Ethernet & Cisco

Most popular Ethernet standards:

• IEEE 802.3u – Fast Ethernet
• IEEE 802.3z – Gigabit Ethernet over fiber optic
• IEEE 802.3ab – Gigabit Ethernet over twisted pair
• IEEE 802.3ae – 10 Gigabit Ethernet over fiber
• IEEE 802.3an – 10 Gigabit Ethernet over UTP

Some facts:

• There are no collisions in full-duplex mode.
• A dedicated switch port is required for each full-duplex node.
• The host network card and the switch port must be capable of operating in full-duplex mode.
• Runts are packets that are discarded because they are smaller than the medium's minimum packet size. Any Ethernet packet that is less than 64 bytes is considered a runt. In half-duplex environments, it is possible for both the switch and the connected device to sense the wire and transmit at exactly the same time and result in a collision. Collisions can cause runts, FCS, and alignment errors, caused when the frame is not completely copied to the wire, which results in fragmented frames. Runts are the result of collisions, faulty NIC's, duplex mismatch, IEEE 802.1Q (dot1q), or an Inter-Switch Link Protocol (ISL) configuration issue.
• Microsegmentation - segmentation of a collision domain into as many segments as there are circuits, minus one (#segments = #circuits - 1). This microsegmentation performed by the switch cuts the collision domain down so that only two nodes coexist within each collision domain. This way, collisions are decreased and only the two NICs which are directly connected via a point-to-point link are contending for the medium.
• If you want to implement a network medium that is not susceptible to elec-tromagnetic interference (EMI), fiber-optic cable provides a more secure, long-distance cable that is not susceptible to EMI at high speeds.
• Hub and switch can enlarge the area covered by a single LAN segment.
• If the duplex settings do not match on the ends of an Ethernet segment, the switch interface will still be in a connect (up/up) state. In this case, the interface works, but it may work poorly, with poor performance, and with symptoms of intermittent problems.
• Only show interfaces status shows whether or not auto negotiation is on.
• Autonegotiation is disabled if speed and duplex is configured.
• If the speed is not known, use 10 Mbps, half duplex.
• If the speed is somehow known to be 10 or 100 Mbps, default to use half duplex.
• If the speed is somehow known to be 1000 Mbps, default to use full duplex.
• Cisco switches can determine speed in a couple of ways even when IEEE standard autonegotiation fails. First, the switch knows the speed if the speed interface subcommand was manually configured. Additionally, even when IEEE autonegotiation fails, Cisco switches can automatically sense the speed used by the device on the other end of the cable, and can use that speed based on the electrical signals on the cable.

Lan problems

• They usually can be found using counters of  show interfaces
• Excessive interference on the cable - can cause the various input error counters to keep growing larger, especially the CRC counter. In particular, if the CRC errors grow, but the collisions counters do not, the problem may simply be interference on the cable.
• Duplex mismatch - collisions and late collision counters could keep growing
• Jabber - collisions and late collision counters could keep growing
• In particular, a significant problem exists if the collision counters show that more than 1% of all the output frames have collided.

MAC

MAC address is 48 bit or 6 bytes:

• Individual Group Bit
• Global/local bit or universal/local bit: 0 administered by IEEE, 1 by local
• 22 bits left of Organizational Unique Identifier (OUI)
• 24 bits vendor assigned

The most popular type of frame is Ethernet II:

• Dest addr 6 bytes
• Source addr 6 bytes
• Type 2 bytes - type of protocol
• Data
• FCS – frame check sequence - crc

Three types of twisted pair:

• Straight-through cable (PC, router, access point to switch, hub)
• Crossover cable (PC to PC, PC to router, switch to switch, hub to hub, switch to hub, router to roter)
• Rolled cable (PC to cisco console)