I said to a colleague recently, “you can’t get 100% link utilisation on an Ethernet link”. When I tried to explain myself I wished I could link to a simple blog post with a nice graph. So here’s a quick blog post with a nice graph. I have talked a little about link speed in a previous post, but I wanted expand on this and add a quick graph to back up the argument.
MTU mismatches are the primary reason an OSPF adjacency becomes stuck in the EXSTART state. After hellos are exchanged and the routers become neighbors, each OSPF speaker advertises the IP MTU of it’s local interface in a Data Base Description (DBD) LSA. If there is a mismatch you’ll probably just adjust the configuration to be identical on both ends of the link and be done. However, when you try to peer two OSPF routers with different network operating systems, things start to fall apart fairly quickly.