NSSA, Totally Stubby Not-So-Stubby Area +++

Hey,

I just clicked on a link from Anthony and it is a little Goldmine of a link
with more Nuggets that probably another other single link I can think of.

Well you are probably wondering what I'm referring to...

Hmm..

Kewl

Here it is:

Q: Which OSPF area type is used to prevent only Type-4 and Type-5 LSAs
from entering from area 0, but allows the origination of Type-7 LSAs?
A: not-so-stubby area
More Info:
http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094a88.shtml

Now the question gives itself away due to the mention of Type-7 LSA's - that
did not really catch my eye. I kinda know that be heart.

But I'm always looking for a better banana...

Anyway...

Take a trip to the Tech Note - (Hint: If Darby say take a trip to the Tech
Note... One might take that trip... and share a peak).

Anyway I don't have to tell anyone how core the OSPF is to anyone's CCIE
Labs right?

However,

Stubs, NSSA's, NSSA Totally Stubby Areas... "No Redistribution"... Summary
Addresses and "Not Advertise"

These are tools my friend...

Anyone ever seen the X-Men or watched the new Wolverine movie yet?

Well if you did these tools are what I'd liken to "Adamantium" for someone
needed to turn a steel wall into an hot butter...

But I digresss...

Anyway - I could see a few open-ended questions in these numbered examples.
I could also see a few coming back from the lab saying that this is like
"Spinach for Popeye".

Yes - I have been to bootcamps where this was not on the agenda.

1.
Defining a Not-So-Stubby Area

In order to make a stub area into an NSSA, issue this command under the OSPF
configuration:

router ospf 1
 *Area 1 nssa*

This command must be configured on every single router in Area 1. After you
define Area 1 as an NSSA, it must have these characteristics:

   -

   No Type 5 LSAs are allowed in Area 1. This means no RIP routes are
   allowed in Area 1.
   -

   All IGRP routes are redistributed as type 7. This type 7 can only exist
   within NSSA.
   -

   All type 7 LSAs are translated into type 5 LSAs by the NSSA ABR and are
   leaked into the OSPF domain as type 5 LSAs.

2.
Defining an NSSA Totally Stub Area

In order to configure an NSSA totally stub area, issue this command under
the OSPF configuration:

router ospf 1
 *Area 1 nssa no-summary*

Configure this command on NSSA ABRs only. After you define the NSSA totally
stub area, Area 1 has these characteristics in addition to the NSSA
characteristics:

   -

   No type 3 or 4 summary LSAs are allowed in Area 1. This means no
   inter-area routes are allowed in Area 1.
   -

   A default route is injected into the NSSA totally stub area as a type 3
   summary LSA.

3. Filtering in NSSA

There are situations where there is no need to inject external routes into
the NSSA as type 7. This situation usually occurs when an ASBR is also an
NSSA ABR. When redistribution takes place in this scenario, the router
generates type 5 as well as type 7 LSAs. You can prevent the router from
creating type 7 LSAs for NSSA with this command:

router ospf 1
 *Area 1 nssa no-redistribution*

 In the network diagram, Area 1 is configured with the*
no-redistribution*option. This means that all IGRP routes are
redistributed into area 0, but
no type 7 LSAs are generated for Area 1. Only configure this command on an
NSSA ASBR that is also an ABR.

Another case of filtering is when you need to prevent the type 7 LSAs from
being translated outside the NSSA. In other words, when you want to control
which type 7 LSAs get translated into type 5. For example, you have a RIP
learned route 141.108.10.0/24 that is injected into the OSPF NSSA Area 1.
You do not want this route to be leaked into the rest of the OSPF areas. Use
this configuration on either the NSSA ASBR or the NSSA ABR in order to
accomplish this:

router ospf 1
 *summary-address 141.108.10.0 255.255.255.0 not-advertise*

This configuration generates a type 7 LSA that is not translated into type 5
by the NSSA ABR.

4.
Default Route in NSSA

There are two ways to have a default route in an NSSA. When you configure an
area as NSSA, by default the NSSA ABR does not generate a default summary
route. In the case of a stub area or an NSSA totally stub area, the NSSA ABR
does generate a default summary route.

5.
Default Summary Route

By defining an area as a NSSA totally stub area, the NSSA ABR generates a
default summary route. As mentioned, if the NSSA area were not defined as
totally stub, then a default summary route is not generated by NSSA ABR.
This configuration generates a default summary route for a NSSA totally stub
area.

router ospf 1
 *Area 1 nssa no-summary*

6. Default Type 7

This configuration generates a type 7 default route. You can configure this
command on any NSSA ASBR or NSSA ABR with these rules:

   -

   NSSA ASBR can generate a default only when it has a default route in its
   routing table.
   -

   The default route must be known through non-OSPF protocol
   -

   NSSA ABR can generate a default route with or without a default route in
   its own routing table.

This command is used in order to generate an NSSA default route:

router ospf 1
 Area 1 nssa default-information-originate

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Received on Thu Apr 23 2009 - 23:10:16 ART