A C B E0
S0
S0
10.1.0.0 10.4.0.0 10.2.0.0
S0
S1 E0
10.3.0.0 X
Update After
Hold-Down Time
Update After
Hold-Down Time
Network 10.4.0.0
is unreachable.
Network 10.4.0.0 is down,
then back up, and
then back down.
112 Chapter 3: Medium-Sized Routed Network Construction
Figure 3-19 Triggered Updates
Triggered updates would be suf?¬?cient with a guarantee that the wave of updates reached every
appropriate router immediately. However, two problems exist:
?– Packets containing the update message can be dropped or corrupted by some link in the
network.
?– The triggered updates do not happen instantaneously. A router that has not yet received the
triggered update can issue a regular update at just the wrong time, causing the bad route to be
reinserted in a neighbor that had already received the triggered update.
Coupling triggered updates with holddowns is designed to get around these problems.
Route Maintenance Using Hold-Down Timers with Triggered Updates
Because the hold-down rule says that when a route is invalid, no new route with the same or a
higher metric will be accepted for the same destination for some period, the triggered update has
time to propagate throughout the network.
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