Re: Multiple connections and metrics?

[Yun Mao <yunmao gmail com>]

On Wed, 30 Mar 2005 10:04:49 -0800, Joe Barnett <thejoe gmail com> wrote:
> A co-worker was showing me the neat way that windows XP deals with
> mutiple connections.  Instead of bringing connections up and down as
> cables get plugged in/waps get too far away/etc, all interfaces are
> brought up whenever they can be connected.  So, for example, a laptop
> with wireless, ethernet, and a bluetooth/gprs connection will have 3
> interfaces up.  The routing table is set up such that all 3 routes
> exist, but have different metrics.  So (hyptothetically) the ethernet
> will have a metric of 0, the wlan 20, and the bluetooth 60.  When a
> packet is routed, it chooses the route with the lowest metric.  This
> means that instead of losing connection, and then having to wait for a
> new connection, the network connection just gets re-routed to the next
> lowest metric connection without a break in connectivity (or at least
> with a noticably shorter break).
> 
> 2 questions:
> 
> 1) can routes  in linux be set up this way? (I've only seen metrics of
> 0 when i run the route command, man route says the metric is not used
> by recent kernels...)
> 
I think windows keeps the interfaces up only when they have
connections. The main difference is windows allows multiple interfaces
to be up at the same time while NM prefers only one is up.

Now the question is when mutiple interfaces are up, whether linux is
capable to setup several default routes with different priorities.
Using metric is one way and I actually believe metric is used,
otherwise the ifmetric [1] project might have been much less useful.
Even if metric were of no use, linux still allow you to setup many
default routes. The key is someone (NM?) should take care of the order
(when new NICs are up) because only the first one will be chosen.

Yun

[1] http://0pointer.de/lennart/projects/ifmetric/