Hello Konstantinos
The term downheap needs a bit of explaining. The term comes from heap data structure theory, a theory that involves how binary trees are manipulated. Because OSPF uses the Shortest Path First (SPF) methodology using Dijkstra’s algorithm, it uses heap data structure theory.
When the topology changes, the nodes that comprise the tree structure are modified. This modification must be reflected in the tree construct that OSPF has created. The terms “upheap” and “downheap” are used to refer to how nodes are kept in order when changes occur to the tree.
Upheap refers to the process by which the order of the nodes in the tree (heap order) is restored after a new node has been added.
Downheap refers to the process by which the order of the nodes in the tree (heap order) is restored after a node is removed.
For more info on heaps and their operations, take a look at this quick summary on heaps as constructs.
https://cs.brown.edu/courses/cs016/2010/Lecture/spikeLec/lecture_12.pdf
To be honest, I’m surprised that such a specific term like “downheap” has made its way into the output of a syslog entry. Usually, such algorithm-specific terminology doesn’t make its way into networking jargon and is typically kept as a black box, but not in this case.
To answer your question then, in your topology, since a better path was found, a node was removed (the old path) and thus a downheap operation had to take place.
I hope this has been helpful!
Laz