Dijkstra's algorithm creates a shortest path tree from a graph in unicast routing.

Introduction:
Unicast routing is a type of network communication where data is transmitted from a single source to a single destination. In unicast routing, the goal is to find the shortest path between the source and destination nodes in a network. Dijkstra's algorithm is a commonly used algorithm for solving this problem.

Dijkstra's Algorithm:
Dijkstra's algorithm is a graph search algorithm that is used to find the shortest path between a source node and all other nodes in a graph. It works by iteratively selecting the node with the minimum cost from a set of unvisited nodes and updating the costs of its neighboring nodes. This process continues until all nodes have been visited.

Create a Shortest Path Tree:
When Dijkstra's algorithm is applied to a graph in unicast routing, it creates a shortest path tree. A shortest path tree is a subgraph of the original graph that contains only the nodes and edges necessary to reach all other nodes from the source node using the shortest paths.

Steps to Create a Shortest Path Tree:
1. Initialize the algorithm by setting the cost of the source node to 0 and the costs of all other nodes to infinity.
2. Select the node with the minimum cost as the current node.
3. Update the costs of its neighboring nodes by adding the cost of the current node and the weight of the edge connecting them. If the updated cost is smaller than the current cost, update the cost.
4. Mark the current node as visited.
5. Repeat steps 2-4 until all nodes have been visited.
6. The resulting shortest path tree will contain the shortest paths from the source node to all other nodes in the graph.

Graph vs Tree:
- A graph is a collection of nodes (vertices) and edges, where the edges represent the connections between the nodes.
- A tree is a special type of graph that does not contain any cycles. It consists of nodes and edges, where each node has a unique parent except for the root node, which has no parent.

Conclusion:
In unicast routing, Dijkstra's algorithm is used to find the shortest path between a source and destination node. It creates a shortest path tree, which is a subgraph of the original graph that contains the shortest paths from the source node to all other nodes. This tree helps in determining the optimal path for routing the data in a network.