Routing in general is based of the premise of "my decision, my control" and therefore you have some (albeit limited) controls about how YOU can influence someone else's routing decision. So any time you have more than one connection to the collective ('Net) then you simply run the risk of you make one decision to send a packet out a particular link, but a bunch of other people make decisions about routing as well and it may very well come back another path. Presumably you have your IP addressing as a constant. If you are NATting, you may have some interesting problems with this, but that would be a design problem on your end. Same with stateful firewalls.
From an appplication viewpoint though, it really shouldn't make any difference. Packet goes out. Packet comes back. Life is good.
In short though, you have some choices with this, but they are all design choices on your end. If you want to be multihomed, this is the way life is. HTH, Scott -----Original Message----- From: owner-nanog@merit.edu [mailto:owner-nanog@merit.edu] On Behalf Of Drew Weaver Sent: Monday, January 14, 2008 10:31 AM To: nanog@merit.edu Subject: Asymmetrical routing opinions/debate Pardon me if I am using the wrong term, I am using the term Asymmetrical routing to describe a scenario in which a request packet enters a network via one path and the response packet exits the network via a different path. For example an ICMP ping request enters a network via ISP A and the reply leaves via ISP B (due to multi-homing on both networks, and or some kind of manual or automatic 'tweaking' of route preferences on one end or the other). I haven't noticed too many instances of this causing huge performance problems, but I have noticed some, has anyone noticed any instances in the real world where this has actually caused performance gains over symmetrical routing? Also in a multi-homed environment is there any way to automatically limit or control the amount of Asymmetrical routing which takes place? (should you?) I have read a few papers [what few I could find] and they are conflicted about whether or not it is a real problem for performance of applications although I cannot see how it wouldn't be. Has there been any real community consensus on this issue published that I may have overlooked? Thank you, -Drew