## 1 Introduction  
Traditional routing protocols in computer networks always select a single optimal path between the source and destination nodes. This path is calculated based on the weights of the links. When a given path in the network becomes overloaded, the weight of the overloaded link (or several links) should be increased so that the path from the given source to the given destination avoids the overload after recalculation. None of the existing routing protocols can dynamically change the weights.  
In a traditional network, in order to force the path recalculation link weights have to be changed manually. After recalculation of the paths, all packets will be redirected to the newly calculated paths. This will result in a change in the flow paths, which can lead to new overloads in other parts of the network. Modification of even one of the weights in the network can cause many problems: (a) recalculation and updating of routing tables by all routers takes relatively a long time, (b) it is very likely that temporary routing loops will be introduced, causing packet losses, overloading of links that have been slightly loaded so far, packets arriving in the wrong order, etc. [1]. It is clear that the more such changes occur, the more chaos is introduced into the network.  
On the other hand, in almost any network, there is at least one other (competitive) path connecting the same source and destination nodes. Using it gives the possibility of bypassing the overload and realizing the transmission understood by us as multipath transmission. Packets belonging to previously existing flows move along the existing paths, while new flows move along the newly designated paths. Many multipath transmission methods have been proposed for different layers. Their overview can be found in [2,3]. Currently, the most commonly used technique that allows breaking the default routing and traffic engineering is MPLS. However, the paths established using this technique do not allow one to follow the high dynamics of traffic changes in networks (especially in data centres).  
The introduction of the concept of software-defined networks (SDN) has enabled simultaneous multipath transmission between the same end nodes for different flows [4]. Since this solution was originally designed for campus networks, the introduction of SDN to data centres and operator networks highlighted the problems with its scalability: the limited number of entries in fast TCAM memories (Ternary Content Addressable Memory) [5], the overhead associated with communication between the controller and the nodes [6], or delays associated with the installation of new flows [7].  
This article is an extension of the publication [8], in which a new flow aggregation mechanism using multipath transmission was proposed. The significant changes introduced to the previous version are:  
1. The proposal of new methods for calculating link weights;
2. Experimental comparison of the implemented methods;
3. The introduction of indicators that determine the level of reduction in the number of entries in switches (both measured in real time and comparative--post processing);
4. Tests and validation using UDP traffic;
5. Comparison of the results with a 'pure' SDN network operating in reactive mode.  
This paper is organized as follows. Section 2 describes the basis of the general mechanism which was defined in [8]. Section 3 introduces the proposed modifications to the mechanism described in the previous section. Section 4 details the testbed setup, the evaluation methodology, and key performance indicators used to assess the mechanism. This section also includes an analysis of the achieved results. Related works are discussed in Section 5. Finally, Section 6 provides a summary of the findings.