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Cisco CCNP / BSCI Tutorial The Role Of The OSPF ASBR |
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To pass the BSCI exam and earn your CCNP certification, you've
got to master the (many) details of OSPF. You might have thought
there were quite a few OSPF details in your CCNA studies,
but you'll now build on that foundation on the way to earning
your CCNP.
One such detail is the role of the Autonomous System Border
Router (ASBR) in OSPF. The name itself raises some eyebrows,
since you learned in your CCNA studies that OSPF doesn't use
autonomous systems! Just as an OSPF Area Border Router borders
multiple OSPF areas, the ASBR borders the entire OSPF domain
and another source of routes. This can be another dynamic
routing protocol, or directly connected networks that are
not being advertised into OSPF by the network command.
Let's say we have a router running both OSPF and RIP version
2. By default, the RIP process will not contain any OSPF-discovered
routes, and vice versa. The two separate routing processes
are just that - separate. If we want the other OSPF routers
to know about the RIP routes, route redistribution must be
configured. When the RIP routes are redistributed into OSPF,
that router is then an ASBR.
In the below example, RIP subnets have been redistributed
into OSPF. A seed metric is not necessary when redistributing
routes into OSPF. The command "show ip ospf" confirms
that this router is now an ASBR.
R1(config)#router ospf 1
R1(config-router)#redistribute rip subnets
R1#show ip ospf
Routing Process "ospf 1" with ID 1.1.1.1
Supports only single TOS(TOS0) routes
Supports opaque LSA
It is an autonomous system boundary router
The ASBR can also perform route summarization on the routes
being injected into OSPF with the summary-address command.
(To configure OSPF inter-area summarization, use the area
range command.) By mastering route summarization and route
redistribution, you're well on your way to passing the BSCI
exam and earning your CCNP certification!
About the Author:
Chris Bryant, CCIE #12933, is the owner of The Bryant Advantage,
home of free CCNP and CCNA tutorials, The Ultimate CCNA Study
Package, and Ultimate CCNP Study Packages. For a FREE copy
of his latest e-books, “How To Pass The CCNA” and
“How To Pass The CCNP”, just visit the website!
You can also get FREE CCNA and CCNP exam questions every day!
Pass the CCNP exam with The Bryant Advantage!
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