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OSPF Route Determination Using SPF Trees
(Page 3 of 4)
To construct the second level, we
look for all devices that the devices on the first level can reach directly.
We then add the cost to reach each device on the first level to the
cost of each device at the second level:
- RB: RB has a cost of 5 and
- RC, with a cost of 5, total cost of 10.
- N2, with a cost of 4, total cost of 9.
- N3, with a cost of 5, total cost of 10.
- N2: N2 has a cost of 3 and
- RA, with a cost of 0, total cost of 3.
- RB, with a cost of 0, total cost of 3.
- RC, with a cost of 0, total cost of 3.
- N3: N3 has a cost of 6 and
- RB, with a cost of 0, total cost of 6.
- RC, with a cost of 0, total cost of 6.
- RD, with a cost of 0, total cost of 6.
You probably can see immediately
that we ended up with a number of different paths to the same devices
or networks, some of which make no sense. For example, we don't really
care about any path that goes to RC, since we are RC! Similarly,
we can weed out certain paths immediately because we already have a
shorter path to them; taking a path through RB to N3 with a cost of
10 makes no sense when we can go directly at the first level for a cost
of 6. So, after separating out the chaff, we end up with the following
wheat at the second level:
- N2 to RA, with a cost of 3.
- N3 to RD, with a cost of 6.
We continue the process by looking
for devices that connect to the weeded-out devices we found
on the second level (this time I am only showing the meaningful ones):
- RA: RA connects to N1,
with a cost of 2, total cost of 5.
- RD: RD connects to N4,
with a cost of 4, total cost of 10.
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Version 3.0 - Version Date: September 20, 2005
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