The real world Internet involves many-to-many conenctions.
Routing Symmetry is important for many reasons. For example, some appications (NTP, MBONE) depend on this.
Consider the case where one AS is connected to two ISPs that are both connected to the same NAP. To do load balances, it would appear that two different routing policies may apply to the same AS. There are three approaches to handling this.
Enke proposes a new BGP attribute called the MPP (multi-provider-preference) (Type code 10)...contains the AS that sets the preference.
This attribute would be set when it was needed.
If available MPP, would make it possible to provide full load sharing, backups as well as limited load-sharing.
There are three internet drafts on this idea.
Yakov comments that the MobilIP work will cause asymmetric routes to be common place. This proposal does not help with this. He also says that the AS Number arguement is specious. He also asks about passing other information (like throughput or something).
There was a suggestion about having MED transmit through a boundary. Enke said that was considered, but there is the problem of how to figure out to which provider the MED applies to....
There was some objection to the idea that the AS-PATH length should be used as a descriminator. Others said that eventhough that is not explicitly called out in the standard, it is a useful metric.
Another comment concerns a possible negative impact on route aggregation.
Sean Doran from Sprint argues that asymmetric routing is a reality (or about to be). This change will continue to promote virtual circuit routing.