Ethernet at the Physical Layer
Keep in mind that troubleshooting Ethernet may be a Data Link Layer issue, a Physical Layer issue, or both. IEEE 802.3 Ethernet specifies various media types including coax, twisted pair, and fiber. From a troubleshooting viewpoint, it is critical to be aware of the specifications for the Ethernet media you are deploying. Refer back to Table 5-4 to review the various Ethernet media types. For pinout information, refer to Chapter 1, cable vendor sites, and Cisco.com.
Although not as common anymore, you may have the need to configure the media type on your router as in Example 5-22.
Example 5-22. Media Type Command
r1(config)#interface ethernet 0 r1(config-if)#media-type ? 10BaseT Use RJ45 connector AUI Use AUI connector ...