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Re: [RRG] some musings on PI v. PA, and assumptions, requirements, and tradeoffs

On Jul 13, 2007, at 4:14 AM, marcelo bagnulo braun wrote:


Hi Dave,


El 13/07/2007, a las 0:40, David Meyer escribió:



        I think we all realize that there is no free
	lunch, and that this is a property (such as it is) of the
	fact that aggregation throws away information in the
	interest of computability (a standard technique).
exactly and if we want to restore such functionlity without burdening the routing system we will need additional mechanisms that will have a cost. 
a similar considration can be applied to the TE discussion and  
using PA addresses. I mean, when we only use PA addresses and the  
only one that knows that 2 PA prefixes are assigned to a single  
site is the site itself, then the rest of the network cannot  
perform TE tricks. This is a similar trade off: the rest of the  
network is no longer overloaded with additional routing  
information, but as they no longer have the information about all  
the paths leading to a multihomed site, they can not longer play  
with different paths. If we want to restore such functionality, we  
will need to pay to publish that information, either by bigger  
routing tables, either having a parallel database that contains the  
multiple PA prefixes assigned to each multihomed site

Regards, marcelo

  Another possibility to publish the the information is to put the  
multiple destination addresses in each packet.  Of course the cost is  
sending the extra bits. It seems to me that for devices in the core  
of the network the cost of publishing the info this way is mostly  
independent of the total number of routes or paths in the network but  
would instead be dependent on the number of destination addresses  
that would need to be supported per packet.

  As you have pointed out before an architecture based on multiple  
destination addresses has it's own set of problems but it would allow  
for a future where TE could be done in the core while as you mention  
above the ability to do TE beyond the edge with other architectures  
can be quite expensive.



---
Bruce Curtis                         bruce.curtis@ndsu.edu
Certified NetAnalyst II                701-231-8527
North Dakota State University


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