[101617] in North American Network Operators' Group
RE: Asymmetrical routing opinions/debate
daemon@ATHENA.MIT.EDU (Darden, Patrick S.)
Mon Jan 14 10:43:48 2008
Date: Mon, 14 Jan 2008 10:42:40 -0500
In-Reply-To: <B7152C470C9BF3448ED33F16A75D81C14D261CAEA2@exchanga.thenap.com>
From: "Darden, Patrick S." <darden@armc.org>
To: "Drew Weaver" <drew.weaver@thenap.com>, <nanog@merit.edu>
Errors-To: owner-nanog@merit.edu
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
