Once a node is added to the "labeled set" of the Dijkstra algorithm, it
is never revisited. Hence, you wont be able to discover additional next
hops for it.
Consider a scenario where the shortest path to router C is either
directly over a p2p link with a cost of 1 or through network N, also
with a cost of 1. If you move router C to the labeled set first, you
will never discover the path through N (the outgoing link from N to C
indeed has a cost of 0).
Roch
On 12/27/2010 10:09 AM, Yasuhiro Ohara wrote:
Hi list.
RFC 2328 16.1. "Calculating the shortest-path tree for an area"
(3) says that:
Note
that when there is a choice of vertices closest to the root,
network vertices must be chosen before router vertices in
order to necessarily find all equal-cost paths.
I couldn't think of any possible case that we fail to find
all equal-cost paths when the router vertices are chosen
first. Is there any ?
When I first felt that I understood this, I remember
that this seemed to be due to the fact that the cost is
implicitly ZERO from a network vertex to a router vertex.
But I can't think of any case now.
Even if the network vertices are chosen later, they are added
eventually, and all the equal-cost multipaths are correctly
calculated.
Would someone please explain ?
thanks in advance,
best regards,
yasu
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