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.
Overview
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.
Fig 1- Auto Negotiation in action
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.
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!
- CCNA RnS Article #21: Switch Interface Configuration
- CCNA RnS Article #20: Configure IP to Manage Switch
- CCNA RnS Article #19: Configuring Secure Shell (SSH)!
- CCNA RnS Article #18: Secure Switch Management Plane!
- CCNA RnS #17: Switching Process
- CCNA RnS #16: Campus vs Data Center Network
- CCNA RnS #15: Memory Types - The Network DNA
- CCNA RnS #14 - CLI Introduction! - The Network DNA
- CCNA RnS #13: CSMA/CD - The Network DNA
- CCNA RnS #12 : Ethernet Frame Type & FCS Field!
- CCNA RnS #11: Ethernet/MAC Addressing - The Network DNA
