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Re: Enterprise Multihoming
- From: Howard C. Berkowitz
- Date: Fri Mar 12 12:20:58 2004
At 4:06 PM +0000 3/12/04, Stephen J. Wilcox wrote:
Hoping that I don't sound too much like Bill Clinton, that depends on
what you mean by "it." If "it" is multihoming, with your own ASN, to
two providers, your raise some valid points.
I think its too easy, thats the problem.
Is there an intermediate alternative before you go all out? Yes, I
think so, assuming your current provider has multiple POPs. Let me
examine some of your points if we consider RFC 1998-style
multi-POPping (I just invented that highly technical term) using PA
For <$1000 (excluding bandwidth/ccts)
Alternatively, another POP link, and preferably another router. If
you are more concerned with loop failures than router failures, not a
completely unreasonable assumption, you could get away with one
router that has multiple interfaces, and spend some of the savings on
backup power -- possibly a backup power supply in addition to the
UPS, such as a Cisco RPS on their smaller routers. While you'll
probably take a performance hit, or if you can reduce to critical
traffic on an outage, you might get away with a second smaller router.
you can buy a box, connect to your two providers, get an ASN and IPs
Like everything else, It Depends. My experience is that access links
fail more often than provider routing systems, especially with a
clueful provider. Since you can't guarantee that your physical
connectivity to two different ISPs doesn't involve a shared risk
group in the lines, there are still some things you may not be
I dont agree that connecting to two+ upstreams makes you better. In my
experience end networks have a couple of orders of magnitude more
a PoP in any reasonably large ISP. Ie the percentage theoretical
One option, depending on the plant in your area, is that if you are
considering a second router, consider putting it in a nearby
building, reachable by WLAN (if you are minimizing costs), where that
building minimally has different ducts to the telco end office, and
ideally goes to a different end office. Not always possible, but to
be considered. Longer-range wireless (radio or optical) links get
That's a motivation for PA address space, where the provider
aggregate is less likely to be small and easily damped.
In addition you seriously increase the complexity of your system, chances are
you're using the cheapest kit you could find (or at least cheaper and smaller
than what I would use).. its not great at BGP and may fall over when you get a
minor DoS attack, you probably generate flaps quite a bit from adhoc
if you're announcing a /24 then thats going to get you dampened quickly..
Again looking at intermediate solutions, there are always partial
routes such as customer routes of the provier.
actually create a new weakest link. Also most of the corporates I've
take defaults rather than full tables.. so if the provider does have an issue
you still forward the traffic, theres no failover of outbound routing.
Again, an operational advantage of multiPOPping and working with one
carrier, although you aren't going to be protected against insanity
of their BGP/
Even if you spend (waste) the money on some decent gear, you're on
your own and
when a problem occurs the ISPs are going to be less helpful to you (not by
choice, I mean they dont have control of your network any more..
of whats causing problems is limited to the bit that they provide to you), so
chances are your problems may be more serious and take longer to diagnose and
MHO would be to look at "multihoming" as a spectrum of solutions
rather than a binary choice of single-provider-single-link versus
multiple-provider. In given situations, you might also want to look
at DSL or cable for diversity, tunneling to an ISP since the
broadband provider is unlikely to be willing to speak BGP. Even
dialup/ISDN, sometimes for critical workstations, has its place.
IMHO avoid multihoming. You will know when you are big enough and
you *need* to
do it, if you're not sure or you only want to do it cause you heard everyone
else is and its real cool then I suggest you dont.
Shameless plug: I do go through these options in my book, Building
Service Provider Networks (Wiley). Even there, though, I only run
through the alternatives. You will still have to make your own
cost-benefit decisions based on business policy, budget, clue level
and cost of alternatives.