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Re: Disaster recovery using as-prepend?
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 <[email protected]> <[email protected]> <[email protected]>