redundancy [was: something about arrogance]
Manolo Hernandez
manolo at dialtoneinternet.com
Tue Jul 30 17:29:53 UTC 2002
Yes their is a reason to some /24s advertised to the world. If this a
class on BGP they would tell you that was a nono, but since this is the
real world it happens and is sometimes required. It is required when you
need to give a customer T-1 access at a location seperate from yours and
has a seperate connection to the net and you are using your AS on the
access router. A /24 is a solution that works nicely and still works
with your aggregated /20 address.
On Tue, 2002-07-30 at 13:23, Derek Samford wrote:
>
> I couldn't possibly agree more. In fact, my approach has been to create
> a mesh between different Colo centers, and keep it at about 3 Transit
> carriers. Because of the different methods of interconnection, I haven't
> ever had a long-term outage. Also, I've been able to filter any issues
> that are beyond my carrier's immediate reach (i.e. congested peering
> points.) At the same time, I've been able to maintain aggregation of all
> of my routes, and maintain true stability in my network. There is
> absolutely no excuse to fill up the routing tables with nonsense.
>
> Derek
>
> -----Original Message-----
> From: owner-nanog at merit.edu [mailto:owner-nanog at merit.edu] On Behalf Of
> Phil Rosenthal
> Sent: Tuesday, July 30, 2002 12:52 PM
> To: 'Pedro R Marques'; brad.knowles at skynet.be
> Cc: nanog at merit.edu
> Subject: RE: redundancy [was: something about arrogance]
>
>
> I have in the past single-homed to Level(3) and Verio, each in their own
> facility in NC.
> In that time, both carriers had about 1 solid hour a month of solid
> downtime (some months were worse, some were better). Some of the outages
> were on the order of 8 solid hours (verio) or 4 hours (level3).
>
> We did not run HSRP with Level3, so it may be difficult to guarantee the
> uptime of one gige handoff... But we ran HSRP with verio, and of all the
> outages (about 20 of them) -- Maybe two of them were avoided because of
> HSRP.
>
> Other than that, it was all downtime.
>
> At this point, I couldn't conceive single-homing to any uplink anymore.
>
> --Phil
>
> -----Original Message-----
> From: owner-nanog at merit.edu [mailto:owner-nanog at merit.edu] On Behalf Of
> Pedro R Marques
> Sent: Tuesday, July 30, 2002 6:23 AM
> To: brad.knowles at skynet.be
> Cc: nanog at merit.edu
> Subject: redundancy [was: something about arrogance]
>
>
>
> Brad writes:
> > I'm probably demonstrating my ignorance here (and my stupidity
>
> in
> > stepping into a long-standing highly charged argument), but I'm
> > completely missing something. For reasons of redundancy &
> > reliability, even if you were to buy bandwidth in only one location,
> > wouldn't you want to buy it from at least two different providers?
>
> > If you buy bandwidth from two different providers at two
> > different locations, this would seem to me to be a good way to
> > provide backup in case on provider or one location goes
> > Tango-Uniform, and you could always backhaul the bandwidth for the
> > site/provider that is down.
>
> Several other posters have mentioned reasons why redundancy between 2
> different connections to separate providers are not, in most situations,
>
> the preferable aproach but i would like to add another point/question...
>
> When considering redudancy/reliability/etc it is important to think
> about what kind of failures do you want to protect against vs cost of
> doing so.
>
> It is my impression, from reading this list and tidbits of gossip, that
> the most common causes of failure are:
> - link failure
> - equipment failure (routers mostly), both software and hardware
> - configuration errors
>
> All of those are much more frequent than the failure of an entire ISP (a
>
> transit provider). It is expected, i believe, of a competent ISP to
> provide redudancy both within a POP and intra-POP links/equipment and
> its connections to upstreams/peers.
>
> As such, probably the first level of redundancy that a origin AS
> (non-transit) would look at would be with the intent to protect from
> failures of its external connectivity link and termination equipment
> (routers on both ends).
>
> To do so, one can look at:
> - 2 external links to distinct providers
> - 2 external links to the same provider
>
> While i can't speak to the economics part of the equation (although i
> would expect it to be cheaper to buy an additional link than connect to
> a different provider) from a point of view of restoration, protecting a
> path with an alternate path from the same provider is certainly an
> aproach that gives you much better convengence times.
>
> This comes from the fact that in terms of network topology, the distance
>
> between 2 links to the same upstream is much shorter than 2 links to
> different upstreams. While, if you protect a path with an alternate path
>
> to the same ISP you can expect convergence to occur within the IGP
> convergence times of your provider, with 2 different providers you need
> global BGP convergence to occur.
>
> This gets to be longer dependent on how topologically distant your 2
> upstreams are... for instance attempting to protect a path to an ISP
> with very wide connectivity with a protection path from one with very
> limited connectivity would be a particularly bad case as you would have
> to wait for the path announced by the larger ISP to be withdrawn n times
>
> from all its peering points and the protection path to make its way
> through in replacement.
>
> It is counter-intuitive to me what i perceive to be the standard
> practice of attempting to multi-home to 2 distinct providers by
> origin-only ASes... from several points of view: convergence times, load
>
> on the global routing system, complexity of management, etc, dual
> connectivity to different routers of the same provider (using distinct
> physical paths) would seem to me to make more sense.
>
> Unless the main concern is that the upstream ISP fails entirely... which
>
> given the fact that it tends to have frontpage honors on the NYTimes
> this days does not apear to be an all to common occurence (i mean
> operationally, not financially - clarification added to dispel potential
>
> humorous remarks).
>
> So, my question to the list is, why is multi-homing to 2 different
> providers such a desirable thing ? What is the motivation and why is it
> prefered over multiple connections to the same upstream ?
>
> Is the main motivation not so much reliability but having a shorter
> as-path to more destinations ? This would apear to me to be a clear
> advantage since that doesn't necessarily reflect in better qualitify of
> interconnection.
>
> My apologies in advance if these seem to be stupid questions...
>
> thanks,
> Pedro.
>
>
>
>
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