Resilience - How many BGP providers

David Hiers hiersd at
Thu Nov 12 00:05:55 UTC 2009

It is wise to stack the deck in your favor, but you'll never really
know how much real redundancy you've purchased:


On Wed, Nov 11, 2009 at 3:41 PM,  <adel at> wrote:
> I suppose I could take the whole resilience thing further and further and further.  One of the replies used a phrase which I thing captured the problem quite nicely: "diminishing returns".
> Basically I could spend lots and lots of money to try and eliminate all single points of failure.  Clearly I don't have the money to do this and what I'm really trying to establish is at what
> point do the returns start to diminish with regards to obtaining multiple transit providers.  The answer appears to be "it depends".  So if getting a third BGP peering with divergent paths,
> separate last mile, separate facility and separate router will increase costs by 5x but only increase resilience by 0.001% is it really worth it?  I'm trying to quantify the resilience of my
> Internet connectivity and quantify the effects of adding more providers.  Now to run through my case:
> - I have one facility to locate BGP routers at.  Thats not changing for the moment.
> - I can afford two BGP routers.
> - The facility I'm located at tell me they have divergent fibre paths and multiple entries into the facility. (Still need to verify this by getting them to walk the routes with me)
> - I am going to take transit from two upstreams.
> - I could ask the question as to whether I can peer with separate routers on each of the upstreams.  i.e. to protect against router failures on their side.
> - I will make sure that neither upstream peers with the other directly. (Does this give me some AS path redundancy?)
> So from the above:
> - I have no resilience with regards to datacentre location.  i.e. if a plane fell out of the sky etc., I'm done.
> - I can afford some BGP router resilience on my side.  So I should be able to continue working if a router failure which only affects one of my routers occurs.
> - I have some resilience in terms of actual fibre paths to the facilites where I will be picking up the BGP feeds from. (to be verified)
> - I have some "AS resilience" if this is the right term.  So if the AS of one of my upstreams drops off the face of the Internet, I can still get to the Internet through the AS of my other
> provider
> - Peering with separate routers may give me some resilience for router failure on the side of my upstreams? (not totally sure on this)
> In this situation, if I add another peering with another upstream, am I really getting much return in terms of resilience?  Or should I spend this money examining the many other SPOFs in
> my architecture?  I'm perfectly sure there is absolutely no point me peering with 6 providers, but maybe some gains in peering with 3?  I'm trying to figure out at what point is adding
> another peering in my case a waste of money.
> I haven't gone into switch and power redundancy, because I "think" I understand it.  I wanted to concentrate on the multiple upstreams question.  Heads starting to whirl right about now.
> Adel
> On Wed   5:27 PM , "Dylan Ebner" dylan.ebner at sent:
>> You question has many caveats. Just having two providers does not
>> necessarily get you more resiliency. If you have two providers and they are
>> terminating on the same router, then you still have a SPOF problem. You
>> also need to look at pysical paths as well. If you have two (or three)
>> providers and they are using a common carrier, then you have a problem as
>> well. For example, GLBX has a small prescence in the Minneapolis metro. If
>> I were to use them as a provider, they would use Qwest as a last mile. If
>> my other provider is Qwest (which it is), I may not have path
>> divergence.Facilities are important too. We have three upstreams; Qwest, MCI and ATT.
>> The facility only has two entrances, so that means two of these are in the
>> same conduit. IF you only have one entrance, all you connections are going
>> to run through that conduit, and that makes you susceptable to a rouge
>> backhoe.
>> You are on the right track to question your resilancy. Some upstreams can
>> offer good resilancy with multiple feeds. Others cannot. I would start with
>> your provider and see what you are getting. Maybe you already have path
>> divergence, sperate last miles, and multiple paths in the isp core.  If you
>> go with multiple providers, you want to make sure you don't risk losing
>> something you already have.
>> -----Original Message-----
>> From: adel at [[email protected]
>>] Sent: Wednesday, November 11, 2009 11:14 AM
>> To: nanog at nanog.o
>> rgSubject: Resilience - How many BGP providers
>> Hi,
>> After recent discussions on the list, I've been thinking about the
>> affectsof multiple BGP feeds to the overall resilience of Internet
>> connectivityfor my organisation.  So originally when I looked at the design
>> proposals, there was a provision in there for four connections with the
>> same Internet provider.  Thinking about it and with the valuable input
>> ofmembers on this list, it was obvious that multiple connections from the
>> same provider defeated the aim of providing resilience.
>> So having come to the decision to use two providers and BGP peer with
>> both, I'm wondering how much more resilience I would get by peering
>> with more than two providers.  So will it significantly
>> increase myresilience by peering with three providers for example, as both of the
>> upstreams I choose will be multihomed to other providers.  Especially
>> asI am only looking at peering out of the UK.
>> Hope the above makes sense.
>> Adel

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