1. Reasons for unnecessary route changes: (1) not knowing the causeb c
c withdraws its route
f withdraws its route;
but the withdrawal by e is delayed d e f
Solution:
propagate information about fault and its location
g knows the invalidity of [e, c, a],
and adopts [d, b, a] immediately g
g will mistakenly regard route
[e, c, a] as valid, and adopt it h

2. Reasons for unnecessary route changes : (2) uncertainty about fault type
the fail-stop of links <a, b> and
<a, c>, instead of the fail-stop
of a, is detected b c
b, c, e, f withdraw their routes;
but the withdrawal by d is delayed d e f
d withdraws its route
Solution:
G waits for certain time
g will mistakenly regard [d, b, a]
as valid, and adopt it g
g withdraws its route h

3. Reasons for unnecessary route changes : (3) obsolete fault information
<a, c> re-joins
Information m0(<a, c>) is generated
to signify the fail-stop of link <a, c> b c d e f
m0(<a, c>) reaches f, but is delayed
in reaching e
c, f, and g change their routes back
Delayed m0(<a, c>) reaches e, and then g
g rejects the delayed m0(<a, c>), and
does not change route g
Solution:
reject obsolete information by attaching
seq. number to fault information
g changes its route to [d, b, a]
g changes its route to [d, b, a] h

4. Reasons for unnecessary route changes : (4) fault propagation
state corruption at c:
c withdraws its route b c
c corrects its state
Before the fault propagates to f,
c corrects the state of itself
Solution:
execute “correction” action quicker than
“propagation” action
before c corrects its state,
f withdraws its route.
Thus, the fault at c propagates to f d f
before f corrects its state,
g changes its route.
Thus, the fault at c propagates to g g h