CCNA RnS Article #22: Interface Auto-negotiation in Action!


CCNA RnS Article #22: Auto-negotiation

We talked about auto-negotiation briefly in our last article. In this article, we are going to cover what is auto-negotiation and how it works in different scenarios.


A switch has multiple interfaces, and those interfaces can work on different speeds or duplex settings. For example, a 10/100/100 interface type can work on 10Mbps, 100Mbps, or 1Gigabit speed. Cisco switches have the default setting for speed and duplex which is auto. It means deciding the speed and duplex settings automatically when a host is connected to the interface.

In a campus network environment, it is acceptable to have an auto setting. However, in a Data Center environment, duplex and speed settings are manually set to avoid sub-optimal performance over Ethernet links.

Auto-negotiation in Action

Ethernet devices connected to the switch port should use the same speed or duplex configuration otherwise, they cannot perform data transfer correctly. For example, if a device connected to the switch interface is configured to speed 100Mbps but on the switch end if it is configured to use 1000Mbps of speed, the device will not be able to work. That’s why Auto-negotiation was designed to solve this speed or duplex mismatch problem.

Auto-negotiation is a signaling protocol that runs between the device and the switch to negotiate on common speed and duplex settings. Each side of the cable informs what it can do and then both devices agree to use the best setting – highest speed and full duplex.

In Figure 1, Host 1 can support a maximum of 100Mbps speed. The switch can support a maximum of 1000Mbps speed. In this case, the common highest speed is 100Mbps and the full duplex is always better than the half duplex. Therefore, a switch will configure the interface speed to 100Mbps and Duplex to full when Auto-negotiation is enabled on the port.

Figure 1: Auto-negotiation in Action

 Auto-negotiation works well in most of the scenarios in the campus access scenario. However, there are scenarios when the network admins need to configure the speed and duplex settings manually when the connected hosts don’t support Auto-negotiation like IoT devices – cameras, HVAC systems, etc.

Manual changes sometimes can be error-prone. Let’s see what happens when there is one side is configured to use a manual setting and the other end is configured to use Auto-negotiation. In this case, chances are the link will not work or there is poor performance over the link.

IEEE defined some rules for such cases when Auto-negotiation fails. The speed and duplex settings will be as follow –

  • Speed – use the lowest speed supported
  • Duplex – if the speed is 10 or 100Mbps use half-duplex and for more than 100Mbps, use full-duplex.

Cisco logic in this case slightly different – in the case of speed, it tries to sense the speed and configured the same on the port else, uses the IEEE logic if speed cannot be sensed.

In the case of LAN Hubs – it is always IEEE rules that are followed which is the slowest speed and the full duplex for greater than 100Mbps speed else half-duplex.

Hope you find this informative!

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