[Xorp-hackers] Xorp newbie: dynamic BGP policies

[Marko Zec]

On Friday 27 April 2007 06:44, Andreas Voellmy wrote:
> Hi all,
>
> I'm just getting started with Xorp and have a question about BGP
> policy (or maybe xorp routing policy in general):
>
> I've noticed in a couple papers (such as "Decoupling Policy from
> Protocols") dynamic BGP configurations for Xorp, such as something
> like
> "metric=metric*2" or "MED += 1". However I haven't seen this in the
> official manual - there I've only come across static values in
> configurations. Are these sorts of dynamic settings (such as
> incrementing, multiplying, etc.) valid policies for the BGP
> implementation that Xorp provides? Or is this something I'd need to
> extend the basic BGP code to do? If so, could someone point me in the
> right direction in the code; i.e. how does one add a new stack
> operation to a protocol?

Hi,

I think at the moment our configuration / policy parser cannot support 
constructs such as "metric = metric * 2", but simulating such a 
functionality using a slightly different syntax should be relatively 
simple to achieve.  For example, adding support for an equivalent of a 
"MED += N" statement that would take syntax form of "med-incr N" can be 
achieved by solely extending the bgp configuration template (see the 
attached patch).  Note that "add" and "sub" operators are already 
implemented in the policy parser and stack machinery, so adding support 
for "med-decr N" operator should also be trivial.  For adding new 
operators to the parser / stack machine you would have to tackle both 
the router manager and policy parsers, and that probably wouldn't be 
that most straightforward thing to do...

Hope this helps (for the start),

Marko



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