5.5 Handling Forwarding Loops During Routing Transients

5.5 Handling Forwarding Loops During Routing Transients

As we discussed in Section 5.2 , because routing protocols used by the Control Component may produce temporary (transient) forwarding loops, we need mechanism(s) to contain the adverse effects of these loops. In this section we describe the mechanisms used by Tag Switching.

For the cases where tag information is carried in a packet in a shim, Tag Switching uses the time-to-live as the loop mitigation mechanism. The shim header contains the TTL field; each TSR that forwards a packet decrements this field; if a TSR receives a packet with TTL in the shim equal to 0, the TSR discards the packet. The effectiveness of this mechanism is comparable to the TTL-based loop mitigation used in IP. Moreover, use of a common mechanism by both IP and Tag Switching makes coexistence and interoperation between loop mitigation mechanisms used by Tag Switching and IP a nonissue.

For the cases where tag information is carried as part of the link layer header (e.g., ATM, Frame Relay), use of time-to-live?based loop mitigation is not a viable option because there is no TTL field in the ATM or Frame Relay header. For these cases, Tag Switching uses a two-pronged approach. First, all the traffic exchanged by the Tag Switching Control Component (e.g., routing traffic, Tag Distribution Protocol) is segregated from the rest of the traffic by using a separate tag (e.g., separate VCI/VPI for the case of ATM) and allocating resources (e.g., buffers, link bandwidth) for that...

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5.5 Handling Forwarding Loops During Routing Transients

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