Re:[isp-bgp] splitting the network and BGP

["Ejay Hire" <ejayhire@...>]

> My question is, what happens if/when I severe the conneciton between the
> two sites?  Can SITE-B still advertise networks that are provided from
> ISP-1 and vice-versa?  Can the two sites still use the same (public) ASN
> even though there is no direct connection between the two?
Hello.

I see two seperate questions here.

1.  Can two networks that are usually interconnected use the same asn, even 
if they multihome to different providers.
Yes, but you'll need to make provisions to workaround BGP's loop prevention 
mechanism.
Options:
- Cisco's allow-as-in feature allows you to recieve your own as 
advvertisements
- A tunnel that fully encapsulates the traffic, gre, ip-in-ip, or ipsec ESP.
- Accept a default route from your isp's, and then you'll use it to reach 
the remote site

2.  Can you advertise space from one isp to another isp from a diverse 
location.
- Yes, but there is a small chance that you will experience reachability 
issues from users that aggressively filter.  For netblocks larger than a 
/24, and if you connect to large ISP's, this should not be a problem.

I know #2 will draw some fire, so I'll go ahead and defend it.
If I have MCI ip space, and advertise it to MCI and At&t, I'm multihomed 
with MCI's ip space.  If my MCI link goes down, I'm single-homed to At&t 
temporarily, and it still works.

Regarding filtering, this shouldn't be an issue as long as the ISP's 
exchange specifics with each other.  I.e. you announce your MCI /24 from 
at&t.  As long as MCI recieves that /24 announcement then it should work 
fine.

Regarding whether or not this is a legitimate use of the assigning ISP ip 
space, as long as you remain a customer and have a legitimate allocation 
from them, they should not complain.


Ejay Hire
Network Engineer

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