5.4 Handling Tag Faults

5.4 Handling Tag Faults

When a TSR receives a packet with a tag, but either (a) there is no entry in the TFIB maintained by the TSR with the incoming tag equal to the tag carried in the packet or (b) there is such an entry, but the entry doesn?t indicate local delivery and the outgoing tag component of the entry is empty, we call this condition a tag fault. In this section we describe how Tag Switching handles this problem.

When a TSR encounters a tag fault caused by a packet, one possible option for the TSR is to strip the tag information from the packet and try to forward the packet based on the information carried in the network layer header of the packet. However, this option may not always be feasible. For example, when a packet carries a stack of tags, the TSR may not have sufficient routing information to forward the packet (e.g., TSR X in Figure 5.2 has routing information only about destinations within its own routing domain and thus doesn?t know how to forward a packet destined to a host in some other routing domain). When the TSR is an ATM-TSR, requiring the TSR to forward the packet based on the network layer header would require the ATM-TSR to reassemble ATM cells that form the packet into a packet and then process the network layer header of the packet. Clearly, this may be a rather unreasonable requirement.

Because handling tag faults by stripping...

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Switching in IP Networks: IP Switching, Tag Switching, & Related Technologies

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5.4 Handling Tag Faults

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