Configuring the Link Characteristics
By default, the router’s management Ethernet interface, fxp0 or em0, autonegotiates whether to operate in full-duplex or half-duplex mode. Fast Ethernet and J Series router Gigabit Ethernet interfaces can operate in either full-duplex or half-duplex mode, and all other interfaces can operate only in full-duplex mode. For Gigabit Ethernet, the link partner must also be set to full duplex.
 | Note: When you configure the Tri-Rate Ethernet copper interface to operate at 1 Gbps, autonegotiation must be enabled. |
| Note: When you manually configure Fast Ethernet interfaces on the M Series and T Series routers, link mode and speed must both be configured. If both these values are not configured, the router uses autonegotiation for the link and ignores the user-configured settings. |
| Note: When the Fast Ethernet interface on Juniper Networks routers with autonegotiation enabled interoperates with a device configured to operate in half-duplex mode (autonegotiation disabled), the interface defaults to half-duplex mode after the PIC is taken offline and brought back online. This results in packet loss and cyclic redundancy check (CRC) errors. |
To explicitly configure an Ethernet interface to operate in either full-duplex or half-duplex mode, include the link-mode statement at the [edit interfaces interface-name] hierarchy level:
