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OSPF – LSInfinity != LSInfinity

OSPF – LSInfinity != LSInfinity

LSInfinity has two different values I have mentioned ‘LSInfinity’ in the max-metric post and in an earlier post on withdrawing OSPF routes.   However if you look closely at each post you’ll see that LSinfinity had a different value in each of those posts .   What gives?  It turns out that the term LSinfinity has been overloaded somewhat in the RFCs and has different values and meanings for different LSAs.

OSPF – how max-metric works

OSPF – how max-metric works

When you configure an OSPF interface with a cost, you can do it directly using ip ospf cost or have the cost calculated for you using auto-cost reference bandwidth.  Whichever method you choose, the OSPF RFC 2328 calls it “the cost of sending a packet out this interface”.  The Router LSA, or type-1 LSA, has a 16-bit field (65535 in decimal) to represent the “interface output cost”.  An interface cost of 65535 is also known as “LSInfinity”.

OSPF – How to withdraw an OSPF route

OSPF – How to withdraw an OSPF route

Todays post will cover how OSPF withdraws a route after a link failure or deconfiguration. There are some fantastic resources out there describing how to originate and advertise links and prefixes, but I haven’t found a good summary of OSPF link failure handling.

OSPF – Why are Router LSA P2P links so complicated?

OSPF – Why are Router LSA P2P links so complicated?

I earned my CCNA a good few years back,  but I still remember all the pencilled notes in the OSPF LSA section of my Todd Lammle study guide. I found the Router LSA information hard to learn, mainly because I didn’t understand why it was so complex.   In this post we’re going to take a closer look at the Router LSA,  specifically the point-to-point (P2P) link.  On completion, you should better understand why the Router LSA seems complex and you will…

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