1.2 Duplex Communication

What are Output Errors?

"Output errors" is the sum of all errors that prevented the final transmission of datagrams out the interface that is being examined. The reported output errors from the show interfaces command include the following: Collisions - Collisions in half-duplex operations are normal. However, you should never see collisions on an interface configured for full-duplex communication. Late collisions - A late collision refers to a collision that occurs after 512 bits of the frame have been transmitted. Excessive cable lengths are the most common cause of late collisions. Another common cause is duplex misconfiguration. For example, you could have one end of a connection configured for full-duplex and the other for half-duplex. You would see late collisions on the interface that is configured for half-duplex. In that case, you must configure the same duplex setting on both ends. A properly designed and configured network should never have late collisions.

Late Collisions

A late collision refers to a collision that occurs after 512 bits of the frame have been transmitted. Excessive cable lengths are the most common cause of late collisions. Another common cause is duplex misconfiguration. For example, you could have one end of a connection configured for full-duplex and the other for half-duplex. You would see late collisions on the interface that is configured for half-duplex. In that case, you must configure the same duplex setting on both ends. A properly designed and configured network should never have late collisions.

How are fiber optic ports preset?

All fiber-optic ports, such as 1000BASE-SX ports, operate only at one preset speed and are always full-duplex.

What can be useful when the speed and duplex are unknown?

Autonegotiation is best used.

What are best practices when connecting knows devices such as servers, workstations, or network devices?

Best practice is to manually set the speed and duplex settings

What can be done if the interface is down?

Check to make sure that the proper cables are being used. Additionally, check the cable and connectors for damage. If a bad or incorrect cable is suspected, replace the cable. If the interface is still down, the problem may be due to a mismatch in speed setting. The speed of an interface is typically autonegotiated; therefore, even if it is manually applied to one interface, the connecting interface should autonegotiate accordingly. If a speed mismatch does occur through misconfiguration, or a hardware or software issue, then that may result in the interface going down. Manually set the same speed on both connection ends if a problem is suspected.

Collsions

Collisions in half-duplex operations are normal. However, you should never see collisions on an interface configured for full-duplex communication.

Useful switch verification commands

Display interface status and configuration. - S1# show interfaces [interface-id] Display current startup configuration. - S1# show startup-config Display current running configuration. - S1# show running-config Display information about flash file system. - S1# show flash Display system hardware and software status. - S1# show version Display history of command entered. - S1# show history Display IP information about an interface. - S1# show ip interface [interface-id] OR S1# show ipv6 interface [interface-id] Display the MAC address table. - S1# show mac-address-table OR S1# show mac address-table

Example steps to set up duplex and speed on an interface

Enter global configuration mode. - S1# configure terminal Enter interface configuration mode. - S1(config)# interface FastEthernet 0/1 Configure the interface duplex. - S1(config-if)# duplex full Configure the interface speed. - S1(config-if)# speed 100 Return to the privileged EXEC mode. - S1(config-if)# end Save the running config to the startup config. - S1# copy running-config startup-config

Giants

Ethernet frames that are larger than the maximum allowed size are called giants.

Runt Frames

Ethernet frames that are shorter than the 64-byte minimum allowed length are called runts. Malfunctioning NICs are the usual cause of excessive runt frames, but they can also be caused by collisions.

What command is used to enable auto-MDIX?

In an interface in configuration mode you use the commoan: mdix auto

What are some media errors that might not cause a circuit to fail but do cause network performance issues?

Input Errors - Total number of errors. It includes runts, giants, no buffer, CRC, frame, overrun, and ignored counts. Runts - Frames that are discarded because they are smaller than the minimum frame size for the medium. For instance, any Ethernet frame that is less than 64 bytes is considered a runt. Giants - Frames that are discarded because they exceed the maximum frame size for the medium. For example, any Ethernet frame that is greater than 1,518 bytes is considered a giant. CRC - CRC errors are generated when the calculated checksum is not the same as the checksum received. Output Errors - Sum of all errors that prevented the final transmission of datagrams out of the interface that is being examined. Collisions - Number of messages retransmitted because of an Ethernet collision. Late Collisions - A collision that occurs after 512 bits of the frame have been transmitted.

