Protection switching between primary and secondary paths in a packet-based
network involves table entries that are pre-programmed with a primary
path, a secondary path, and a value that identifies the primary path,
referred to as a primary path identifier (PPI). When table entries are
accessed to make forwarding decisions, the PPI is compared to a field
that identifies that a particular path is down, referred to as a down
path identifier (DPI). If the two fields match, (i.e., PPI=DPI), then the
secondary path is selected instead of the primary path as the path on
which the traffic should be forwarded.