A method and a system using the method enabling to decrease required resource for the bandwidth reservation in an inter-site connection network used for communication between communication sites. The aggregation of bandwidth is considered for allocating bandwidth in the inter-site connection network. The network consists of a plurality of nodes each having a packet switch, and a plurality of user sites each constituted by a host or an internal network respectively connected to the plurality of nodes via access lines. With respect to the plurality of user sites, when one user site is to be connected to the other site through the network, a necessary and sufficient bandwidth for interconnecting between the user site and the other is calculated according to the bandwidth of the access line connecting the user site to the network considered as a minimal bandwidth. Based on the calculated result, the bandwidth is allocated to the plurality of nodes.