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BGP Tuning Attributes

Finished with my BGP video series and BGP module on the BSCI guide I’m overwhelmed, for the first time during my BSCI studies.

In my BGP Lab post I said it is not that bad (so far – I did say so far). Well, now it is bad. Tuning Attributes are lords of details and kings of optional parameters.
While each attribute make sense and nothing seem to be too complicated, it is the full concept with all its details that left me lost and unfocused.

Selecting BGP path is different and a much more complex process than any other protocol (at least in this exam). While RIP use hop-count, EIGRP uses Bandwidth & Delay (and break ties with Reliability, Load and MTU) and OSPF use cost calculations, BGP has 14 parameters:

  1. Ignore routes with an inaccessible next hop address
  2. Prefer the path with the highest WEIGHT
  3. Prefer the path with the highest LOCAL_PREF
  4. Prefer the path that was locally originated via a network or aggregate BGP subcommand or through redistribution from an IGP
  5. Prefer the path with the shortest AS_PATH
  6. Prefer the path with the lowest origin type
  7. Prefer the path with the lowest multi-exit discriminator (MED).
  8. Prefer eBGP over iBGP paths
  9. Prefer the path with the lowest IGP metric to the BGP next hop
  10. Determine if multiple paths require installation in the routing table for BGP Multipath
  11. When both paths are external, prefer the path that was received first
  12. Prefer the route that comes from the BGP router with the lowest router ID
  13. Prefer the path with the minimum cluster list length
  14. Prefer the path that comes from the lowest neighbor address

My study guide did not offer too many detail on each attribute and I found cisco.com to be useful and easy to read. For this one topic this is the source for details (and we all know that at exam day, passing is in the little details).

Now don’t get me wrong, this is the easy part…
When start using route maps it is cool as Jeremy repeatedly reminded me but there are so many options and scenarios…

I’m close to completing the study guide and video with only two (relatively) short topics: Multicast and IPv6. I hope to wrap it up before the end of the weekend and start reviewing the notes and make sure the black holes from round I are fully understood.

Exam tip:  Jeremy Cioara (of CBT Nuggets) said only attributes 1-6 are BSCI materials and that route-map configuration (for attributes) is CCIE level requirements.

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  1. February 22, 2010 at 9:01 am

    Rofi, the exam tip from Jeremy isn’t totally correct. I just passed the BSCI today and I had a question whose answer was number 12, the router ID.

    • February 22, 2010 at 9:11 am

      congrats on passing the exam – was it a reasonable?
      and thanks for the tip correction (though I’m not sure Cisco will approve it ;))

      • February 22, 2010 at 12:10 pm

        Thanks 🙂 The exam was ok, I got hit by a lab from the first question and that was the hardest one, which I was almost sure I didn’t completely solve. But I did and apparently I screwed some multicast question.
        And regarding the tip, I haven’t told you any question from the exam, just warned you not to skip this topic. Anyway, the first 7 should be known well and the rest you can learn like poetry.
        Btw, from your list, 13 is surely not on the BSCI list and 14 is made useless by 12. And the BSCI Cisco Press Exam Cert Guide gives the full list (1-12) for study anyway.

        Good luck!

  2. Peter
    February 19, 2010 at 9:44 am

    Don’t forget #0

    0. Most specific route wins

    I’ve seen to many instances of a /24 trumping a summary route or a static /32 that causes unintended routing table entries. You can then throw in some redistribution for bonus points and added fun…

  3. February 19, 2010 at 8:30 am

    You can simplify it as:

    1. Don’t send traffic through a destination I don’t know how to get to.
    2-5. Process user settable attributes
    6-9. Get traffic out of my network efficiently (BGP being more hot potato routing)
    10-14. Tie breakers

    The first section is just obvious. The second section follows the normal “small knobs” to “big knobs” progression. For example weight is non transitive, LP is transitive, so put weight before LP. AS_PATH is a very big knob so it goes at the end. The third section is more about “I’ve decided that from a global perspective it doesn’t matter how to get the traffic there, so get it out as fast as possible”. Finally come the “OK, so it really doesn’t matter, but the algorithm has to be deterministic so figure it out”

    Sean

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