Cisco CCNA Exam: Half-Duplex Network Access Configuration

Half-Duplex Network Access Configuration

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Which feature allows a device to use a switch port that is configured for half-duplex to access the network?



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A. B. C. D.


Ethernet began as a local area network technology that provided a half-duplex shared channel for stations connected to coaxial cable segments linked with signal repeaters. In this appendix, we take a detailed look at the half-duplex shared-channel mode of operation, and at the CSMA/CD mechanism that makes it work.

In the original half-duplex mode, the CSMA/CD protocol allows a set of stations to compete for access to a shared Ethernet channel in a fair and equitable manner. The protocol's rules determine the behavior of Ethernet stations, including when they are allowed to transmit a frame onto a shared Ethernet channel, and what to do when a collision occurs. Today, virtually all devices are connected to Ethernet switch ports over full-duplex media, such as twisted-pair cables. On this type of connection, assuming that both devices can support the full-duplex mode of operation and that Auto-Negotiation (AN) is enabled, the AN protocol will automatically select the highest-performance mode of operation supported by the devices at each end of the link. That will result in full-duplex mode for the vast majority of Ethernet connections with modern interfaces that support full duplex and AN.

The correct answer to this question is A. CSMA/CD.

CSMA/CD (Carrier Sense Multiple Access with Collision Detection) is a networking protocol that allows devices to share the same network medium (such as a switch port) and avoid data collisions. In a half-duplex communication mode, a device can either transmit or receive data at a time, but not both simultaneously.

When a device wants to send data, it first listens to the network medium to check if it is clear (i.e., no other devices are transmitting). If the medium is busy, the device waits for a random period before retrying. If the medium is clear, the device starts transmitting its data.

However, in a shared network medium, two devices may attempt to transmit data at the same time, resulting in a data collision. When a collision is detected, both devices stop transmitting and wait for a random period before retrying. This is where the "collision detection" part of CSMA/CD comes into play.

In summary, CSMA/CD allows a device to use a switch port configured for half-duplex by ensuring that it listens to the network medium before transmitting data, and by detecting and resolving data collisions when they occur.

IGMP (Internet Group Management Protocol) is used for multicasting on IP networks. Port security is a feature used to restrict access to a switch port based on the MAC address of the connected device. Split horizon is a routing technique used to prevent routing loops in distance-vector routing protocols. These features do not directly relate to the question being asked.