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Re: Confussion over multi-homing

  • From: Alex Pilosov
  • Date: Fri Sep 15 17:27:02 2000

On Fri, 15 Sep 2000, David Lott wrote:

(for some reason RFC2260 refers to RFC1773 as describing encapsulation
over non-direct-ebgp-learned-routes, I assume that they meant GRE,

> RFC2260 essentially states how routers using BGP can be configured to detect
> failure of connectivity to a particular ISP and then advertise that address
> out to another ISP.  However, if the ISP that is still up filters at a /20
> you still end up with the same problem.
That's not a problem, since you have a contractual relationship with this
ISP, you can thwack them with the money-stick, and tell them to blow a
hole in the filter for you. 

The real problems I see with it are following:
1. You need to get both upstreams to agree to run GRE on their edge
router. That requires bigger money-stick than fixing their filtering :)

2. It only protects you from failure of a link from you to upstream, not
from upstream losing their connectivity, power, or flapping like crazy and
getting dampened. In my experience, latter happened more often than first.

> Actually the are two issues that must be addressed by any multi-homing
> solution.
> 1) How does someone get to me? (outside connectivity in)
> 2) If I use more than one address pool, how do I NAT with the correct
> (meaning up) ip address (inside connectivity out)
> While RFC 2260 does not solve these problems, it does lead me to think that
> a solution is possible - give me a few days to work on this and I'll report
> back.
This RFC doesn't address cases where you use NAT. See this if you do:
It uses DNS trickery to accomplish it. You CAN use RFC2260 trickery with
it (half of RFC2260 is included there), but you don't have to, if you
don't care about broken connections when one of links fails.

Alex Pilosov            |
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