LSA 7 - NSSA External LSA - The master of disguise

- Why does he need a diguise? you might ask.

Because LSA 5 is not allowed inside a Not-So-Stubby Area, or NSSA. An NSSA is a special type of stub area that still allows external routes to be redistributed into OSPF. But since Type 5 LSAs are not allowed inside the area, we need another way to carry that external route information.

Enter LSA 7. A Type 7 LSA, also called an NSSA External LSA, is generated by an ASBR inside an NSSA. Just like LSA 5, it carries information about routes that came from outside OSPF.

But Type 7 stays inside the NSSA. When that external route needs to be advertised to the rest of the OSPF domain, the ABR can translate the Type 7 LSA into a Type 5 LSA.

So it goes a little something like this:

External route information approaches the NSSA.

NSSA: Sorry, LSA 5:s are not allowed in here!

puts on fake moustache

Hello. I am LSA 7!

NSSA: Welcome!

And when it is time to leave the NSSA, the ABR can remove the moustache and turn that Type 7 information into a Type 5 LSA.

So if you only remember three things about LSA 7:

1. It is generated by an ASBR inside an NSSA.
2. It carries external route information, just like LSA 5.
3. An ABR can translate it into a Type 5 LSA when the information leaves the NSSA.

LSA 7 is basically LSA 5's kind of information in disguise.

And apparently a fake moustache is enough to get into an NSSA.