Multiprotocol Label Switching
The metro Ethernet rings that did not route IP traffic, instead used one of the various proprietary spanning tree protocol implementations, so that each MAN ring had a root bridge. Because layer 2 switching can not operate if there is a loop in the network, the protocols to support L2 MAN rings all need to block redundant links and thus block part of the ring. Capsuling protocols, such as Multiprotocol Label Switching (MPLS), were also deployed to address the drawbacks of operating L2 metro Ethernet rings.