On Fri, 17 Feb 2006, Todd Vierling wrote:
I might be crazy, but couldn't you just prepend the route enough to effectively poison it at ingress to 'backup-isp' ?
Some route decision override schemes don't care what the path length is at all, or factor it in with such a low weight, such that no reasonable amount of prepending will change the situation.
And while I actually misunderstood what you said, this is still partly correct -- ISP_B will often put a preference onto their own route at a level that outweighs path length absolutely. If ISP_B has a "don't prefer" community, that could work for ISP_B's own customers, but short of removing the advertisement from ISP_B's peers too, that doesn't always move traffic completely off of the ISP_B pipe. This is because traffic entering ISP_B destined for the network *will* usually prefer ISP_B's link in spite of the community, thanks to ISP_B not desiring to transit packets in and out of their network without a revenue point somewhere in the path.
With the development of source traffic engineering schemes, prepending is no longer reliable as a means of affecting routing on the remote side. That perception will have to die with it (hopefully sooner rather than later).
-- -- Todd Vierling <tv@duh.org> <tv@pobox.com> <todd@vierling.name>