RE: Asymmetrical routing opinions/debate

["Darden, Patrick S." <darden () armc org>]

I'm not sure I understand.  If a routing protocol such as BGP is being used, this is considered normal behavior, and 
the routing determination is made usually wrt either best route or best bandwidth.  In the first case, a return packet 
would usually follow on the same interface.  In the second case it would be determined by however you have set things 
up (round robin, 2/3rds on one int and 1/3rd on the other, whatever.)

If you are multi-homed with two backbone providers with static routes, then it is also normal behavior for some packets 
to enter thru either of your two interfaces, and then to exit on the preferred interface (if no preference is made 
clear via routing, then the default outbound interface is the one with the lower IP address--e.g. 201.x.y.z would be 
preferred over 202.x.y.z).

Does that help?

--Patrick Darden
--ARMC


-----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