AC Interface Receiving a Loop Detection Packet

Figure 1 AC interface receiving a loop detection packet

In Figure 1, PE1's AC1 is an Ethernet interface. After AC1 goes physically Up, it proactively sends a loop detection packet. If AC2 receives this packet, a loop occurs on the network. PE1 then sets the link layer protocol of AC1 to Down and reports an alarm to the NMS. This mechanism prevents AC1 from sending or receiving any packets.