PW Side Receiving a Loop Detection Packet

Figure 1 PW side receiving a loop detection packet

In Figure 1, PE1's AC interface is an Ethernet interface. After the AC interface goes physically Up, it proactively sends a loop detection packet. If this packet loops back to PE1 through Switch, PE2, and the PW between PE1 and PE2, a loop occurs on the network. PE1 then sets the link layer protocol of the AC interface to Down and reports an alarm to the NMS. This mechanism prevents the AC interface from sending or receiving any packets.