[nsp] Methods for Non-BGP multihoming

Pekka Savola pekkas@netcore.fi
Wed, 24 Jul 2002 09:13:46 +0300 (EEST)

On Tue, 23 Jul 2002 CARL.P.HIRSCH@sargentlundy.com wrote:
> Given the shakiness of WorldCom, I'm looking into implementing multihoming
> for our internet connection. Currently leaning towards a metro ethernet
> provider.
> I've read a couple of Avi Freedman's BGP tutorials as well as a number of
> documents from Cisco (BGP Case Studies, the ISPCon BGP presentation,
> Configurations for Load Sharing with BGP in Single and Multihomed
> Environments, etc). I feel like I've got a basic grasp of BGP but for
> simplicity's sake I'm still inclined to go with the option of taking static
> routes from each ISP and allowing them each to advertise our /16. I'm not
> interested in load-balancing, only redundancy.
> If both ISPs were to advertise our /16 address space (registered by us, not
> an ISP), we wouldn't necessarily need an AS number, would we? Given that we
> wouldn't be actually talking BGP with anybody and the ISPs would be
> handling the route advertisements.  We'd put a metric on our static routes
> outbound so only one link would be used unless it failed.
> Are there any serious gotchas to this sort of approach? Outbound routing
> would be very simple, but I'm wondering about the logistics of having ISP A
> and ISP B both advertising routes. When traffic comes inbound to our
> network, is there any way to make sure that one provider is always used?
> Does it even matter? It's the issue of how traffic finds its way to your
> network when you're multihomed that I'm not quite clear on.
> If any body knows a better way to do this, I'd be happy to hear about it.
> I'm also curious about the pros and cons of running BGP with limited
> routing tables or even BGP with full routing tables. At this point the
> primary benefit of running full BGP appears to be improved visibility into
> traffic patterns and easier troubleshooting.

Yes, there are serious problems with this w.r.t. real failover and 

The best solution considering the requirements is to run BGP with private 
AS numbers to your ISP's and have them remove them in the advertisements.  
That way you can have control on outbound/inbound traffic but don't need 
an ASN.

Pekka Savola                 "Tell me of difficulties surmounted,
Netcore Oy                   not those you stumble over and fall"
Systems. Networks. Security.  -- Robert Jordan: A Crown of Swords