CCNP ROUTE Exam Tutorial:
OSPF Design Guidelines For The 642-902 Exam
From "The Computer Certification Bulldog" -
Chris Bryant, CCIE #12933!
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During your CCNA studies, you worked with basic and intermediate OSPF configurations and networks. Those fundamental skills are very important, but there's one part of OSPF that's left out of your CCNA studies - working with multi-area OSPF networks.
Hey, we had to leave something for your CCNP! :)
In this new series of CCNP tutorials designed for those of you working on the CCNP ROUTE exam, we'll take a detailed look at multi-area OSPF. We'll go beyond the theory and go into detail about why we use multiple areas rather than one giant Area 0.
Before that, we need to review some OSPF design guidelines. The emphasis there is on "guidelines"; no two networks are alike and there are few hard-and-fast design rules that will apply to every single network.
Design rules are generally subjective, but these OSPF design rules are particularly important since they are designed to keep the demand on the router's CPU and memory resources to a reasonable level.
(The term "reasonable level" is subjective as well - generally, your more powerful routers will be at the core of your network while the lesser routers will be at the outskirts.)
Cisco OSPF Design Guidelines:
Don't put a router in more than three OSPF areas
Don't put more than 50 routers in a single area
Don't allow a router to have more than 60 OSPF neighbors
If you allow a router to become a DR and/or BDR for multiple OSPF segments, keep a close eye on the CPU usage on that router as this multiple-DR role can hammer the CPU
Finally, don't run more than one OSPF process on an Area Border Router (ABR)
Of all the OSPF design rules, it's my experience that you really have to watch the one regarding having too many routers in a single area.
This ends up causing more LSA traffic than you really need, which in turn means you've got more routers having to recalculate their routing tables more and more often, which in turn puts an unnecessary load on the CPU of the routers - and that overload is exactly what these guidelines are designed to help us avoid.
So why are we building these multiple OSPF areas rather than just putting all of our routers into one large Area 0?
Excellent question - and I'll answer it for you a little further down the page. Take just a moment to read this notice and learn how to prepare for CCNP ROUTE exam success with yours truly!
I'm paying it forward to all CCNP candidates:
My CCNP ROUTE Bulldog Boot Camp DVD is only $97!
 
"Chris, I passed my CCNP ROUTE exam yesterday. YEAH!! Thanks for your Devil-Dog quality instruction in your videos. Each time I watched them I learned a little bit more!"
-- Jack Boyle, Raleigh, North Carolina
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Thanks! Now on to those reasons we build multiple OSPF areas ....
Using a hierarchical approach to OSPF gives us these benefits:
Route summarization is much easier. When you're able to allocate addresses in a layered approach, it truly makes route summarization much easier to work with. (This hierarchical approach to addressing is what makes summarizing IPv6 routes more efficient than IPv4 routes.)
In turn, that route summarization makes our routing tables concise yet complete, which lessens the load on a router's CPU during the routing process. Smaller routing tables are better than larger routing tables, but they still have to be complete! Using multiple OSPF areas helps us accomplish that.
By creating multiple areas, LSA flooding upon a change in the network is localized. For example, LSA Types 1 and 2 don't leave the local area. This results in fewer Link State Updates traveling across the OSPF network as a whole.
Finally, fewer Shortest Path First (SPF) recalculations are needed.
Those are all pretty good reasons, I'm sure you'll agree!
For additional CCNP ROUTE exam tutorials and videos, just click that link - and over 400 additional CCNA and CCNP tutorials are ready for you as well!
I'm paying it forward to all CCNP candidates:
My CCNP ROUTE Bulldog Boot Camp DVD is only $97!
"Chris, I passed my CCNP ROUTE exam yesterday. YEAH!! Thanks for your Devil-Dog quality instruction in your videos. Each time I watched them I learned a little bit more!"
-- Jack Boyle, Raleigh, North Carolina
Save $100s, Get Over $200 Of Free Ebooks, And Receive Free Worldwide Shipping With My CCNP Video Boot Camp Bundle!
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