Simple Loop Prevention Protocol (SLPP) in computer networking is a layer 2 protocol invented by Nortel (now acquired by Avaya) to protect against network loops. SLPP uses a small hello packet to detect network loops. The SLPP protocol checks packets from the originating switch and the peer switch in a SMLT configuration. Sending hello packets on a per VLAN basis allows SLPP to detect VLAN based network loops for un-tagged as well as tagged IEEE 802.1Q VLAN link configurations. Once a loop is detected, the port is shutdown.