CCIE Pursuit Blog

July 14, 2009

Core Knowledge Question of the Day: 14 July 2009

Does an OSFP stub explicitly filter Type-4 LSAs, or is their absence in an OSPF stub area simply due to being unnecessary because the Type-5 LSAs have been filtered?

Highlight for answer: Actually, I don’t know the answer to this question.  I was thinking about it today.  In an OSFP stub area Type-5 LSAs are explicitly filtered.  There are no Type-4 LSAs present either.  I don’t know if they are explicitly filtered, or they are just never generated because the Type 5 LSA is filtered/never created?  It’s my understanding (possibly a misunderstanding) that the ASBR generates the Type-4 LSA, so…it must be explicity filtered at the ABR, right? <–This is WRONG! :-)

—-

Thank you for the comments (big ups to Ivan P, Zeeshan, and Pavel Sefanov).  I think that I have this cleared up in my head now:

The ABR generates the Type-4 LSA. If the area is configured as a stub area, the ABR filters the Type-5 LSAs(generated by the ASBR) and does not generate a Type-4 LSA. So, technically, an OSPF stub configuration only explicitly filters Type-5 LSAs, but it implicitly filters Type-4 LSAs as well as there is no need for the ABR to generate a Type-4 LSA.

So if you were to tell a co-worker that both Type-5 and Type-4 LSAs are filtered, you would be technically wrong.  :-(

Ivan Pepelnjak from Cisco IOS Hints and Tricks wrapped it up nicely:

To make it more explicit: the type-4 LSA is the glue that ties together a type-5 LSA originated by an out-of-area ASBR with the ABR flooding type-5 into the area. If there are no type-5 LSAs, type-4 LSAs are not needed (you will also not see them for ASBRs in the same area).

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10 Comments »

  1. The ASBR does not Generate the Type-4 LSA. Its Actually the ABR.
    Because If you only have OSPF AREA 0 and there is an ASBR redistributing Routes,It will only generate Type-5 LSA. There will not be any Type-4 LSAs.
    Moreover the Adv Router field in the Type-4 LSA is the Router ID of ABR

    Comment by Zeeshan — July 14, 2009 @ 11:05 am | Reply

  2. Type 4 LSAs are not generated in stub areas and if you think about it, there is not reason to have them in the area because of the lack of type 5 LSAs. I’ve come across statements in INE’s workbooks and other places where they say that only type 5 LSAs are filtered and type 4 are generated in stub ares, but this is entirely untrue.

    This is actually specifically written in a bullet in RFC 2328 in 12.4.3:
    o Else, if the destination of this route is an AS boundary
    router, a summary-LSA should be originated if and only
    if the routing table entry describes the preferred path
    to the AS boundary router (see Step 3 of Section 16.4).
    If so, a Type 4 summary-LSA is originated for the
    destination, with Link State ID equal to the AS boundary
    router’s Router ID and metric equal to the routing table
    entry’s cost. Note: these LSAs should not be generated
    if Area A has been configured as a stub area

    Comment by Pavel Stefanov — July 14, 2009 @ 11:21 am | Reply

  3. @Zeeshan and Pavel Sefanov – Thanks guys. I stumbled a bit on the “ASBR creates the Type-4 LSA” because I had recently read this post: http://packetlife.net/blog/2008/apr/21/where-are-type-4-lsas-generated/ Reviewing that post I realize that the setup is different than what I had in my head (final area in not a stub).

    Thanks for the quote from the RFC. If I understand this correctly, the ABR generates the Type-4 LSA. If the area is configured as a stub area, the ABR FILTERS the Type-5 LSAs(generated by the ASBR) and DOES NOT CREATE a Type-4 LSA. So, technically, an OSPF stub configuration only explicitly filters Type-5 LSAs, but it implicitly filters Type-4 LSAs as well as there is no need for the ABR to generate a Type-4 LSA.

    Comment by cciepursuit — July 14, 2009 @ 11:42 am | Reply

  4. To make it more explicit: the type-4 LSA is the glue that ties together a type-5 LSA originated by an out-of-area ASBR with the ABR flooding type-5 into the area. If there are no type-5 LSAs, type-4 LSAs are not needed (you will also not see them for ASBRs in the same area).

