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
r1(config-if)#end
					

Today, media type is primarily autosensed. In the past, you used to have to decide whether you wanted to use the AUI or RJ-45 connector and hard code the media type appropriately. Decisions like these are also required with 100-Mbps standards. For example, you may need to choose between 100BASE-TX for RJ-45 or medium-independent interface (MII). The MII is a 40-pin, high-density D-connector that carries various signals to support 10/100-Mbps transceivers that use different encoding schemes. The Gigabit MII (GMII) leverages off of the design of the MII to allow a gigabit controller to connect 1000BASE-X and 1000BASE-T transceivers. The MII enables you to substitute the Layer 1 of your choice.