Multiple default routes on multihome host

[Bill Moran]

In response to Tom Judge <tom at tomjudge.com>:

> Bill Moran wrote:
> > In response to Eygene Ryabinkin <rea-fbsd at codelabs.ru>:
> > 
> >> Bill,
> >>
> >> Mon, Feb 18, 2008 at 04:36:18PM -0500, Bill Moran wrote:
> >>> I would suggest you ask yourself (and possibly the list) _why_ you think
> >>> multiple default routes is necessary ... what is it that you're hoping
> >>> to accomplish.  I'm guessing your looking for some sort of redundancy,
> >>> in which case something like CARP or RIP is liable to be the correct
> >>> solution.
> >> I had faced such situation once: I had multihomed host that was
> >> running Apache daemon that was announced via two DNS names that
> >> were corresponding to two different IPs, going via two different
> >> providers.  When the first provider's link goes down, the second
> >> provider is still alive, and when both providers are alive, the
> >> traffic is balanced via DNS round-robin alias.  Do you see some
> >> better way to do it via CARP, RIP, something different?  I am still
> >> interested in other possibilities.
> > 
> > The canonical way to do this is with BGP.  I can be done with CARP
> > if both providers support it and are willing to work together.
> 
> Unfortunately businesses tend to get bundled PA address space when 
> purchasing leased lines off of ISP.  This means that a some what simple 
> transition from provider A to provider B can not be done with BGP.  Also 
>   as the OP states one the the address blocks that he has is a /25 which 
> most ISP's will filter from the BGP address table because it is to small.

You're confusing issues.  The OP is in the process of a migration, in
which case the packet rewriting via pf/ipfw/etc is probably the best
approach.  Eygene is describing a different scenario with a permanent
multihomed system, in which case BGP is probably the best option, but
CARP _may_ be an option.

> I think the cost of learning BGP, getting an AS number and a suitable 
> large block of PI address space, getting 2 routers that can do BGP, 
> coupled with the consultancy costs charged by the ISP to setup the BGP 
> feed totally out way the cost of just multihoming a box for a few 
> days/weeks while the required changes take affect..  Ok so this is not 
> ideal but hey it works and its simpler..

Agreed.  In fact, if you read the prior messages, I never disagreed.

Personally, I prefer to do this type of migration as a "flag day" where
everything just gets switched over all at once ... but that's not always
possible.  The OP seems to have a number of systems with public IPs,
and it's harder to do a flag day with a lot of systems.

-- 
Bill Moran
Collaborative Fusion Inc.
http://people.collaborativefusion.com/~wmoran/

wmoran at collaborativefusion.com
Phone: 412-422-3463x4023