I have 4 different WAN links for redundancy one of them is 15mbps MPLS and the other ones are 30mbps Internet , 50 mbps Internet and 10 mbps MPLS , the way that it is set up is that we have load balancing across the 4 links with TINA (Traffic Intelligence over IPSEC) , now the primary link is the MPLS link because of lower latency that it offers , and secondary is the 50 mbps and both load balance the traffice sent out , now the problem and question is the following there are occassions where the link of of the 15mbps gets saturated the interfaces that connects the mpls router and my equipment are both 1Gig interfaces but since the bandwidth provided by ISP is 15mbps, and I'm trying to understand if there is a way that the interface of the ISP router can let my interface know that its limit is 15mbps ?
There is no interface rate that runs at precisely 15 Mbit/s. The most common handover interface is Ethernet which exists for 10, 100, 1000 Mbit/s or faster.
Your plan's data rate is implemented as the forwarding rate of the router, possibly by the ISP uplink's data rate (between CPE and COE) or a software limitation. That is of no consequence to you, however.
The router simply doesn't transport packets that exceed these limits but queues them or drops them when the queue is filled up.