Re: BGP dampening [7:65086]
At 07:39 PM 3/11/2003 +, Oliver Hensel wrote: >Hi! > >Can someone point me to a document which explains >what happens with a prefix that is dampened if >it's distributed via two providers. Hi Oliver, Here is a link to a doc from Randy Bush that covers damping in some detail. http://psg.com/~randy/021028.zmao-nanog.pdf (handily posted to NANOG today :) For technical info on damping in general, check rfc 2439, and RIPE 229 for recent best practise config settings (which are put into serious question by the above PDF) Damping was brought into existence as a means to protect routers which could be overwhelmed by a large amount of BGP updates to the extent where they would would either crash, or drop BGP sessions themselves thereby exacerbating the route churn issue. At present, newer routers and better BGP implementations are able to deal with large amounts of BGP updates without any impact to other processes in the router and thus the need to protect them via damping isn't a huge priority. Further, as Randy points out, damping may do more harm than good to route convergence in the global Internet. As a result, I think it is safe to say that the need for damping in general is in serious question. >Will only the penalized route dampened, that is >will we still have connectivity if one link is >flapping. I think so, but I'd like to have some >confirmation for that. BGP prefixes (NLRI) are damped individually, however damping really only impacts you on more remote AS's. In your case, you have a situation like the below: you / \ transit1transit2 | \ / | remote1 - - remote2 | \ / | remote3 --- remote4 When you advertise 10/8 to transit1 and transit2, assuming these folks are clueful and automatically pref customer routes above peer/transit, both of them will always prefer the direct route to you. This is important as implicit withdrawals are penalized in the same way as direct withdrawals. This fact, coupled with the fact that damping stats are cleared on EBGP sessions when the peer resets will tend to make damping irrelevant between neighboring AS's. However, as you get more and more remote, things get worse. To expand on this, consider remote3. Assuming you advertise 10/8 to both transits, imagine that the update from transit2 gets to remote1 first and on to remote3. In this case, remote3 hits you with an advert penalty and posts the route 10/8 via as-path "r1,t2, you" Shortly thereafter, the update from transit1 shows up in remote1 and by virtue of a better AS-PATH becomes the best path in remote1. Remote1 therefore sends an update with the new path info to remote3. This update includes an implicit withdrawal of the old path and a subsequent damping penalty applied to 10/8 in remote3.Likely these two updates appeared in remote 3 in a pretty narrow time window and thus you have a 10/8 prefix that has suffered a nice penalty without ever really flapping. Consider also that depending on AS size, router types, BGP advertisement intervals and such, remote 3 may have seen an r1,r4,r2,t2 path first, then an r1.r2,t2, then an r1,t1 path and may have penalized you once for the initial advert and two more times for the implicit withdrawals which might get you damped in remote3 right off the bat. This issue gets worse as you consider ASes more and more remote from you. For what it's worth, I may have this entirely wrong :-) But this is my understanding of the behavior. The networks I have designed used graded damping and are not tremendously aggressive. I am however considering removing damping from the configs for the few networks I have some impact in as I really don't see it serving much of a role. Pete >Thanks and best regards, > >Oliver > > >-- >Oliver Hensel >telematis Netzwerke GmbH >mailto: [EMAIL PROTECTED] >Siemensstrasse 23, D-76275 Ettlingen >Tel: +49 (0) 7243-3448-0, Fax: -498 >visit us: http://telematis.com >3 Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=65302&t=65086 -- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
RE: BGP dampening [7:65086]
