North American Network Operators Group

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Re: What is multihoming was (design of a real routing v. endpoint id seperation)

  • From: Stephen Sprunk
  • Date: Mon Oct 24 16:14:51 2005

Thus spake <[email protected]>
> the market wouldn't
> feel the need to have to dual home.

the internet model is to expect and route around failure.
Seems to me that there is some confusion over the meaning
of "multihoming". We seem to assume that it means BGP multihoming
wherein a network is connected to multiple ASes and uses BGP
to manage traffic flows.
AFAICT, that is the accepted definition in this forum. Anything less is best called by a different, more precise term to avoid confusion.

Other people use this term in very different ways. To some people
it means using having multiple IP addresses bound to a single
network interface. To others it means multiple websites on one
That is virtual hosting in a NANOG context. Some undereducated MCSEs might call it multihoming, but let's not endorse that here.

A single tier-2 ISP who uses BGP multihoming with several
tier 1 ISPs can provide "multihoming" to it's customers
without BGP. For instance, if this tier-2 has two PoPs
in a city and peering links exist at both PoPs and they
sell a resilient access service where the customer has
two links, one to each PoP, then it is possible to route
around many failures. This is probably sufficient for most
people and if the tier-2 provider takes this service seriously
they can engineer things to make total network collapse exteremely
I bet customers who bought two links to Cogent no longer believe they're "multihomed"; policy failures are disturbingly frequent in Tier 2s, particularly those wanting to join the Tier 1 club. Total network failures are rarer, but even folks like UUNET, WorldCom, AT&T, MCI, etc. have them from time to time. With restoral times measured in days on both types of occasions, you can't discount them as "extremely unlikely" if your business can't function without a network. Ask the folks at Starbucks how many millions of dollars of coffee they gave away when their cash registers didn't work for a couple days... and how many customers (i.e. future revenue) they would have lost if they hadn't.

Two links to the same provider is merely "redundancy" or "link/POP diversity", not multihoming. Don't let your marketing department override your common sense or engineering clue.


Stephen Sprunk "Stupid people surround themselves with smart
CCIE #3723 people. Smart people surround themselves with
K5SSS smart people who disagree with them." --Aaron Sorkin