OSPF

In this fairly short post, id like to address a topic that came up on IRC (#cciestudy @ freenode.net). Its about how you select a route thats being redistributed into an OSPF NSSA area and comes into the OSPF backbone area 0.

I wanted to share a very interesting tidbit i figured out the hard way.

Again, its about NSSA areas in OSPF. When redistributing a route into an NSSA area, how is the forwarding address chosen?

Some say the lowest IP address of any loopback interfaces thats advertised into the NSSA area. Others say the highest. Which one is correct?

Welcome back to part 3 about OSPF and external routes.

In this post I will focus on type 7 LSA’s. These are LSA’s thats created within an NSSA area. An NSSA area is, as you may know, an area which is meant as a stub area (no type 4 and 5’s), but will still allow for external routes to be introduced in the area. Instead of having type 5’s, which is not allowed in a stub area, it is allowed to have a special type of LSA called a type 7 LSA.

Welcome back to the 2nd post about OSPF and external routes.

Last time we spoke of OSPF and how it behaves in regard of external routes in a single “normal” area.

By normal, it implies an area that accepts all types of LSA’s.

I want to create some posts about the actual behavior of OSPF in regards to external routes in different areas and how everything behaves. It might take more than a few posts, but I hope you’ll keep reading them 🙂

The first one, is about redistributing external routes into a normal OSPF area. A “normal” OSPF area is an area which allows all kinds of LSA’s. This means we can have an external source in this area. In a “normal” external routes will be present as a type 5 LSA. This type 5, will either be an E1 or an E2 route. The difference being that the metric associated with the LSA will increase with an E1 and it will not with an E2 route. The default is an E2 route and the default metric is 20.

Been a few days since my last post.

During that time, I have taken quite a few practice tests to gauge where im at. I am also slowly progressing through the Exam Guide.

Also, in the near future im switching over to a new hosting service. Need some features which wordpress doesnt allow me to use, such as advertising. I got a friend in germany who has a colocated server which he will provide some access to. Very grateful for this.

Even though my next post was going to be about some enhancements to STP, I found this little nugget about OSPF behavior, that I thought I would share with everyone.

As we know OSPF, when crossing an area boundary, the ABR (Area Border Router), will gather type 1 (router and its links) and type 2 (network information provided by the DR on a transit network) and form a type 3 LSA, which is basically a boiled down version of the two other types of LSA’s. It will send this LSA into the area (assuming its not a totally stubby/nssa area).