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

Mon Oct 24 20:12:53 UTC 2005

Thus spake <Michael.Dillon at btradianz.com>
>> > 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
> server.

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
> unlikely.

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.

S

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