Today I decided to take the new class of Cisco Certification for the CCNP Enterprise Track. I booked the 300-401 Exam for the end of April. I am not certain if this is enough time to study for the exam but should motivate me to review everyday. The ultimate goal is for a CCIE Number, which is no easy undertaking. If I pass the 300-401, I would need to take the lab exam inside of 18 months.

This mornings topic was OSPF Stub areas. The ability to limit SPF runs in OSPF is the key to scaling out the solution. OSFP Stub areas prevent either LSA Type 4 (ASBR) or Type 5 (External) from being announced into a stub area. Instead a default route is announced to the ABR. OSPF allows further traffic engineering with blocking Type 3 (Summary) routes as well. This is known as a Totally Stubby Area.

OSPF Traffic Engineering allows for Not So Stubby Area and Totally Not So Stubby Areas. I believe I understand how these work. The principle behind both is that there can be a need to redistribute external protocols into OSPF and to announce to other areas of the OSPF graph.

In NSSA or TNSSA , redistributed routes are introduced into OSPF. The ABR for OSFP for the NSSA/TNSSA will announce there routes as a Type 7 LSA internally. When the ABR is flooding this information into another area is will convert the Type 7 to a Type 5 (External) LSA.

I learned about traffic engineering in a topology where there is a stub area that can learn a default route from two different ABRs in the network. Although the “Stub” Flag is a negotiated feature in OSPF Neighbors, the “no-summary” option has not bearing on this. This allows for traffic engineering to have one ABR to filter LSA Type 3 Routes for one of the ABR. This has the effect of allowing the longest match prefix to be matched on the other ABR in the topology and to have the redundancy of the default route to the “no summary” ABR.