BGP - bogus AS path entries

Unanswered Question
Mar 16th, 2007


I'm using multihoming to 2 providers, one primary and one backup (full routing).

Currently to the backup provider I prepend the AS with 3 bogus entries, to ensure that traffic routes via the primary.

However, I am finding that there is a small amount of traffic that still routes via the backup, one of which is a customer that is having connectivity issues.

It's strange because the traceroutes that route over the backup have at least 14 or so hops whereas the ones over the primary have more like 5.

Anyway I would like to get all traffic to route on the primary - would adding another 2 or 3 bogus path entries realistically do this? I don't really want to be adding 100s to get it to work!

I'm interested in whether this is a realistic solution....



I have this problem too.
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flashsplash Fri, 03/16/2007 - 08:48

I don't know what "bogus path entries " exactly are but i know that u can manipulate which path bgp must use or don't use by making use of the MED or Local Preference options...

ciao flash

jigsaw2026 Fri, 03/16/2007 - 08:59

Many thanks for your response flash, unfortunately MED/local pref are nontransitive so won't propagate beyond or direct upstream provider, so that won't have any effect.

Bogus path entries means that I advertise to my backup provider my network with a path of - for example if my as was AS100 - 100 100 100 100 to make it appear further away and thus less preferable than my primary route.



ilya.varlashkin Fri, 03/16/2007 - 09:17

Effect of your AS-PATH prepend (that's what you're doing) might easily be canceled by your (backup) upstream provider if they set higher local-pref on all customers (commonplace). Ask both providers to show you routes towards troubled network as seen from their routers.

As a rule you need to work closely with your providers to figure out which of the attributes you need to tune in order to achieve desired routing policy. Often using communities allows to achieve better results than simple AS-PATH prepend.

jigsaw2026 Fri, 03/16/2007 - 09:22

Thanks for your response Ilya, I am already working with my providers but will investigate communities.

jason.plank Sat, 03/17/2007 - 06:55

The easiest way to see how your prefixes look upstream is to login to a free route servers / looking glasses and see. Go to, there is a huge list of these so you can see if your prepending is getting dumped by the provider. Most providers will allow prepending as long as you are prepending your own AS and not someone elses (obviously).


jason.plank Sat, 03/17/2007 - 06:56

furthermore, depending on who the two providers are, you may need to add a few more prepends to the route-map. I assume you are talking about prepending when you mention bogus entries :)

jigsaw2026 Mon, 03/19/2007 - 07:56

Thanks for your help Jason :)

p.s. Yes, I was talking about prepending :)


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