Navigating the OSPF LSDB to find relevant LSAs can be tricky. Even when you find the right LSA, there is no guarantee that OSPF will include that LSA in it’s SPF calculation. Thankfully Cisco routers will tell you if the router advertising that LSA is reachable or not.
Network engineering can be really frustrating sometimes. Sometimes the root cause is technology, sometimes it’s broken process or poor systems. Mostly, the frustrations stem from interaction with humanoids, otherwise known as people.
How does a Type-5 E1 path calculation work? Does the E1 auto-magically have it’s cost incremented is passes from the ASBR to the receiving router? If not, how does it work? If you read this blog long enough you’ll notice that I get stuck easily and often. When that happens I like to lab stuff up and share my lessons learned.
Type-4 LSAs always seemed like an ugly afterthought to me. I know it’s irrational and ignorant but the type-4 seemed to disturb the symmetry of OSPF. I cursed the type-5 for needing this kludgy type-4 helper. However, time was short, so I acknowledged my ignorance, rote-learned the type-4 and moved on. When I later revisited OSPF for a deeper understanding I got confused and questioned if a type-4 really was necessary. [Hint: John Moy is considerably smarter than me! ] In…
It is a common myth that OSPF ABR generates a type-4 ASBR summary ‘when it sees a type-5’ from an ASBR. The ABR does generate the type-4, but it isn’t the type-5 that triggers the ABR to originate it. I’ve setup a quick lab in GNS3 to help track down the true trigger.