allows for faster convergence when a link fails because it is looped. (Links are typically looped
for testing purposes.) When this occurs, a router continues to receive the looped Cisco
proprietary keepalive messages, so the router might not think that the link has failed. For
instance, the absence of routing updates from a neighbor for a certain length of time is used to
drive convergence. Waiting on such an event when the link is looped increases convergence
time.
router sends PPP messages instead of keepalives; these messages include a magic number,
which is different on each router. If a line is looped, the router receives a message with its own
magic number instead of getting a message with the magic number identifying the router on the
other end of the link. A router receiving its own magic number knows that the frame it sent has
been looped back. If configured to do so, the router can take down the interface, which speeds
convergence.
Monitoring (LQM). PPP at each end of the link sends messages describing the number of
correctly received packets and bytes. This is compared to the number of packets and bytes sent
to calculate a percentage loss. The router can be configured to take down the link after a
configured error rate has been exceeded so that future packets are sent over a longer--but
hopefully better--path.
detection
other with a different magic number. If you ever receive
your own magic number, the link is looped. A
configuration setting determines whether the link
should be taken down when looped.
to authenticate the device on the other end of the link.
links. This feature is more often used with dial. The
section "Multilink PPP," later in the chapter, covers this
concept in greater detail.
Feature