Otherwise it silently discards the lsa because that

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Unformatted text preview: that starts at 0 when the node turns on, and increments by 1 each time the node sends an LSA. This information is used by the flooding process, as follows. When a node receives an LSA that originated at another node, s, it first checks the sequence number of the last LSA from s. It uses the “origin addr” field of the LSA to determine who originated the LSA. If the current sequence number is greater than the saved value for that originator, then the node re-broadcasts the LSA on all its links, and updates the saved value. Otherwise, it silently discards the LSA, because that same SECTION 18.5. A SIMPLE LINK-STATE ROUTING PROTOCOL 11 or later LSA must have been re-broadcast before by the node. There are various ways to improve the performance of this flooding procedure, but we will stick to this simple (and correct) process. For now, let us assume that a node sends out an LSA every time it discovers a new neighbor or a new link gets added to the network. The next lecture will refine this step to send advertisements periodically, in order to handle failures and packet losses, as well as changes to the link costs. ￿ 18.5.2 Integration step: Dijkstra’s shortest path algorithm The competent programmer is fully aware of the limited size of his own skull. He therefore approaches his task with full humility, and avoids clever tricks like the plague. —Edsger W. Dijkstra, in The Humble Programmer, CACM 1972 You probably know that arrogance, in computer science, is measured in nanodijkstras. —Alan Kay, 1997 The final step in the link-state routing protocol is to compute the minimum-cost paths from each node to every destination in the network. Each node independently performs this computation on its version of the network topology (map). As such, this step is quite straightforward because it is a centralized algorithm that doesn’t require any inter-node coordination (the coordination occurred during the flooding of the advertisements). Over the past few decades, a number of algorithms for computing various properties over graph...
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This document was uploaded on 02/26/2014 for the course CS 6.02 at MIT.

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