Help us improve your experience.

Let us know what you think.

Do you have time for a two-minute survey?




Hierarchy Level


For Gigabit Ethernet interfaces on EX Series devices enable auto-negotiation. Auto-negotiation is supported in some ACX devices. In ACX5448, ACX5448-D, ACX5448-M and ACX710, auto-negotiation is supported on 1G interface. Beginning with Junos 21.1R1 and beyond, no-auto-negotiation configuration through CLI is supported on ports 24 onwards on ACX5448 variants and port 16 onwards on ACX710 due to hardware limitation for SerDes lanes.

  • auto-negotiation—Enables autonegotiation. This is the default.

  • no-auto-negotiation—Disable autonegotiation. When autonegotiation is disabled, you must explicitly configure the link mode and speed.

When you configure Tri-Rate Ethernet copper interfaces to operate at 1 Gbps, autonegotiation must be enabled.


On EX Series switches, an interface configuration that disables autonegotiation and manually sets the link speed to 1 Gbps is accepted when you commit the configuration; however, if the interface you are configuring is a Tri-Rate Ethernet copper interface, the configuration is ignored as invalid and autonegotiation is enabled by default.

To correct the invalid configuration and disable autonegotiation:

  1. Delete the no-auto-negotiation statement and commit the configuration.

  2. Set the link speed to 10 or 100 Mbps, set no-auto-negotiation, and commit the configuration.

On EX Series switches, if the link speed and duplex mode are also configured, the interfaces use the values configured as the desired values in the negotiation. If autonegotiation is disabled, the link speed and link mode must be configured.

Note: 1G optical transceivers do not support auto-negotiation on EX4100 1G/10G uplink module (PIC 1/PIC 2) and EX4400 1G/10G interface. 1G-T supports auto-negotiation on these ports.

On T4000 routers, the auto-negotiation command is ignored for interfaces other than Gigabit Ethernet.


On ACX Series routers, when you configure fiber interfaces (fiber media mode) to operate at 1 Gbps, autonegotiation is enabled by default to negotiate the speed and duplex settings. You can disable autonegotiation on ACX Series routers, except ACX5448 routers, in the fiber media mode, by using the (no-auto-negotiation) statement, and commit the configuration. In copper interfaces (copper media mode), autonegotiation is enabled by default. To disable autonegotiation, you need to explicitly configure the link speed to 10 or 100 Mbps, set no-auto-negotiation, and commit the configuration.


Auto-negotiation is not supported on ACX710 and ACX5448 at 10G speed.

1G mode is not supported on pic level port profile and port level port profile.

1G is supported in no-autonegotiation mode. This indicates link partner must disable autonegotiation.

Timing is not supported.


The auto-negotiation parameter link partner speed is displayed based on local speed even when auto-negotiation is disabled on ACX710 and ACX5448.


For Gigabit Ethernet interfaces on NFX350 devices, auto-negotiation is enabled by default. The no-auto-negotiation command is ignored for Gigabit Ethernet interfaces on NFX350 devices.

For SRX Series Firewalls, when autonegotiatiation is disabled, you can set the mdi-mode to enable it in case of non-cross table.

From Junos OS Evolved Release 21.2R1, we support autonegotiation and link training using 400G DAC cable for PTX10001-36MR. This is not a default behavior. Use auto-negotiation under [edit interfaces interface-name ether-options] hierarchy level to enable autonegotiation and link training. Enabling autonegotiation automatically enables link training. This advertises configured speed on the interface hierarchy. It does not upgrade or downgrade speed based on peers advertised capability.

Autonegotiation and link training is supported only on one 400-Gigabit Ethernet interfaces (1x400G), two 200-Gigabit Ethernet interfaces (2x200G), and four 100-Gigabit Ethernet interfaces (4x100G) channelization.

Starting in Junos OS Release 22.1R1, we support auto-negotiation for 1G interface on ACX5448, ACX5448-D, ACX5448-M and ACX710. Use the existing auto-negotiation | no-auto-negotiation statement at the [edit interfaces interface-name gigether-options] hierarchy level to enable and disable auto-negotiation for 1G interfaces.

No auto-negotiation configuration through CLI will be supported from port 24 onwards on ACX5448 variants and port 16 onwards on ACX710 due to hardware limitation for serdes lanes. No-auto-negotiation configuration on unsupported ports will be ignored and alerted to the user with a PFE log message.

Table 1 shows the behaviour of auto-negotiation in different combinations.

Table 1: Behavior of auto-negotiation

Device (ACX5448/ACX710)

Peer device

Link status

auto-negotiation ON

auto-negotiation ON


auto-negotiation ON

auto-negotiation OFF

Results in anomalous behaviour due to duplex mismatch

auto-negotiation OFF

auto-negotiation ON

Results in anomalous behaviour due to duplex mismatch

auto-negotiation OFF

auto-negotiation OFF

UP (If the speed and duplex mode matches on both the ends)

DOWN (If the speed or duplex mode does not match on both the ends)


Autonegotiation is automatically enabled. No explicit action is taken after the autonegotiation is complete or if the negotiation fails.


remote-fault (local-interface-online | local-interface-offline)—(Optional) For M Series, MX Series, T Series, TX Matrix routers, and ACX Series routers only, manually configure remote fault on an interface.

  • Default: local-interface-online

Required Privilege Level

interface—To view this statement in the configuration.

interface-control—To add this statement to the configuration.

Release Information

Statement introduced in Junos OS Release 7.6.