Light-trail could be a promising technology for minimizing active switching and maximizing wavelength utilization. This paper will discuss a new reliable and practical light-trail architecture called a closed light-trail in which a light-trail originates and terminates at the same node. The closed light-trails create logical rings on physical ring or mesh topology networks, e.g., ROADM-enable optical metro networks. Since designing a medium access protocol for high speed closed light-trail is a challenging problem, a polling-based MAC protocol is proposed to guarantee the fairness among nodes while maintaining the wavelength efficiency even for highly dynamic traffic patterns. The proposed MAC protocol leaves the nodes to determine autonomously their transmission windows not only to reduce overhead but also to avoid the request and grant mismatch problem, whereas the fair bandwidth share is assured by pre-assigned maximum transmission window for each node. The trouble encountered by the pre-assigned fixed size window is that variable-length frames may not be able to completely occupy the window. The proposed MAC employed the technique called the window fitness algorithm to efficiently handle variable-length frames such as Ethernet frames. Its performance in terms of overhead, packet delay and packet delay variation is evaluated to prove its acceptability to the metro networks according to the ITU recommendation Y.1541.