6.4 Calculating the failure time
In our example, the master router fails. What we need to calculate is the time until
the backup router registers the failure of the master router.
Figure 6-5
The Master Down Interval corresponds to 3 times the advertisement interval plus
the skew time.
πππ π‘ππ π·ππ€π πΌππ‘πππ£ππ = 3 β π΄ππ£πππ‘ππ πππππ‘ πΌππ‘πππ£ππ + ππππ€ ππππ
where the skew time is equal to 256 minus the priority of the backup router divided
by 256.
ππππ€ ππππ =
(
256 β πππππππ‘π¦
)
256
If the master router fails, then the backup routers have received no advertisement
from it within the time period 3xAdvIn. Each backup router now calculates its
Master Down Interval as follows:
Router 2
ππππ€ ππππ =
256 β 100
256
=
156
256π
πππ π‘ππ π·ππ€π πΌππ‘πππ£ππ =
(
3 β 1
)
+
156
256
= 3 +
156
256
= 3.6093 π
Router 3
ππππ€ ππππ =
256 β 50
256
=
206
256π
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