This document is presented as a series of Questions and Answers, discussing various aspects of OSPF protocol designed to prevent inter-area routing loops and related issues. The discussion covers ABR functions, Virtual-Links, OSPF Super-backbone, OSPF Sham-Links, BGP Cost Community. Reader is assumed to know these concepts already, as this publication focuses on complex interaction features arising in MPLS/BGP VPN scenarios. The discussion is culminated by analyzing a number of issues arising in complex multi-area multi-homed OSPF site deployed in MPLS VPN environment. Please download the following document to read the publication: Loop Prevention in OSPF

About Petr Lapukhov, 4xCCIE/CCDE:

Petr Lapukhov's career in IT begain in 1988 with a focus on computer programming, and progressed into networking with his first exposure to Novell NetWare in 1991. Initially involved with Kazan State University's campus network support and UNIX system administration, he went through the path of becoming a networking consultant, taking part in many network deployment projects. Petr currently has over 12 years of experience working in the Cisco networking field, and is the only person in the world to have obtained four CCIEs in under two years, passing each on his first attempt. Petr is an exceptional case in that he has been working with all of the technologies covered in his four CCIE tracks (R&S, Security, SP, and Voice) on a daily basis for many years. When not actively teaching classes, developing self-paced products, studying for the CCDE Practical & the CCIE Storage Lab Exam, and completing his PhD in Applied Mathematics.

Find all posts by Petr Lapukhov, 4xCCIE/CCDE | Visit Website

You can leave a response, or trackback from your own site.

10 Responses to “Understanding Inter-Area Loop Prevention Caveats in OSPF Protocol”

  1. Michal says:

    hello Petr , wow nice write-up ! Multi-Homed Multi-Area OSPF Site section – page 17 is just the right thing to read for every rs/sp candidate! I had some doubts on that part so many thanks for clearing up things , on the appropriate architecture examples.

  2. Michal says:

    PS Happy New Year ! :-)

  3. Hemanth Raj says:

    Superb post first up Petr Started 2011 wid ospf cool
    Rockin post

  4. Jeferson says:

    As usual, AWESOME writeups! Thanks for making the CCIE communitty better!

  5. Richard says:

    Is there a typo: bottom of page 4

    “Therefore, even though both R1 and R3 will have the type-5
    LSAs, they will not be able to use them because there is no corresponding type-5 LSA for the ASBR.”

    Should the second “type-5 LSA” be “type-4 LSA”?

    “…they will not be able to use them because there is no
    corresponding type-4 LSA for the ASBR”

  6. @Richard,

    Yes, it should be type-4 LSA the advertises presence of ASBR

  7. Vinoth Kumar Arumugam says:

    Great start for 2011 with a wonderful explanation …. Good job !!!!

  8. Tony says:

    Hi Petr,
    Fantastic paper! I’ve read plenty of books on OSPF, but never come across much of this before. I can only assume you’ve been delving into RFCs!

    If you run BGP between your CE router and service provider MPLS PE, is there any novel way to extend a sham link out to the CEs? I can only think of using a GRE tunnel between CEs, but it’s not a very scalable or neat solution.


  9. Daniel says:

    These are by far the best 40 pages about OSPF I’ve ever read in my life. It cleared all sorts of confusions, this is just golden.

  10. Jerome says:

    It is a really good document ! Thank you.


Leave a Reply


CCIE Bloggers