Cisco Systems 8600 Series manual Tag Forwarding Information Base Tfib in an ATM Environment

Tag Switching in an ATM WAN

Figure 9-2 Tag Forwarding Information Base (TFIB) in an ATM Environment

Control

ATM-TSRs use the downstream-on-demand allocating mechanism. Each ATM-TSR maintains a forwarding information base (FIB) that contains a list of all IP routes that the ATM-TSR uses. This function is handled by the routing engine function which is either embedded in the switch or runs on an outside controller. For each route in its forwarding information base, the ATM Edge TSR identifies the next hop for a route. It then issues via TDP a request to the next hop for a tag binding for that route.

When the next hop ATM-TSR receives the route, it allocates a tag, creates an entry in its TFIB with the incoming tag changed to the allocated outgoing tag. The next action depends on whether the tag allocation is in an optimistic mode or a conservative mode. In optimistic mode, it will immediately return the binding between the incoming tag and the route to the TSR that sent the request. However, this may mean that it is not immediately able to forward tagged packets which arrive, as the ATM-TSR may not yet have an outgoing tag/VCI for the route. In conservative mode, it does not immediately return the binding, but waits until it has an outgoing tag.

In optimistic mode, the TSR that initiated the request receives the binding information, it creates an entry in its TFIB, and sets the outgoing tag in the entry to the value received from the next hop. The next hop ATM TSR then repeats the process, sending a binding request to its next hop, and the process continues until all tag bindings along the path are allocated.

In conservative mode, the next hop TSR sends a new binding request to its next hop, and the process repeats until the destination ATM edge TSR is reached. It then returns a tag binding to the previous ATM-TSR, causing it to return a tag binding, and so on until all the tag bindings along the path are established.

Tag Switching 9-5