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.
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”.
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.
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…