[AusNOG] Write up - Big ISP, little ISP, local internet exchanges

Matthew Moyle-Croft mmc at internode.com.au
Mon Sep 15 08:10:02 EST 2008

Edwin Groothuis wrote:
> The solution is a pure technical one: adjust the network to 
> reflectwhat you are offering. Or say you are offering. Or what you 
> said you were going to be offering.
> Edwin`16
Ultimately all we're looking for is a way of communicating a single bit 
of information as meta data along with each packet.   All we need to 
know is "did this packet enter our network on a transit or peering 
interface?".    If a packet enters via a peering point then it shouldn't 
leave via peering or transit.  If a packet enters via transit it 
shouldn't leave via a transit or peering port.

So, you could, assuming your network is MPLS enabled use the MPLS TE 
bits (assuming you're not using them for QoS - even if you are I reckon 
you probably aren't using all 3 bits).   If a packet enters via peering 
or transit, set a bit.  On exit at a peering or transit bit, use a 
policy map or your partitcular router vendor's mechanism to enforce 
this.  You could also mangle the packets and use the DSCP/ToS bits if 
you don't mind doing that.

The great thing about this solution is it doesn't require updating any 
filters or creating a whole lot of tunnels or wierd things.  It should 
be a set and forget on just the external interfaces.

Maybe it's possible to do something more clever with MPLS - maybe 
prevent various label paths existing would also prevent the packets even 
having to be carried.

A whole lot of our networking issues would probably be a lot easier if 
we could carry some meta data with packets and have it interact with the 
forwarding policy.  I guess TE bits are an example, but a more generic 
version would be nice.


More information about the AusNOG mailing list