Re: OSPF LSA type 3 filtering

Something that popped to my mind when I read this is a hidden gotcha in
IOS regarding type-3 LSAs going across areas.

May be you know already, but it is worth bringing it back: when an ABR
copies a T3 from area 0 to a non area 0, it does a RIB check to see if
the route is actually in there. If it is not, the T3 is not copied.
It works just like if you were redistributing OSPF(A0) into OSPF(Ax).

-Carlos

Marko Milivojevic @ 04/01/2013 06:44 -0300 dixit:
> LSA summarization is not done in/from the RIB. Original Type-3 is created from the computed SPF tree, which in turn is based on the information from Type 1 and Type 2 LSAs. "The routing table structure" in the context quoted is what I called the "computed SPF tree", not the RIB, as the RFC has no control over how RIB is constructed.
>
> --
> Marko Milivojevic - CCIE #18427 (SP R&S)
> Senior CCIE Instructor - IPexpert
>
> :: This message was sent from a mobile device. I apologize for errors and brevity. ::
>
> On Jan 4, 2013, at 1:20, shiran guez <shiranp3_at_gmail.com> wrote:
>
>> Hi All
>>
>> I have to agree with Brian and add some to back it up:
>>
>> 1) LSA type 1 or LSA type 2 does not need to be filtered or summarized
>> simply because that are locally significant to the area that they where
>> generated
>> RFC2328
>> "
>>
>> 12.4.1. Router-LSAs
>>
>> ...
>>
>> The LSA is flooded throughout the particular area, and no further.
>>
>> ...
>>
>> 12.4.2. Network-LSAs
>>
>> ...
>>
>> The network-LSA is flooded throughout the area that contains the
>> transit network, and no further.
>>
>> "
>>
>> 2) LSA type 3 is generated from the RIB using the SPF algorithm
>>
>> "12.4.3. Summary-LSAs
>>
>> ...
>>
>> The precise summary routes to advertise into an area are
>> determined by examining the routing table structure (see
>> Section 11) in accordance with the algorithm described
>> below.
>>
>> "
>>
>>
>> - Any filtering and / or summary is done on the RIB not on the database
>> it self!
>>
>> Personal Note: I much prefer the semantics story being told. if you wish to
>> follow someone all your professorial life you can take the easy way and
>> learn configurations and some of the basic or even advanced designs and you
>> will do fine in your work, however if you wish to be followed upon and be
>> that one that people turn to for answers, you MUST know and understand the
>> semantics even if it mean that you need to waist more time and effort.
>>
>>
>>
>>
>> On Thu, Jan 3, 2013 at 11:46 AM, Brian McGahan <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>
>>>
>>> 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>> 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>> 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>> 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/>
>>> Sr. Technical Instructor
>>> YES! We take Cisco Learning Credits!
>>> A Cisco Learning Partner
>>>
>>>
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>>
>>
>> --
>> Shiran Guez
>> MCSE CCNP NCE1 JNCIA-ENT JNCIS-ENT CCIE #20572
>> http://cciep3.blogspot.com
>> http://www.linkedin.com/in/cciep3
>> http://twitter.com/cciep3
>>
>>
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-- 
Carlos G Mendioroz  <tron_at_huapi.ba.ar>  LW7 EQI  Argentina
Blogs and organic groups at http://www.ccie.net