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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:
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
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 email@example.com
Certified NetAnalyst II 701-231-8527
North Dakota State University
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