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.
𝑀𝑎𝑠𝑡𝑒𝑟 𝐷𝑜𝑤𝑛 𝐼𝑛𝑡𝑒𝑟𝑣𝑎𝑙 = 3 ∗ 𝐴𝑑𝑣𝑒𝑟𝑡𝑖𝑠𝑒𝑚𝑒𝑛𝑡 𝐼𝑛𝑡𝑒𝑟𝑣𝑎𝑙 + 𝑆𝑘𝑒𝑤 𝑇𝑖𝑚𝑒
where the skew time is equal to 256 minus the priority of the backup router divided
by 256.
𝑆𝑘𝑒𝑤 𝑇𝑖𝑚𝑒 =
(
256 − 𝑃𝑟𝑖𝑜𝑟𝑖𝑡𝑦
)
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
𝑆𝑘𝑒𝑤 𝑇𝑖𝑚𝑒 =
256 − 100
256
=
156
256𝑠
𝑀𝑎𝑠𝑡𝑒𝑟 𝐷𝑜𝑤𝑛 𝐼𝑛𝑡𝑒𝑟𝑣𝑎𝑙 =
(
3 ∗ 1
)
+
156
256
= 3 +
156
256
= 3.6093 𝑠
Router 3
𝑆𝑘𝑒𝑤 𝑇𝑖𝑚𝑒 =
256 − 50
256
=
206
256𝑠
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