RE: OSPF LSA type 3 filtering

For LSA 3 filter the syntax can be confusing since you can apply it both
inbound and outbound, and can apply it to area 0 and non-backbone areas.
However this is done purposely in the feature to give you more control over
which areas learn which LSAs.

Suppose you have an ABR R1 that has three links, one in each of areas 0, 1,
and 2. Internal routes that R1 receives from area 0 will be summarized in LSA
3, and then advertised to areas 1 and 2. Likewise in the reverse, internal
routes that R1 receives from areas 1 or 2 will be summarized in LSA 3 and
advertised to area 0. The result is that we have the following possible
combinations of the filter:

1. Area 0 in

2. Area 0 out

3. Area 1 in

4. Area 1 out

5. Area 2 in

6. Area 2 out

The first, area 0 in, is the equivalent of both area 1 out and area 2 out. In
other words, routes out from other areas going in to area 0. The second, area
0 out, is the equivalent of both area 1 and area 2 in. In other words, routes
going out from area 0 and going into both areas 1 and 2.

The next is where the distinction is made, as area 1 in would affect routes
going out of area 0 and area 2 into area 1, but would not affect routes going
out of area 0 into area 2, and likewise it would not affect routes going out
of area 2 into area 0.

It's sometimes hard to visualize it without drawing it out in your diagram.
The best way to think about it is that the ABR is the exit point out of the
area. Routes come from inside area 0 to the ABR, and flow outside to other
areas. From the other areas perspective this is their routes flowing in from
the ABR.

Make sense?

Brian McGahan, CCIE #8593 (R&S/SP/Security)
bmcgahan_at_INE.com<mailto:bmcgahan_at_INE.com>

Internetwork Expert, Inc.
http://www.INE.com

From: Sarad [mailto:tosara_at_gmail.com]
Sent: Thursday, January 03, 2013 5:56 AM
To: Brian McGahan
Cc: Cisco certification
Subject: Re: OSPF LSA type 3 filtering

Hi Brian,

If I understood correctly, as per your explanation area-range and filter-list
commands come in to effect after ABR construct the type 3 LSA to be generated
in to the other Area.

When I lab this up I noticed filtering with these commands work differently
when filter type 3 LSA from one Area to another area. Is there a technical
explanation for why this happening?

Thanks
Sara

On Thu, Jan 3, 2013 at 8:46 PM, Brian McGahan
<bmcgahan_at_ine.com<mailto:bmcgahan_at_ine.com>> wrote:
If you want to continue this as a technical discussion that's fine, just don't
freak out again after reading my response ;)

You said:

> What if in area 1 there are some LSA type-1 and type-2? Can you not filter
them or summarize them with the "area range" command?
No, you can not. This is a fundamentally incorrect notion about OSPF. First,
both LSA 1 and 2 are area local scope. The ABR cannot pass them between areas
hence there is no filtering or summarization that can affect them. Secondly,
the *topology* information described by these LSAs is automatically summarized
by the ABR into LSA 3. The *reachability* information is not.

The reachability information described in multiple LSA 3s can summarized
together with the "area range" command. Additionally the reachability
information described in LSA 3 can be filtered with either "area range" or
"area filter-list".

"area range" and "area filter-list" do not affect LSAs 1 or 2, they affect LSA
3. You can argue this is semantics if you want, but in binary there are only
two values, TRUE and FALSE.

Brian McGahan, CCIE #8593 (R&S/SP/Security)
bmcgahan_at_INE.com<mailto:bmcgahan_at_INE.com<mailto:bmcgahan_at_INE.com>>

Internetwork Expert, Inc.
http://www.INE.com
On Jan 3, 2013, at 3:25 AM, "Narbik Kocharians"
<narbikk_at_gmail.com<mailto:narbikk_at_gmail.com><mailto:narbikk_at_gmail.com<mailto:
narbikk_at_gmail.com>>> wrote:

Unbelievable,

You are saying that LSA Type-2s don't provide reachability information, I am
saying and showing you that they do provide the subnet mask, you then say that
you should NOT say LSA filtering because we can not theoretically filter LSAs,
especially when you are going to take the CCIE lab, let me tell you something,
they will probably say "LSA Type 3 Filtering" as the header, they mention that
in every Doc CD i have read, now whose student/s will miss out on the
terminology? You guys use it because it is "commonly used" (Based on Petr) or
Cisco says it that way in their DOC-CD, but if I say it, you claim that I do
not understand basics of OSPF or routing and I should be teaching CCNA.

Then, you agree with Paul about my explanation, and then you ask him what does
that have to do with "Area range" or the other commands, so why is it OK with
you to use the term "LSA Filtering" and Not anyone else? Check how quick you
agreed with Paul, and he was basically repeating what I mentioned, that tells
me that you are agreeing with me but you like to argue. I even said at the end
of my post "I am not disagreeing with you", but I guess it did not click.

Once again, stop doing that. Do you know how to unsubscribe a person from a
thread? You are very good with google, try it one more time.
On Thu, Jan 3, 2013 at 1:04 AM, Brian McGahan
<bmcgahan_at_ine.com<mailto:bmcgahan_at_ine.com><mailto:bmcgahan_at_ine.com<mailto:bmc
gahan_at_ine.com>>> wrote:
You need to relax Narbik. I'm not sure how you made this leap in the
discussion, but thanks for once again ruining a potentially helpful and
intellectual thread on the list. My apologies if I somehow offended you.

On Jan 3, 2013, at 2:34 AM, "Narbik Kocharians"
<narbikk_at_gmail.com<mailto:narbikk_at_gmail.com><mailto:narbikk_at_gmail.com<mailto:
narbikk_at_gmail.com>>> wrote:

You are VERY WRONG. Picking words and acting as though you are an attorney did
not convince me a bit, but your immaturity is what you definitely proved here
today. You are in a routing loop my friend, we made a full circle.

Unsubscribe me from further responses. Paul B the owner of this forum forgot
to put a disclaimer about people under legal age.

If this continues, I will ignore your replies or comments all together, or i
will be very rude.

How do you connect this discussion about my students failing because in many
words they attended my class? What does that have to do with this discussion?
A student of mine told me that you guys in your volumes say "filtering LSA
Type 3", so what gives you the right to use the terms that you disagree with?

I even commented in your blog, when Petr wrote an article "ospf route
filtering demystified" right after I released a 10 minute VoD on OSPF
Filtering, and he admitted in the blog that he uses that same term because
Cisco uses it in their documentation, but if I use it, I don't know what I am
talking about? Here incase you forgot:
http://blog.ine.com/2009/08/17/ospf-route-filtering-demystified/

As I said before unsubscribe me from this thread.

--
Narbik Kocharians
CCSI#30832, CCIE# 12410 (R&S, SP, Security)
www.MicronicsTraining.com<http://www.MicronicsTraining.com><http://www.micron
icstraining.com/>
Sr. Technical Instructor
YES! We take Cisco Learning Credits!
A Cisco Learning Partner
Blogs and organic groups at http://www.ccie.net