3Com CoreBuilder 2500, User Manual

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7-8 C
HAPTER
7: R
OUTING WITH
OSPF
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Area border routers — Area border routers connect directly to
networks in two or more areas. An area border router runs a separate
copy of the OSPF algorithm and maintains separate routing data for
each area connected to it (including the backbone area).
In Figure 7-1, four area border routers link the areas in autonomous
system A.
Area border routers send configuration summaries for their attached
areas to the backbone area, which distributes this information to other
OSPF areas in the autonomous system.
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Autonomous system boundary routers — Autonomous system
boundary routers exchange their autonomous system topology data
with boundary routers in other autonomous systems. Every router
inside an autonomous system knows how to reach the boundary
routers for its autonomous system.
In Figure 7-1, two autonomous system boundary routers control
traffic between two autonomous systems.
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Designated routers (DRs) — Designated routers advertise network
link states for attached network segments. A link state advertisement
lists all routers connected to a segment.
The DR exchanges routing data with all routers that are connected to its
network segment.
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Backup designated routers (BDRs) — Backup designated routers
are given a lower priority value than the DR and take over DR
functions if the DR fails.
Router IDs
The OSPF router ID identifies a router to other routers within an
autonomous system. OSPF uses three types of router identifiers, which
take the form of an IP address:
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Default — An arbitrary ID that the system generates and uses as the
default router ID
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Interface — The address of an IP interface on the router
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Address — An arbitrary user-defined ID in the form of an IP address