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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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