Cisco CCNA Study Guide - Router Alley

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CCNA Study Guide v2.62 – Aaron Balchunas 28 Speed and Duplex Autonegotiation Fast Ethernet is backwards-compatible with the original Ethernet standard. A device that supports both Ethernet and Fast Ethernet is often referred to as a 10/100 device. Fast Ethernet also introduced the ability to autonegotiate both the speed and duplex of an interface. Autonegotiation will attempt to use the fastest speed available, and will attempt to use full-duplex if both devices support it. Speed and duplex can also be hardcoded, preventing negotiation. The configuration must be consistent on both sides of the connection. Either both sides must be configured to autonegotiate, or both sides must be hardcoded with identical settings. Otherwise a duplex mismatch error can occur. For example, if a workstation’s NIC is configured to autonegotiate, and the switch interface is hardcoded for 100Mbps and full-duplex, then a duplex mismatch will occur. The workstation’s NIC will sense the correct speed of 100Mbps, but will not detect the correct duplex and will default to halfduplex. If the duplex is mismatched, collisions will occur. Because the full-duplex side of the connection does not utilize CSMA/CD, performance is severely degraded. These issues can be difficult to troubleshoot, as the network connection will still function, but will be excruciatingly slow. When autonegotiation was first developed, manufacturers did not always adhere to the same standard. This resulted in frequent mismatch issues, and a sentiment of distrust towards autonegotiation. Though modern network hardware has alleviated most of the incompatibility, many administrators are still skeptical of autonegotiation and choose to hardcode all connections. Another common practice is to hardcode server and datacenter connections, but to allow user devices to autonegotiate. Gigabit Ethernet, covered in the next section, provided several enhancements to autonegotiation, such as hardware flow control. Most manufacturers recommend autonegotiation on Gigabit Ethernet interfaces as a best practice. * * * All original material copyright © 2013 by Aaron Balchunas (aaron@routeralley.com), unless otherwise noted. All other material copyright © of their respective owners. This material may be copied and used freely, but may not be altered or sold without the expressed written consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
CCNA Study Guide v2.62 – Aaron Balchunas 29 Gigabit Ethernet Gigabit Ethernet operates at 1000 Mbps, and supports both twisted-pair (802.3ab) and fiber cabling (802.3z). Gigabit over twisted-pair uses all four pairs, and requires Category 5e cable for reliable performance. Gigabit Ethernet is backwards-compatible with the original Ethernet and Fast Ethernet. A device that supports all three is often referred to as a 10/100/1000 device. Gigabit Ethernet supports both half-duplex or fullduplex operation. Full-duplex Gigabit Ethernet effectively provides 2000 Mbps of throughput. Common Gigabit Ethernet physical standards include: IEEE Standard Physical Standard Cable Type Speed Maximum Cable Length 802.3ab 1000baseT Twisted-pair 1 Gbps 100 meters 802.3z 1000baseSX Multimode fiber 1 Gbps 500 meters 802.3z 1000baseLX Multimode fiber 1 Gbps 500 meters 802.3z 1000baseLX Singlemode fiber 1 Gbps Several kilometers In modern network equipment, Gigabit Ethernet has replaced both Ethernet and Fast Ethernet. 10 Gigabit Ethernet 10 Gigabit Ethernet operates at 10000 Mbps, and supports both twisted-pair (802.3an) and fiber cabling (802.3ae). 10 Gigabit over twisted-pair uses all four pairs, and requires Category 6 cable for reliable performance. Common Gigabit Ethernet physical standards include: IEEE Standard Physical Standard Cable Type Speed Maximum Cable Length 802.3an 10Gbase-T Twisted-pair 10 Gbps 100 meters 802.3ae 10Gbase-SR Multimode fiber 10 Gbps 300 meters 802.3ae 10Gbase-LR Singlemode fiber 10 Gbps Several kilometers 10 Gigabit Ethernet is usually used for high-speed connectivity within a datacenter, and is predominantly deployed over fiber. * * * All original material copyright © 2013 by Aaron Balchunas (aaron@routeralley.com), unless otherwise noted. All other material copyright © of their respective owners. This material may be copied and used freely, but may not be altered or sold without the expressed written consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
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