[c-nsp] Inbound redundancy with two ISPs

[Pete Templin]

Steve Bertrand wrote:

>> BGP is the method.  If for some reason someone or something says "BGP is 
>> not an option", that someone or something needs to be eliminated from 
>> the puzzle.  I've done BGP with a fractional T1 customer, they can do it 
>> too.
> 
> So, if I understand you right, this is what you would propose, assuming
> that you were provider 'A', and it was your /24 you were assigning to
> the OP:
> 
> - 'A' routes a /24 to OP

'A' assigns a /24 to OP.  Better if they don't static route it; let BGP 
do its thing.

> - OP advertises the /24 to provider A and B via BGP with personal local
> preferences in place
> - A advertises it's aggregate including the /24 to the 'net

A advertises its aggregate, and propagates OP's /24 announcement.

> - B advertises the more specific /24 prefix to the 'net
> - OP uses an ASN from the private range, which both A & B must agree to
> honour

Use a public ASN.  That's what they're there for.

> I'd really like to understand this, because I've never had to deal with
> a situation where I've received a prefix from one ISP and needed to
> multi-home with it. (I have my own ARIN prefix).
> 
> Will the more specific /24 route advertised to the 'net by B not be the
> preferred route at all times?

If A propagates OP's announcement, it'll load dribble/share based on the 
rules of BGP.

Fairly quick and easy; your routers don't care who provided the /24 and 
they don't care about the AS path once they've selected a path.  We had 
a church do this...I probably spent more time explaining it to them AND 
to the other ISP than it would have taken to lab it up and test it.

pt