Cisco CCNP / BSCI Exam Tutorial: OSPF Router TypesPage: 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 or Go To our Certification Articles Section
When you're preparing to pass the BSCI exam on the way to
the coveted Cisco CCNP certification, you can be quickly overwhelmed
by the sheer amount of BGP and OSPF knowledge you must demonstrate
a mastery of. One set of details that some BSCI and CCNP candidates
underestimate are the differences between the OSPF router
types. The OSPF router types seem straightforward enough,
but what you must keep in mind is that a router can fill more
than one of these roles!
An OSPF Internal router has one rule - it must have all its
interfaces in a single area. It does not mean that area has
to be Area 0.
An OSPF Backbone router is a router with at least a single
area in the OSPF backbone area, Area 0. A router can be both
an Internal and Backbone router if all its interfaces are
in Area 0.
An Area Border Router has at least one interface in Area
0 and another interface in a non-backbone area. ABRs are also
one of two router types that can perform OSPF route summarization.
(To advertise a summary route from one OSPF area to another,
use the area range command on the ABR.)
Finally, an ASBR is an OSPF router that is performing route
redistribution by injecting routes from another source into
the OSPF domain. This is the other OSPF router type that can
perform route summarization; to summarize routes being redistributed
into OSPF, use the summary-address command on the ASBR.
There are several commands you can use to determine the router
types in a given OSPF area. The command "show ip ospf"
will display quite a bit of information regarding the local
router, and this includes whether that router is acting as
an ABR or ASBR. To see the routes to the ABRs and ASBRs from
the local router, run "show ip ospf border-routers".
About the Author:
Chris Bryant, CCIE #12933, is the owner of The Bryant Advantage,
home of free CCNA and CCNP tutorials, The Ultimate CCNA Study
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