Somewhere in Basham Halabi's "Internet Routing Architectures" (Cisco Press) he mentions that two different origins for a prefix is an "illegal configuration". Unfortunately I just can't find the exact phrase in the book.
Anyway, I am sure that announcing the same prefix under 2 different origins should work although I never did something like this.
Indeed, it happens today, and my guess is that it works, or networks wouldn't do it (not necessarily a valid assumption, I know). :)
Shane
This can probably be made to work, but it is quite easy to create a routing loop, as by default an external BGP learnt route is preferred (admin dist) over an iGP or iBGP: so provider 'A' would prefer to route via provider 'B', rather than via the direct customers interface. This can be got around by not carrying this route in IGP, but only in iBGP with local preference higher than any eBGP peerings. i.e. carry customer static routes in iBGP with your highest local-prefernce. Which is usually the best way anyhow. Both providers would need to implement it this way, or one provider would end up carrying all the traffic. Not that I have tried this, just thought it ought to work. It's getting a bit away from the original question ... -- ---- Steve Alvey Email: steve.alvey@ip-engineering.bt.com Global IP Design Specialist