Since I've been getting a few private email questions on this - here is what I was what I was able to find out...Basically it can work, but its dependent upon on you selecting appropriate ISP providers and that these provider peer (or I would assume are not too many ASs away)... Chris Williams had it correct all along in his mail: http://www.merit.edu/mail.archives/nanog/msg02315.html Here is how I understand it: Yes, the /24s will be filtered out by some [big "meanies" - but give'm lots of $$ and they co-loc for ya - its for the greater good of the Internet...;-)...]. If Provider A (primay) has a /19 or larger, and you get you /24 from them, then when your link to them goes down - your /24 is withdrawn by A but Provider A's /19 or larger is still globablly visible and packets will still be sent to Provider A for your /24. If provider B is peered with Provider A, provider B is advertising your /24, then packets will be routed from A through B to your site. Note: In this case, Provider A must not filter /24s from his peers to their own address space. A must also export your /24 when your link is up with your AS (in addition to their /19). See the "BGP Multi-Homing to > 1 ISP Help Required" thread in the comp.protocols.tcp-ip if you want more info. Cheers... Todd Sandor wrote:
This brings me back to my original "operational" question of whether a non-aggregated /24 will be globablly visiable when our primary link fails...The situation:
"When a customer in one location is using a multi-homed setup to two providers A and B, with A being the primary (using a /24 from loaned from provider A) and B being the secondary (updates via B would have a longer AS_path - using default routes with local-pref on the primary). When the customers link to A fails, will the /24 that needs to be now globably visible via B (a non-aggregate IP address for B) NOT be globably visible because of the BGP filtering policies of some other provider somewhere, say C ?