching. In the context of destination-based routing, a FEC is associated with an address prefix. Using the information provided by unicast routing protocols (e.g. OSPF, EIGRP, BGP), a TSR router constructs mappings between FECs (address prefixes) and their corresponding next hops. The Tag Switching control component uses this mapping to construct its TFIB; the TFIB is used for the actual packet forwarding, not like conventional routers, which uses the FEC to next-hop mapping to do the actual forwarding of packets. 6

 

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