quick comment in line. At 04:53 PM 5/27/2002 -0400, Chuck wrote: >I have a question, Howard - in line: > > >""Howard C. Berkowitz"" wrote in message >[EMAIL PROTECTED]">news:[EMAIL PROTECTED]... > > This is one of those posts where the attributions have gotten very > > confused. Comments inline. > > >snip for brevity > > > > It can be done, if both ISPs agree to it and coordinate their routing > > policies. A public AS, however, is justified in this circumstance. > > > > While doesn't quite describe this situation, look at RFC 2270 for the > > general strategy. Both ISPs have to remove private AS. This will > > also cause more than one ISP to appear to originate the route, which > > is a technical violation of BGP (i.e., it's an "inconsistent route"), > > but that isn't that uncommon and doesn't seem to break anything. > > > >Question: in an ideal world, what would happen when an "inconsistant route" >shows up? idealy, would that route be black holed? >Since it is "common" and since it "doesn't seem to break anything" in ral >terms, what happens? BGP advertises reachability to other BGP routers, be >they internal or external. But in terms of a packet traveling from my house >to a destination that is "inconsistant" what happens? What matters? My >packet continues to be passed from here to there until some directly >connected router receives it. I'm assuming that "inconsistant" does not >imply "loop" > >thanks.
You are correct in that inconsistent advertisements do not represent looped routes. In the case of a prefix seemingly existing in two AS's, a remote router simply passes that prefix through the basic BGP path selection algorithm and selects the more preferable of the two for export to the main routing table. Once a route hits the routing table, transiting packets are forwarded as usual. Any potential concern lies in the handling of routes that show up as inconsistent. I have seen discussions from various communities (RIPE comes first to mind) about specifying a globally accepted behavior for such routes, but haven't seen a consensus on this issue other than to leave it alone. Howard probably has somewhat more detailed insight here. At present, inconsistent advertisements are accepted and many feel are valid and should not be handled differently from normal announcements. Customers who think that connecting to two providers is generally better than two pops from a single provider and providers who are too about nervous about losing customer revenue to force customers to properly multi-home (PI space/ASN) or not multi-home to different providers at all are likely the cause of this situation. So long as this continues to be the norm, we'll likely see more and more of these type announcements and the likelihood of routers dealing with them differently (dropping for example) will similarly decrease. Hit a route server (say route-server.exodus.net) and do a show ip bgp incon and you'll see just how many of these routes we are dealing with. Pete > >snip for brevity< Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=45169&t=45169 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
