Реферат: Cisco Tag Switching Essay Research Paper Abstract

In downstream allocation, a TSR is responsible for creating tag bindings that apply to incoming data packets, and receiving tag bindings for outgoing packets from its neighbors.

In upstream allocation, a TSR is responsible for creating tag bindings for outgoing tags, such as tags that are applied to data packets leaving the TSR, and receiving bindings for incoming tags from its neighbors. The following summaries focus on the operational differences between all three methods.4

+ Figure 3. Destination based forwarding with tag switching. 6

Downstream Tag Allocation

When using downstream tag allocation, the TSR allocates a tag for each route in its routing table, creates an entry in its TFIB with the incoming tag set to the allocated tag. Then advertises the binding between the (incoming) tag and the route to other adjacent TSRs. The advertisement can be accomplished by either piggybacking1 the binding on top of the existing routing protocols, or by using the Tag Distribution Protocol (TDP).2

When a TSR receives tag binding information for a certain route, and that information was originated by the next hop for that route, the TSR places the tag (carried as part of the binding information) into the outgoing tag of the TFIB entry associated with the route. This creates the binding between the outgoing tag and the route. 4

Incoming Tag

Outgoing Tag

Next Hop Outgoing Interface

On TSR A 100 ? TSR B If1

On TSR B 6 ? TSR E If1

On TSR C 17 ? TSR D If2

On TSR D 5 ? TSR E If0

On TSR E 6 ? TSR E If0

+ Figure 4. Initial TFIB entries. (Example related to figure 3) 6

Downstream Tag Allocation on Demand

When using downstream on demand tag allocation, tags are allocated by a downstream TSR device and conveyed upstream as with the downstream technique. However, the allocation of tags by the downstream TSR is only performed upon a specific request from the upstream TSR. This technique is appropriate for TSR devices that have an ATM switch component. Typically, ATM switches have a finite set of tags ( VPI/VCI labels) they are able to support. 1

When an entry is made in a TSRs routing table index, the upstream TSR generates a request for a tag binding and transmits it to the next hop toward the destination. After the downstream TSR representing the next hop toward the destination receives the allocation request, the TFIB allocates a tag and updates the incoming entry index. A tag binding with [ address prefix, tag ] is created and transmitted to the upstream TSR.

When the upstream TSR receives the tag binding, it replaces the tag in the outgoing tag entry of the TFIB along with any outgoing link layer information. 7

Incoming Tag

Outgoing Tag

Next Hop Outgoing Interface

On TSR A 100 6 TSR B if1

On TSR B 6 6 TSR E if1

On TSR C 17 5 TSR D If2

On TSR D 5 6 TSR E If0

On TSR E 6 ? TSR E If0

+ Figure 5. TFIB entries after tag distribution. (Example related to figure 3) 6

Upstream Tag Allocation