[Xorp-users] Fwd: from and to blocks of policy terms

[Andreas Voellmy]

Hi all,

I originally posted my message (see below) to the xorp-hackers list, but I
think it might be more appropriate on this list, so I am sending it here
also.

-Andreas

---------- Forwarded message ----------
From: Andreas Voellmy <andreas.voellmy at gmail.com>
Date: Feb 6, 2008 2:25 PM
Subject: from and to blocks of policy terms
To: Xorp-hackers at icir.org


Hi,

I've read the XORP user manual and Bittau & Handley's paper on "Decoupling
Policy from Protocols" and I am still a bit confused as to how the policy
terms work.

I understand that policies that do route redistribution, like "from
{protocol:rip} to {neighbor: 192.168.1.2} then {accept}" make sense as an
export policy, but it's not clear to me why a policy term has both from and
to blocks when it is not doing route redistribution. For example, take the
following policy term:

from {} to {neighbor: 192.168.1.2} then {accept}

As an export policy I understand that it would advertise all routes to
neighbor 192.168.1.2. However, if it were an IMPORT policy, what would it
mean? More generally, what do any conditions in the to block mean in an
import policy?

Similarly, what does "from {neighbor: 192.168.1.2} to {} then {accept}" mean
as an export policy? What does any condition in the from block of an export
policy mean?

Thanks!

-Andreas
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