Реферат: Cisco Tag Switching Essay Research Paper Abstract

After a TFIB entry is populated with both incoming and outgoing tags, the TSR can forward packets for routes bound to the tags via the tag-switching algorithm. 8

The upstream TSR first allocates a tag for each entry in the routing table that contains a next hop address reachable by a point-to-point connection. Next, the upstream TSR updates the appropriate outbound TFIB entry by placing the allocated tag in the outgoing tag entry field and also places specific per-interface link layer information with that entry. The TSR creates the tag binding and then transmits the tag binding to the downstream TSR representing the next hop toward the destination.

The downstream TSR receives the tag binding and places the tag in the incoming TFIB entry for the destination network. 1

When a TSR creates a binding between an outgoing tag and a route, the TSR will populate its TFIB and update all necessary table entries with binding information. Note the following:

A TSR can add tags to previously untagged packets.

The total number of tags a TSR must maintain can be no greater than the number of routes in the TSRs routing table.

A single tag can be associated with a group of routes, not just a single route.

In general, a TSR will try to populate its TFIB with incoming and outgoing tags for all reachable routes, allowing all packets to be forwarded by simply using label swapping. Tag allocation is driven by topology (routing), not traffic. 7

The use of tags associated with routes, rather than flows, also means that there is no need to perform flow-classification procedures for all the flows of data to determine whether to assign a tag. This simplifies the overall routing scheme and produces a more robust and stable environment. 4

In conclusion, when tag switching is used to support destination-based routing, the need for normal network layer forwarding is not eliminated. First, to add a tag to a previously untagged packet requires normal network layer forwarding. This function can be performed by the first hop router, or by the first router on the path that is able to participate in tag switching. 7 In most cases, a packet can be forwarded by using the tag-switching algorithm.

Hierarchy of Routing Knowledge

The hierarchy of routing knowledge is one aspect used to improve the scaling properties of the routing system, which is one of the essential goals of tag switching. First, to understand the hierarchy of routing knowledge, we must review certain parts of the Internet routing architecture.

The IP routing architecture used today in the Internet is hierarchical and represents a collection of routing domains. Below we define those routing domains.

X Routing within individual domains is provided by intra-domain routing protocols (e.g. OSPF, RIP, EIGRP).

X Routing across multiple domains is provided by inter-domain routing protocols (e.g. BGP). Routing across these inter-domain systems is usually referred to as routing between Autonomous Systems. 2

One advantage to partitioning routing into intra- and inter-domain components is the reduction in the volume of routing information that has to be maintained by routers, which is essential to providing a scalable routing system. 6 The initial partitioning of routing information is not complete. The following information describes some routing issues and resolutions:

X Transit Routing Domain – a domain that carries traffic that neither originates in the same domain nor is destined for a node in the same domain. 6

X Every router within a transit routing domain has to store in its forwarding tables all the routes provided by the inter-domain routing, regardless of whether this router is an interior router or a border router. 5

X The amount of routing information is not insignificant. This places additional demand on the resources required by the routers in order for them to maintain all necessary routes. This increase in the volume of routing information would tend to increase routing convergence time, which leads to degradation of the overall performance of the routing system. 3

X Since interior routers in a transit domain basically transfer packets, from one border router to another, it seems wasteful to have them maintain complete routing tables for all inter- and intra-domain routes.

Tag Switching provides a means by which interior routers can store only the routing information they really need. The border routers still maintain full routing information. Tag switching allows the decoupling of intra-domain and inter-domain routing, so that only TSRs at the border of a domain would be required to maintain routing information provided by the inter-domain routers. However, all other intra-domain routers in that domain would only maintain routing information associated with the interior routing protocols. Now the routing load on non-border routers has been reduced and the convergence time has been shortened.

To support this separation of interior and exterior topologies mentioned above, Tag switching allows a packet to carry not just one but a set of tags, organized in a stack.

+ Figure 6. Simple example of Tag Switching with a hierarchy of routing knowledge. 6

A TSR could either swap the tag at the top of the stack, or pop the stack, or swap the tag and push one or more tags into the stack. Inter-domains are connected via border TSRs. When a packet is forwarded between inter-domains, the tag stack in the packet contains only one tag.

When a packet is forwarded in an intra-domain, the tag stack in the packet contains two tags. The intra-domains ingress border TSR pushes the second tag onto the stack. The tag at the top of the stack provides packet forwarding information to the appropriate egress border TSR, while the next tag in the stack provides correct packet forwarding information at the egress TSR. When the packet reaches the egress TSR, or the next to last TSR, the tag stack is popped.

+ Figure 7. Example of tag stack movement within the routing hierarchy. 1

The control component used in this scenario is similar to the one used with destination-based routing. The one difference lies with the fact that this methods tag binding information is distributed both among physically adjacent TSRs and among border TSRs within a single domain. 5

Flexible Routing using Explicit Routes and QoS

Explicit routing is another extremely useful function that is supported by tag switching. In the case of destination-based routing, the destination address is the only information that is used to forward a packet. This particular function does enable highly scalable routing, but it also limits the capability to influence the actual paths taken by packets. When a need arises to evenly distribute traffic among multiple links in order to relieve the load off of those links that are over