[c-nsp] Multihoming

[Jon Lewis]

On Wed, 15 Sep 2010, Heath Jones wrote:

> Jon there seems to be a bit of a common belief that advertising a /24 or
> some prefix that has been assigned by a provider, out to another provider,
> is bad practise. I don't get it either and haven't seen issues myself.

This is done quite a bit, as (again, I'm only familiar with practices in 
the ARIN region) this is basically the only way a small organization (ISP 
or end user) could multihome in the past.  ARIN just adopted a policy 
allowing multihomed end users to get a PI /24.  So that's an option now as 
well.

> The only scenario I can think of is (in some odd configurations) when the
> original provider sees part of their own network being advertised by another
> ISP, they filter it and it breaks connectivity, or the original provider's
> igp contains that prefix somehow already.. ?

Ideally, both ISPs would be aware of your intentions to multihome and not 
be dumb enough to filter your announcement via the other ISP.  It wouldn't 
surprise me if that sort of filtering has happened though.

Of course, it wouldn't surprise me if your ISP broke your prefix filter, 
didn't use prefix filters, lost your interface config, assigned your 
interface /30 to another customer, or fundamentally altered your 
route-map (if they have one) such that they stopped accepting some of your 
routes.  I've seen all these things happen.

----------------------------------------------------------------------
  Jon Lewis, MCP :)           |  I route
  Senior Network Engineer     |  therefore you are
  Atlantic Net                |
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