On 18-Jul-2007, at 06:30, Anton Smith wrote:
How many operators out there are using strict RPF?
With a lot of interconnection points around the place to me it seems a bit harsh to use strict RPF.
And in fact, I wonder if Telstra Clear are using it, since a lot of times I notice broken connectivity due to what appears to be asymmetric routing, and I then have to work around it.
I think two useful things to remember are: 1. All routing is asymmetric, in general. 2. "Strict-mode" RPF is inappropriate to apply to an interface if the thing at the other end has other connectivity to the Internet. Unless you are absolutely convinced that the device at the far end is not multi-homed, applying strict-mode RPF is an error (and presumably should be corrected just like any other error). So, if $carrier has applied strict-mode RPF checks to their interface facing you, and you are multi-homed, you need to call them to report their configuration error so it can be corrected. If they doubt the existence of the error, point them at RFC 3704/BCP 84. However, if the path from A in your network to B elsewhere is not the same as the path from B back to A, then that's normal, and the observed asymmetry (in isolation) is no reason for a change. (If one or both of the paths is congested, or has some other problem, then obviously those are still problems that might deserve escalation.) Joe