Another thing that I have come across is that some ISP's ignore the prepending of AS paths and local preference settings you send them, so it could be that ISP is doing this. lrmoore is correct, the first point of call would be to talk to your two ISP's and see what they have to say.
An alternative way of doing things is to use a conditional BGP advertisement, whereby you only advertise the route via ISP2 when the link to ISP1 goes down (or more specifically when you lose BGP connectivity to ISP1).
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by: lrmoorePosted on 2007-12-11 at 15:23:01ID: 20453847
If ISP1 does not specifically give you a 'portable' ip subnet, then they are typically sending summary routes to their peers and many other ISP's will simply not accept an individual advertisement of these networks that belong to the larger summary network, or the summary has precidence.
Does ISP1 know that you are trying to advertise their network out through ISP2?