Реферат: Cisco Tag Switching Essay Research Paper Abstract

X As a small “shim” tag header inserted between a layer2 and layer 3 header.

X As a part of the layer 2 header.

X As a part of the layer 3 header.

+ Figure 2. Tag shim header Note: numbers in ( ) equal number of bits1

As a result, tag switching can be implemented over virtually any media type including point-to-point links, multi-access links, and ATM.

Observe also that the forwarding component is network layer independent. Use of control components specific to a particular network layer protocol enables the use of tag switching with different network layer protocols.

Forwarding Component

The forwarding criterion used by tag switching is based on a technique known as label swapping. When a tag switch receives a packet with a tag, the switch uses the tag as an index in its Tag Forwarding Information Base (TFIB). Each entry in the TFIB consists of an incoming tag, and one or more sub-entries of the form:

Outgoing tag

Outgoing interface

Outgoing link level information (e.g. MAC address) 3

Once the tagged packet enters the switch, the switch searches its TFIB index for an entry equal to the incoming packet tag. Then for each outgoing tag, outgoing interface, outgoing link level information, in the entry the switch replaces the tag in the packet with the outgoing tag, replaces the link level information in the packet with the outgoing link level information, and forwards over the outgoing interface. 3

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.) 2

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

Once a TSR has constructed a mapping between a particular FEC and its next-hop, the TSR is ready to construct an entry in its TFIB.

There are three permitted methods that accommodate tag allocation and TFIB management: 4

Downstream tag allocation

Downstream tag allocation on demand

Upstream tag allocation

In all cases, a TSR allocates tags and binds them to address prefixes in its TFIB. 1

Downstream tag allocation – the tag that is carried in a packet is generated and bound to a prefix by the TSR at the downstream end of the link ( with respect to the direction of data flow).

Upstream allocation – tags are allocated and bound at the upstream end of the link.

On-demand allocation means that tags are allocated and distributed by the downstream TSR only when requested to do so by the upstream TSR.