This article is intended to provide supporting documentation related to how auto-negotiation operates in terms of link-mode (duplex mode) and speed on Juniper Networks EX4300 switches.

The customer is reporting that the port of the EX4300 switch grabs “half-duplex” link mode when auto-negotiation is enabled by default.

The documentation below explains that the default behavior includes auto-negotiation enabled and the Juniper EX4300 switch will listed to the proposed link-mode and speed by the far-end device, which means that whenever auto-negotiation is enabled, if the port acquires "half-duplex" link mode and certain speed such as 100mbps, this is a consequence of the far-end side proposing these parameters.

Configuring the Link Settings

EX Series switches include a factory default configuration that enables interfaces with the link settings provided in Table 8.

Table 8 : Factory Default Configuration Link Settings for EX Series Switches. Below is an extract of this table focused on 1G ports.

NOTE: 

On EX4300 switches, there is no link-mode configuration statement. The link-mode setting on an EX4300 switch is handled as follows:

• If the link partner is operating in half duplex, the EX4300interface goes to half duplex.

• If the link partner is not capable of autonegotiation, then the link is established as either half-duplex or full-duplex, based on the physical layer of the link partner and EX4300 switches. Only if the speed is either 10-Gbps or 100-Gbps and the duplexity is Half Duplex on both sides, link will be established successfully.

• If the link partner is capable of autonegotiation and is operating in full duplex, the EX4300 interface also works in full duplex.

• To force an EX4300 interface to stay in full-duplex mode, configure the interface’s speed as 10 Mbps or 100 Mbps and also configure the interface with the no-autonegotiation statement.