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Catalyst 3550 Multilayer Switch Software Configuration Guide
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Chapter8 Configuring Interface Characteristics
Configuring Layer 2 Interfaces
Configuring Interface Speed and Duplex Mode
These sections describe how to configure the interface speed and duplex mode:
Configuration Guidelines, page 8-14
Setting the Interface Speed and Duplex Parameters, page 8-14

Configuration Guidelines

When configuring an interface speed and duplex mode, note these guidelines:
If both ends of the line support autonegotiation, we highly recommend the default autonegotiat ion
settings.
If one interface supports autonegotiation and the other end d oes not, configure duplex and speed on
both interfaces; do not use the auto setting on the supported side.
Note GigaStack-to-GigaStack cascade connections operate in half-duplex mode, and
GigaStack-to-GigaStack point-to-point connections operate i n f ull -dupl ex mode .
Caution Changing the interface speed and duplex mode configuration might shut down and re-enable the
interface during the reconfiguration.

Setting the Interface Speed and Duplex Parameters

You can configure interface speed on Fast Ethernet (10/100-Mbps) and Gigabit Ethernet
(10/100/1000-Mbps) interfaces; you cannot configure speed on G iga bit I nter fac e Conver ter ( GBIC)
interfaces. You can configure duplex mode on any Fast Ethernet or Gigabit Ethernet inte rf aces th at a re
not set to autonegotiate; you cannot configure duplex mode on GBIC interfaces. Because collisions are
major constrictions in Ethernet networks, full-duplex communication is an effective solution. Normally ,
10-Mbps ports operate in half-duplex mode, which means that stations can either receive or send. In
full-duplex mode, two stations can send and receive at the same time. When packets can flow in both
directions simultaneously, effective Ethernet bandwidth doubles to 20 Mbps fo r 10- Mbps in ter faces ,
to 200 Mbps for Fast Ethernet interfaces, and to 2 Gbps fo r Gi gabi t inter fa ces.
Note You cannot configure speed or duplex mode on Gigabit Interface Converter (GBIC) ports, but for
certain types of GBICs, you can configure speed to not n egot iate ( nonegotiate) if connected to a
device that does not support autonegotiation.