Policy-based routing is evil? Discuss.

Mon Nov 25 07:43:54 UTC 2013

On Nov 24, 2013, at 10:36 PM, Eugeniu Patrascu <eugen at imacandi.net> wrote:

> On Fri, Oct 11, 2013 at 8:27 PM, William Waites <wwaites at tardis.ed.ac.uk>wrote:
> 
>> I'm having a discussion with a small network in a part of the world
>> where bandwidth is scarce and multiple DSL lines are often used for
>> upstream links. The topic is policy-based routing, which is being
>> described as "load balancing" where end-user traffic is assigned to a
>> line according to source address.
>> 
>> In my opinion the main problems with this are:
>> 
>>  - It's brittle, when a line fails, traffic doesn't re-route
>> 
> 
> You can always know what IPs are on the other end of the link, add static
> routes for them to make sure they're reachable and based on ping results
> use the link or not. It works fairly well if 1-2 minutes of downtime is not
> an issue. I've done this using Linux and a bash script and it worked to
> balance traffic across two links with up/down detection. iproute2 does
> wonders.
> 
Or you could run FreeBSD with PF and ifstated and it would be an almost instantaneous failover.

> 
>>  - None of the usual debugging tools work properly
>> 
> 
> As long as you don't have asymmetric routing in place, debugging will be
> the same. Even so, you can (at least on Linux) do a "tcpdump -i any" and
> see what goes in/out of your box :)
> 
> 
Asymmetric routing is a fact of life and is fairly common.

>>  - Adding a new user is complicated because it has to be done in (at
>>    least) two places
>> 
>> 
> I agree it's not scaleable, but for when all you have are DSL lines or low
> capacity lines over which you cannot run an IGP, you'll have make it work
> with what you have :)
> 
> 
>> But I'm having a distinct lack of success locating rants and diatribes
>> or even well-reasoned articles supporting this opinion.
>> 
>> 
> I would go for the "right tools for the right job" idea and say that PBR in
> the case you're mentioning of a valid use and probably the most effective
> way of doing business for them.
> 
> Also take into consideration that in many parts of the world, the effort of
> configuring and maintaining a setup like this fall in the the day to day
> job of one or several network admins. Also, most of the time is cheaper to
> hire more people than go and buy let's say professional networking
> equipment.

Hmm, really?  The professional networking equipment required for this type of thing would be in the ~10k new and significantly cheaper used.  That's not a lot of salary.

Mike