CCNP Tutorial For The BSCI Exam

ISIS - OSPF Similarities and Differences

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Both OSPF and ISIS are link state protocols, so they have to have some things in common - but they also have some important differences. In this final ISIS configuration tutorial , we'll take a quick look at both the similarities and differences.

This section is more of a summary than a full-fledged tutorial, so be sure to visit our BSCI Tutorial and CCNP Practice Exam sections for literally dozens of fully-illustrated tutorials and practice tests!

Let's take a look at the similarities first:

• Both are link state protocols.

• Both use Hello packets to form and maintain adjacencies.

• Both allow clear text and MD5 link authentication.

• Both use the Dijkstra / SPF algorithm.

• Both use areas to create a hierarchical network topology.

• Both converge quickly after a change in the network topology.

• Both support address summarization between areas.

• Both use cost as a default metric.

All simple enough, right? Let's take a look at the differences.

• OSPF runs on IP, ISIS runs on CLNS.

• Theoretically, you can have up to 30 separate OSPF processes running on a single router. (This is not always a good idea, hence the term "theoretically".) Multiple ISIS processes are not supported, so you're limited to one ISIS process. That's why you don't see a number in the ip router isis or router isis commands.

• ISIS default timers allow more options than OSPF, making it easier to "fine-tune" ISIS.  As with any Cisco technology, you should have an excellent reason for changing timer values before actually doing it.

• OSPF does have more area types than ISIS, making it easier to limit route advertisement in OSPF.

• ISIS Link State Packets contain TLVs, where OSPF Link State Updates contain Link State Advertisements (LSAs).

• While ISIS uses cost as the default metric, optional metrics include delay, expense , and error .  Since cost is the default metric, this metric is sometimes called "default".

• The ISIS default cost is 10 for all links, regardless of bandwidth.  For this reason, you may want to adjust the costs accordingly.

• ISIS must be configured at the interface level as well as the routing protocol level.

Watch that last one - it's really easy to leave the ip router isis command out!

Be sure to visit our BSCI Tutorial page for dozens of fully-illustrated tutorials, as well as our CCNP Practice Exam page to test and reinforce your knowledge on your way to a successful exam day!

Chris Bryant

CCIE #12933

"The Computer Certification Bulldog"

chris@thebryantadvantage.com

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