BGP Well-Known Communities

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I’m still making slow progress through Narbik’s advanced lab workbooks. I was scheduled to have the BGP book done last Saturday. (Ha ha. I’m funny.) I’m only halfway through the BGP volume at this point, not because it’s harder than I expected, but because I just haven’t had the hours to deal with it. The last BGP lab I did dealt with communities, including well-known communities. It seems that if they are “well-known” communities…well, I should know them and what they are used for.

The names are fairly intuitive, but here’s a handy chart I stole from here that describes them in a tidy way.

Q. What are the well known communities of the BGP community attribute?

A. The community attribute is a transitive, optional attribute designed to group destinations in a certain community and apply certain policies (such as accept, prefer, or redistribute). This table shows the well known BGP communities.

Community Description
Local-AS Use in confederation scenarios to prevent sending packets outside the local autonomous system (AS).
no-export Do not advertise to external BGP (eBGP) peers. Keep this route within an AS.
no-advertise Do not advertise this route to any peer, internal or external.
none Apply no community attribute when you want to clear the communities associated with a route.
internet Advertise this route to the internet community, and any router that belongs to it.

This chart makes a distinction that I should lab out to verify, but still…it’s the distinction between “local-AS” and “no-export”. At a glance, they both do the same thing: routes tagged with BGP community of “local-as” or “no-export” shouldn’t advertise the route to an eBGP peer. But this table implies that “local-as” will keep a route inside of a sub-AS in a confederation scenario, whereas “no-export” would send the route to all sub-AS’s in the confederation. Maybe I’ll lab that up and give it whirl a bit later on. I was just happy to find a chart that explained the difference.

Heh. As I was blogging this, I did a search on my own blog articles from the Official Exam Certification Guide, and discovered some info from here that confirms what I’m thinking.

LOCAL_AS (Cisco) aka NO_EXPORT_SUBCONFED (RFC1997) – FFFF:FF03 – don’t advertise outside the local confederation sub-AS.

I still might lab it at some point, but I feel less concerned about it than I did.

7 comments

  • Ethan
    I just registered for Narbik’s bootcamp in April.
    I need a word of advice though.
    Problem is that due to uncertainty of dates in Dubai, as well financial contraints I could only go to April’s bootcamp.
    But unlike you I wont be as prepared. I would have gone through half of IEWB workbooks (I know that you used NMC), means someway half way in my prep.
    Plus my Lab date is not until September (Just cant find a date).
    Do you think its a good idea to attend Narbik’s bootcamp midway the prep, and then having4 months gap between the bootcamp and Lab.
    My peers seem to be unanimous that bootcamp should be taken just before the lab (not a possibility) and 4 month gap basically will mean that I dont gain anything from bootcamp.
    I dont have options, plus I think bootcamp will be good push towards prep.
    Please comment.

  • A couple of thoughts.

    (1) Narbik allows retakes at essentially no cost to you until you pass the lab exam. If travel and lodging is not cost-prohibitive and you have the time, you could take the bootcamp multiple times between now and your lab date in September.

    (2) You will walk away from the bootcamp with Narbik’s Advanced Lab Workbooks, which will help you in your prep. Going through Narbik’s workbooks a couple of times would help you a lot.

    If you only have the chance to attend the lab one time, I don’t know if April is appropriate or not, considering your date is in September. Narbik would be the best person to advise you on that. I know his first recommendation to me was to attend the bootcamp multiple times. If that’s not an option for you, I’m not sure what he’d recommend.

  • Thank you for reply.
    Lets see if a bootcamp date falls near my scheduled date.
    I emailed Narbik and he told me Dubai schedule post April is still not finalized and website is not correct.
    A free retake will mean a week of the work, and that may be a problem.
    Anyway, the bootcamp is what is in hand :) so better to attend it and see how things shape out.
    Thank you again for quick reply.

  • Barooq…
    As Ethan put it so well….you should take Narbik;s bootcamp as as you so if during the 6 intense non-stop labs Narbik determin that you are not ready then you will have plenty of time of doing his workbook at least two time and then be able to do the bootcamp for second time before taking the lab….That is exacatly what I am doing, going over the workbook very slow and then when i am done take the bootcamp the week before the lab….

  • Ethan off topic question, Have you done any Mock Labs. I do recall you posting something about it on your website. Can you please tell me the vendor name and the lab you attempted
    Thanks

By Ethan Banks

Ethan Banks is a podcaster and writer with a BSCS and 20+ years in enterprise IT. He's operated data centers with a special focus on infrastructure — especially networking. He's been a CNE, MCSE, CEH, CCNA, CCNP, CCSP, and CCIE R&S #20655. He's the co-founder of Packet Pushers Interactive, LLC where he creates content for humans in the hot aisle.

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