and Token Ring is performed more easily at layer 3 or via routing. Devices that interconnect at the layer 3 are traditionally called routers. Where there is a need for a great deal of analysis of network performance and security, switches may be connected between WAN routers as places for analytic modules. Some vendors provide firewall, network intrusion detection, and performance analysis modules that can plug into switch ports. Some of these functions may be on combined modules. Through port mirroring, a switch can create a mirror image of data that can go to an external device such as intrusion detection systems and packet sniffers. A modern switch may implement power over Ethernet (PoE), which avoids the need for attached devices, such as a VoIP phone or wireless access point, to have a separate power supply. Since switches can have redundant power circuits connected to uninterruptible power supplies, the connected device can continue operating even when regular office power fails. Bridging Modern commercial switches primarily use Ethernet interfaces. The core function of an Ethernet switch is to provide multiple ports of layer-2 bridging. Layer-1 functionality is required in all switches in support of the higher layers. Many switches also perform operations at other layers. A device capable of more than bridging is known as a multilayer switch. A layer 2 network device is a multiport device that uses hardware addresses (MAC addresses) to process and forward data at the data link layer (layer 2). A switch operating as a network bridge may interconnect otherwise separate layer 2 networks. The bridge learns the MAC address of each connected device. Bridges also buffer an incoming packet and adapt the transmission speed to that of the outgoing port. While there are specialized applications, such as storage area networks, where the input and output interfaces are the same bandwidth, this is not always the case in general LAN applications. In LANs, a switch used for end-user access typically concentrates lower bandwidth and uplinks into a higher bandwidth. Interconnects between switches may be regulated using the spanning tree protocol (STP) that disables forwarding on links so that the resulting local area network is a tree without switching loops. In contrast to routers, spanning tree bridges must have topologies with only one active path between two points. Shortest path bridging and TRILL (TRansparent Interconnection of Lots of Links) are layer 2 alternatives to STP which allow