At 3:19 AM + 3/12/03, Priscilla Oppenheimer wrote: >I'll take a stab at it since nobody else did. > >Oliver Hensel wrote: >> >> Hi! >> >> Can someone point me to a document which explains >> what happens with a prefix that is dampened if >> it's distributed via two providers. > >I don't think you'll find a document that answers the question explicitly >because the implicit answer is sort of obvious, as I think you realize. > >It's a route that's dampened, not a prefix. With two providers, in most >cases, there would be two distinct routes to your prefix. A BGP route has >path attributes including an AS_Path, which is a list of Autonomous System >numbers. With two providers, the two routes will be distinct and have a >different set of AS numbers in most situations. Otherwise what would be the >point? Depending on how deeply you want to go, there is a very nuanced range of ideas in BGP information and how it is propagated. In our http://www.ietf.org/internet-drafts/draft-ietf-bmwg-conterm-04.txt , there was tremendous effort to clarify, and part of this clarification means that you probably have to understand the differences among a dozen or so seemingly alike concepts. These are also being refined in the new BGP standard, which is at draft 19 when last I looked -- and probably still has a round or two to go. Let's put it this way...we ducked going deeply into route flaps, much less dampening (an important difference -- they are NOT equivalent) in this version of the document. There are concepts that need to be most thoroughly internalized -- grokked if you will -- before dealing with flap control and propagation. Quite a bit of theoretical analysis is available, but the associated global scalability issues are by no means solved. > >> >> Will only the penalized route dampened, that is >> will we still have connectivity if one link is >> flapping. I think so, but I'd like to have some >> confirmation for that. > >If just one of your links is flapping, then just one of your routes will get >dampened. The other one will still be usable. > >If your two links share a circuit to the telco or physically share a path >that has been dug up by the proverbial back-hoe operator, it's possible both >links could be flapping, and then they will both get dampened, but hopefully >you have a network design that avoids that problem. Don't let BGP rain on >your parade and dampen too much at once! Have good physical diversity. ;-) > >Priscilla > >> >> Thanks and best regards, >> >> Oliver >> >> >> -- >> Oliver Hensel >> telematis Netzwerke GmbH >> mailto:[EMAIL PROTECTED] >> Siemensstrasse 23, D-76275 Ettlingen >> Tel: +49 (0) 7243-3448-0, Fax: -498 >> visit us: http://telematis.com Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=65126&t=65086 -- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
RE: BGP dampening [7:65086]
I'll take a stab at it since nobody else did. Oliver Hensel wrote: > > Hi! > > Can someone point me to a document which explains > what happens with a prefix that is dampened if > it's distributed via two providers. I don't think you'll find a document that answers the question explicitly because the implicit answer is sort of obvious, as I think you realize. It's a route that's dampened, not a prefix. With two providers, in most cases, there would be two distinct routes to your prefix. A BGP route has path attributes including an AS_Path, which is a list of Autonomous System numbers. With two providers, the two routes will be distinct and have a different set of AS numbers in most situations. Otherwise what would be the point? > > Will only the penalized route dampened, that is > will we still have connectivity if one link is > flapping. I think so, but I'd like to have some > confirmation for that. If just one of your links is flapping, then just one of your routes will get dampened. The other one will still be usable. If your two links share a circuit to the telco or physically share a path that has been dug up by the proverbial back-hoe operator, it's possible both links could be flapping, and then they will both get dampened, but hopefully you have a network design that avoids that problem. Don't let BGP rain on your parade and dampen too much at once! Have good physical diversity. ;-) Priscilla > > Thanks and best regards, > > Oliver > > > -- > Oliver Hensel > telematis Netzwerke GmbH > mailto:[EMAIL PROTECTED] >Siemensstrasse 23, D-76275 Ettlingen >Tel: +49 (0) 7243-3448-0, Fax: -498 > visit us: http://telematis.com > > Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=65120&t=65086 -- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
BGP dampening [7:65086]
Hi! Can someone point me to a document which explains what happens with a prefix that is dampened if it's distributed via two providers. Will only the penalized route dampened, that is will we still have connectivity if one link is flapping. I think so, but I'd like to have some confirmation for that. Thanks and best regards, Oliver -- Oliver Hensel telematis Netzwerke GmbH mailto:[EMAIL PROTECTED] Siemensstrasse 23, D-76275 Ettlingen Tel: +49 (0) 7243-3448-0, Fax: -498 visit us: http://telematis.com Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=65086&t=65086 -- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
