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OSPF Basic Topology and the Link State Database (LSDB)
(Page 2 of 3)
OSPF Basic Topology and LSDB Example
For example, let's consider the same
autonomous system we looked at in our examination of the RIP route determination
algorithm. This internetwork has four individual networks, connected
as follows:
- Router A (RA) connects Network
1 (N1) to Network 2 (N2).
- Routers B (RB) and C (RC)
connect Network 2 to Network 3 (N3).
- Router D (RD) connects Network
3 to Network 4 (N4).
To make this example more interesting,
I am also going to add a direct link between RB and RC;
the resulting AS is shown in Figure 179.
The LSDB for this autonomous system would look something like Table 123.
Figure 179: Example OSPF Autonomous System This is the same AS that we looked at in RIP (as shown in Figure 172), but with the addition of a link between the two routers RB and RC.
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Table 123: Example OSPF Link State Database (LSDB)
To
Router / Network
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From
Router
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From
Network
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RA
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RB
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RC
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RD
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N1
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N2
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N3
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N4
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RA
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0
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0
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RB
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[
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0
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0
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RC
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[
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0
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0
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RD
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0
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0
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N1
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[
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N2
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[
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[
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[
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N3
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[
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[
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[
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N4
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[
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In practice, each of the
bullets ([)
in Table 123
would be replaced by a metric value indicating the cost to send a datagram
from the particular router to another router or network. Note that the
chart is symmetric, since if RB can reach RC, RC
can reach RB. However, the costs do not have to
be symmetric. It is possible for RB to have a metric that is
higher for it to send to RC than for RC to send to RB.
Note also that there is no cost to
reach a router from a network. This ensures that only
one cost is charged for a router to send to another router
over a networkthe cost to reach the network from the router. This
makes sense, since each router is a member of the network upon which
it is connected.
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The TCP/IP Guide (http://www.TCPIPGuide.com)
Version 3.0 - Version Date: September 20, 2005
© Copyright 2001-2005 Charles M. Kozierok. All Rights Reserved.
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