Cisco CCNP Certification / BSCI Exam Tutorial: OSPF Router Types
Posted: Tuesday, March 07, 2006
by Chris Bryant CCIE 12933
The Bryant Advantage
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.
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 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".
Chris Bryant, CCIE #12933, is the owner of The Bryant Advantage, home of over 100 free certification exam tutorials, including Cisco CCNA certification test prep articles. His exclusive Cisco CCNA study guide and Cisco CCNA exam training is also available!
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