[nsp] Methods for Non-BGP multihoming

CARL.P.HIRSCH@sargentlundy.com CARL.P.HIRSCH@sargentlundy.com
Tue, 23 Jul 2002 14:36:26 -0500

Given the shakiness of WorldCom, I'm looking into implementing multihoming
for our internet connection. Currently leaning towards a metro ethernet

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.

thanks much,
-carl hirsch