Path Selection in OSPF for E1 and E2 routes in GNS3

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Path Selection in OSPF for E1 and E2 routes in GNS3

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Added by August 21, 2013

How do you feel about your understanding of why OSPF routers choose the paths they do?

I read a fantastic article which goes into great detail on exactly why a router would choose one route over another: Understanding OSPF External Route Path Selection.

I give full credit to this very comprehensive article as it was not authored by me.  It was actually posted by Brian McGahan, CCIE #8593.  What I wanted to do with this blog is to use the scenario presented by Brian to create a GNS3 environment which matches it.  I know many of my students prefer a hands-on method of learning, so my post will provide the means to accomplish this.

I know there are other sites online where GNS3 scenarios are made available so I checked.  I do not see this particular scenario posted anywhere, so I wanted to make it available to anyone who wants to use it.  There are several snapshots of the environment contained in the zip file as well, so you can work the scenario using the snapshots, preserving the initial configs, if you wish.  Of course you are welcome to mix and modify the configs however you wish; it is a lab environment after all.

I have zipped all the relevant information into the file, so as long as you have GNS3 installed and an IOS file of your liking (the version info I used is presented in the zipped file), download the Cisco info and have at it!

Feel free to take a look at this series of labs with a common purpose, that of understanding the OPSF external route selection process.  I welcome your feedback on this posting, especially the GNS3 aspect of it.  I would be thrilled if you had any suggestions for improvement to this scenario.

Until next time, happy OSPFing…

Mark Jacob
Cisco Instructor – Interface Technical Training
Phoenix, AZ

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