    Comment by Ivan Pepelnjak — July 14, 2009 @ 12:25 pm | Reply

  5. @ Pavel Stefanov – Not sure what INE’s workbooks you’re referring to, but here are a few excerpts from WB.Vol1.V5′s OSPF section:

    “…stub areas can be used to optimize the OSPF database by replacing the redundant Type-5 External and Type-4 ASBR routing information with default information.”

    “With area 3 configured as a stub area, Type-5 External LSAs and Type-4 ASBR Summary LSAs no longer exist in the area 3 database.”

    @ Mr. CCIE Pursuit – Here’s OEQ for you! How’s the route lookup work for “translated” Type-5 LSA versus “inter-area” Type-5 LSA? You already know one uses Type-4 LSA and one doesn’t, but it’s a bit more involved than that. Not the typical “a few words answer” that Cisco would have you believe ;-). Yes, the answer can be found in the INE’s WB.Vol1.V5′s OSPF section.

    In case you’re wondering…No I don’t work for INE or am affiliated with them in any way. I’m just a regular network engineer squirrel, using INE’s workbooks, to look for his CCIE nertz!

    Comment by Dragons & Faeries — July 14, 2009 @ 11:41 pm | Reply

  6. Hi
    Hope you remember. Its been ages since I used to blog:)
    Anyway, I started a new blog as I starte prep from security.
    please check if you have time.

    Comment by Barooq — July 15, 2009 @ 2:15 am | Reply

  7. Ouch… The blog url is http://iptechtalk.wordpress.com/

    Comment by Barooq — July 15, 2009 @ 2:27 am | Reply

  8. @Dragons & Faeries – Just to show you that I am not making this up – look at the solutions guide for lab 4 (task 4.5) and lab 6 (task 3.4, the table). I used INE as well for my preparation and think their products are wonderful, but mistakes like this, which are not made due to lack of knowledge but because of lack of editing/editors, are sometimes pain in the ass :) Hope they’ve corrected this in v5.

    Comment by Pavel Stefanov — July 15, 2009 @ 5:19 am | Reply

  9. Actually , all depends on the topology
    Imagine something like (ASBR) – area 1(ABR1)-area 0 – (ABR2) – area 2

    In this ,ABR1 will flood the type 5 out into the backbone along with the generated Type4 LSA for the ASBR.
    If area 2 is not a stub , ABR2 will flood the type 5 into area 2 along with the type 4 . Note that the type 4 is generated by ABR1 , not by ABR2 :) ABR2 simply forwards it after adding the cost to reach the ABR1 to the cost in the LSA.

    If area 2 is a stub , ABR2 will indeed *FILTER* both the type 5 and type 4 LSAs.

    So if you were to tell a co-worker that both Type-5 and Type-4 LSAs are filtered, you would be technically correct :) atleast in this case

    Comment by mayurgai — July 17, 2009 @ 8:58 pm | Reply

  10. Here’s an RFC-compliant(TM) answer to the question:

    Type-4 LSA are used to track the cumulative metric to reach an ASBR, and are generated and flooded domain-wide for E1 routes. E1 routes, in contrast to E2, have an added task of calculating a cumulative metric in addition to the metric set at the ASBR through the OSPF domain. Across the domain, ABRs will change and then flood the Type-4 to reflect their cost to the ASBR, along with the Type-5s.

    Assuming the following scenario:

    ASBR – Area 2 – ABR – Area 0 – ABR – Area 1 – RouterA

    The ASBR creates and floods a type 5 that contains the prefix and the metric set through redistribution, and a type-4 that will contain its own router-id and a cumulative metric that will be incremented by routers on its way to RouterA. When RouterA gets both Type-4 and Type-5, it calculates the metric for the E1 prefix by adding the metric to reach the ASBR thats in the Type-4 plus the advertised metric at redistribution thats in the Type-5. If this is clear, we also understand that the Type-4 is useless for E2 type external routes.

    It is correct to assume that the ABR of a stub area filters Type-4 and Type-5 LSAs, if we understand that filters is synonymous with “doesn’t propagate”.

    Comment by Mat — August 4, 2009 @ 3:30 am | Reply


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