North American Network Operators Group

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Re: Moving filters from edge to core

  • From: Peter John Hill
  • Date: Mon Jul 28 11:44:08 2003

--On Monday, July 28, 2003 12:16 AM -0700 Mike Lyon <[email protected]> wrote:

I would tend to keep the filters on the edge, for obvious reasons. Your
management would probably agree with this the first time you get attacked
coming from each of your edge routers with nothing to protect it from

You could always make a script (PERL) to go out and make the modifications
to your edge routers for you.
Got to agree there, the core is not the place to have ACLs. You want the ACL as close to the host as possible, which pretty much means the edge router.

We have a great perl script that we use that uses expect to add and remove deny hosts from our cisco routers. It uses a show route to find the interface where it needs to filter. If it is not directly connected, it fails and informs the script user. It properly removes the ACL statement from the interface, removes, modifies and readds the acl and reapplies the acl to the interface.

I did not write the script, so I won't share it here. If you get a chance to go to LISA this year, you can hear the author of the script talk about even cooler ways to kill a hosts network connectivity.

Peter Hill
Network Engineer
Carnegie Mellon University

On Mon, 28 Jul 2003, Tay Chee Yong wrote:
Hi all,

This might be quite a stupid question. But my management is looking at
moving the filters from the edge to the core, so as to reduce adminstration
of apply filters on all our edge routers, and minimizing the possibility of
non-synchronized filters at the edge.

Does anyone has any advise on this? I believe all the there are many larger
ISP in this list that have a better way to manage your filters at the edge.

Would appreciate all inputs/comments.