[Sussex] Persistent routes

[Jan Henkins]

Hello Steve,

On Tue, May 15, 2012 14:13, Steve Dobson wrote:
> Hi Simon
>
>
> On 15/05/12 12:33, Simon Story wrote:
>
>> On 15/05/12 11:43, Paul Howard wrote:
>> You can't specify 'gateway' twice in an debian/ubuntu interfaces file,
>> that is shorthand for setting the default route. You can only have one
>> default route.
>
> Why can you have only one default route?
>
>
> As far as I am aware the default route is nothing special (in routing
> terms) other than it matches all addresses so it needs to be the last route
> tried by the routing software.  Therefore, a site with two ISP links would
> have two default routes and a packet could be routed via either to get off
> the site.  Useful if one of the links should go down or (more likely)
> become very busy.
>
> True, most sites (esp homes) only have one ISP link.  In this case then
> there can be only one default route.  But while this may be the most common
> configuration I don't think it is a requirement.

Having a multi-homed network without policy routing is non-optimal. I
agree with you that the actual setup promises a redundant way out of your
network, but thinking that two default gateways will automatically solve
your resultant asymmetric routing hassles is unfortunately a fallacy. Here
is a nice (old) Linux Journal article that seems to handle the subject
quite nicely:

http://www.linuxjournal.com/article/7291?page=0,1

> The problem is not the gateway line, I believe it to be the fact that
> the host and the gateway do not share a common network address and thus the
> routing tables can't figure out how to route packets to the other network.

Please see my previous email with the output from ipcalc.


-- 
Regards,
Jan Henkins