A Cisco 2501 includes a single 10Mb Ethernet port. While many Cisco router models now include an integrated 10/100 RJ-45 port, the 2500 series uses what is referred to as a generic attachment unit interface (AUI) DB-15 port instead. The name for this connector (DB-15) comes from the fact that it is physically shaped like the letter ‘D’ and uses a 15-pin connector.
The purpose of providing an AUI port instead of a fixed RJ-45 port is flexibility – AUI ports use an external transceiver, which allow different media types to be connected, according to your network needs. Different transceivers can be attached that allow twisted pair, coaxial, or fiber connections. In that way, a transceiver connects to the DB-15 port, and then provides a port to which an RJ-45 or BNC connection could be made, for example.
But what is a transceiver? Well, its name comes from what it does – transmitting and receiving data. In most NICs that you’ll come across today, the transceiver is built directly into the network card. With older network cards (like those found on Ethernet 10Base5 networks) the transceiver was usually an external device.
When connecting a Cisco 2501’s Ethernet connection to a common RJ-45-based network, a transceiver attaches to the AUI port, and then a patch cable connects the transceiver to a switch or hub. Remember, a router connecting to a switch or hub always uses a straight-through cable, as shown in the figure below.