Resilience - How many BGP providers

Steve Gibbard scg at gibbard.org
Wed Nov 11 13:18:20 CST 2009


The thing to remember about redundancy is that it's a statistical game 
rather than a magic formula.

You can be reasonably sure that any single component will go down at some 
point.  Nothing works perfectly.  Few things last forever.

If you have two fairly reliable components, and if they're suffciently 
isolated from eachother that they won't be broken by the same event, it's 
much less likely that they'll both break at the same time.  That means 
that if one breaks, and you're not unlucky, you'll have time to fix it 
before the other breaks.

If you have three components, the chances of all three being broken at 
once are even less than the chances of two of them being broken at once. 
With four, you're even safer, and so on and so forth.  But once you get 
beyond two, you hit a point of diminishing returns pretty quickly.

That doesn't mean you should always do two of any given component.  Some 
things may be so important that you're not willing to take that level of 
risk and are willing to spend significantly more money to get a small 
amount more protection.  Some things may be sufficiently unimportant that 
you're willing to deal with occasional outages, and you can get by without 
a spare (few people -- with obvious exceptions who we don't need to hear 
about right now -- have fully redundant home connectivity, for instance). 
It's just a matter of understanding the risks, and doing the cost-benefit 
analysis to determine how much protection you need and how much you're 
willing to pay for it.

-Steve

On Wed, 11 Nov 2009, adel at baklawasecrets.com wrote:

>
>
> Hi,
>
> After recent discussions on the list, I've been thinking about the affects
> of multiple BGP feeds to the overall resilience of Internet connectivity
> for 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 of
> members 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 my
> resilience by peering with three providers for example, as both of the
> upstreams I choose will be multihomed to other providers.  Especially as
> I am only looking at peering out of the UK.
>
> Hope the above makes sense.
>
> Adel
>
>


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