[Sussex] Persistent routes

[Jan Henkins]

Hello Simon,

On 15/05/12 16:13, Simon Huggins wrote:
> In practice you can only have one default route.


Correct.


> If you want to route via two lines or to have a fallback you need
> something else which detects that the link has failed and removes the
> failing route which switches everything to the other route.


It is possible to do so without a way to "detect" how to do this by 
employing techniques called "weighted" and "policy" routing. Because 
they are static in nature, there will of course be a limit what one can 
do with them. Still, it's pretty amazing what can be done.


>
> More normally as an ISP or if you were doing routing properly you'd do
> BGP (external or internal) or OSPF or similar. i.e. use proper routing
> software; you can do this on linux with quagga for instance.

Correct. BGP is an acronym that stands for "Border Gateway Protocol", 
and it's one of many such. It's also an open standard, unlike some of 
the proprietary ones employed by vendors like Cisco. It has a 
disadvantage - you need to be the registered owner of a netblock of 
addresses of some size (I'm not quite sure what the minimum size is 
these days, last time I had a run-in with the technology it was a /20, 
which was years ago). Then you had to have an AS number (Autonomous 
System number) registered against this netblock, which is the number 
that gets advertised far and wide across the Intertubes.It is remarkably 
effective, but really only useful to ISP's. OSPF (Open Shortest Path 
First) is something that you would use if you have dynamic multiple LANS 
and WANS within your organisation that needs to interoperate at all 
times. Not for the feint-hearted... :-)

-- 

Regards,
Jan Henkins