At 05:56 AM 6/3/2005, mike shand wrote:
At 16:43 31/05/2005 -0400, Alia Atlas wrote:
Routers SHOULD use the symmetric-link safety condition by default,
MAY attempt to dynamically determine the method that needs to be
applied based on the topological information from the routing
SB> I think that we need to discuss which algorithm should
SB> be the default. Given that many networks that are thought
SB> to be symmetric turn out to be asymmetric, it's not clear
SB> which we should choose and why.
AA> How many of the symmetric networks that actually turn out to be
asymmetric have multi-hop loops in
AA> them? Couldn't this be something that was flagged by a MIB - to
indicate that the "symmetric"
AA> network isn't really. Surely this is something that the network
operators would want to know so that it
AA> can be corrected??
Yes, I wonder how much of a problem this really is. Given that the
algorithm doesn't prevent all loops anyway, then a small increment in the
number of loops caused by incorrectly handled asymmetric cost cases
doesn't seem to be much a price to pay, especially since using the
stronger condition to handle them correctly will result in the overall
coverage being less. i.e. the total number of loops may get WORSE by using
the asymmetric cost fixing algorithm.
I know.... something for me to simulate :-)
AA> Exactly ;-) Please let us know what you find. I suspect that the
total number of loops would be
AA> increased. As I recall, going to the asymmetric safety condition meant
that about only half of the micro-
AA> loops otherwise resolved by PLSN were handled.
AA> A point of interest in the simulation might be not merely the number of
loops but the number of affected
AA> Another related question is how does PLSN work with max-cost
links? How should it work? Is it
AA> acceptable to use a max-cost link to reach a safe neighbor that isn't
a potential primary neighbor on
AA> either the old or new topology? That seems potentially bad to me,
since it could cause additional
AA> traffic loss, depending on why the link was set to max-cost.
I think a max-cost link should be treated as unreachable, since that is
probably why it was set to max cost.
AA> The condition I have for LFA is that if the cost of the link or the
cost of the associated reverse link are
AA> max-cost or (for ISIS) the neighbor is over-loaded, then don't consider
the link as a candidate alternate
AA> next-hop. For PLSN, I'd modify it to the following. "If no links to a
neighbor could be considered as a
AA> candidate alternate next-hop and the neighbor is not either a new
primary neighbor or a old primary
AA> neighbor, then the neighbor cannot be a safe neighbor. "
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