What are input errors?

Input errors" is the sum of all errors in datagrams that were received on the interface being examined. This includes runts, giants, CRC, no buffer, frame, overrun, and ignored counts. The reported input errors from the show interfaces command include the following: Runt Frames - Ethernet frames that are shorter than the 64-byte minimum allowed length are called runts. Malfunctioning NICs are the usual cause of excessive runt frames, but they can also be caused by collisions. Giants - Ethernet frames that are larger than the maximum allowed size are called giants. CRC errors - On Ethernet and serial interfaces, CRC errors usually indicate a media or cable error. Common causes include electrical interference, loose or damaged connections, or incorrect cabling. If you see many CRC errors, there is too much noise on the link and you should inspect the cable. You should also search for and eliminate noise sources.

CRC errores

On Ethernet and serial interfaces, CRC errors usually indicate a media or cable error. Common causes include electrical interference, loose or damaged connections, or incorrect cabling. If you see many CRC errors, there is too much noise on the link and you should inspect the cable. You should also search for and eliminate noise sources.

Which command is particularly useful for detecting common media issues?

The output from the show interfaces command is useful for detecting common media issues. One of the most important parts of this output is the display of the line and data link protocol status, as shown in the example. S1# show interfaces fastEthernet 0/18 FastEthernet0/18 is up, line protocol is up (connected) Hardware is Fast Ethernet, address is 0025.83e6.9092 (bia 0025.83e6.9092)MTU 1500 bytes, BW 100000 Kbit/sec, DLY 100 usec, The first parameter (FastEthernet0/18 is up) refers to the hardware layer and indicates whether the interface is receiving a carrier detect signal. The second parameter (line protocol is up) refers to the data link layer and indicates whether the data link layer protocol keepalives are being received. Based on the output of the show interfaces command, possible problems can be fixed as follows: If the interface is up and the line protocol is down, a problem exists. There could be an encapsulation type mismatch, the interface on the other end could be error-disabled, or there could be a hardware problem. If the line protocol and the interface are both down, a cable is not attached, or some other interface problem exists. For example, in a back-to-back connection, the other end of the connection may be administratively down. If the interface is administratively down, it has been manually disabled (the shutdown command has been issued) in the active configuration.

What commands can be used to verify the switch port configuration?

The show running-config command can be used to verify that the switch has been correctly configured. From the sample abbreviated output on S1, some important information is shown in the figure: Fast Ethernet 0/18 interface is configured with the management VLAN 99 VLAN 99 is configured with an IPv4 address of 172.17.99.11 255.255.255.0 The default gateway is set to 172.17.99.1 The show interfaces command is another commonly used command, which displays status and statistics information on the network interfaces of the switch. The show interfaces command is frequently used when configuring and monitoring network devices. The first line of the output for the show interfaces fastEthernet 0/18 command indicates that the FastEthernet 0/18 interface is up/up, meaning that it is operational. Further down, the output shows that the duplex is full and the speed is 100 Mbps.

What is auto-MDIX?

Until recently, certain cable types (straight-through or crossover) were required when connecting devices. Switch-to-switch or switch-to-router connections required using different Ethernet cables. Using the automatic medium-dependent interface crossover (auto-MDIX) feature on an interface eliminates this problem. When auto-MDIX is enabled, the interface automatically detects the required cable connection type (straight-through or crossover) and configures the connection appropriately. When connecting to switches without the auto-MDIX feature, straight-through cables must be used to connect to devices such as servers, workstations, or routers. Crossover cables must be used to connect to other switches or repeaters. With auto-MDIX enabled, either type of cable can be used to connect to other devices, and the interface automatically adjusts to communicate successfully. On newer Cisco switches, the mdix auto interface configuration mode command enables the feature. When using auto-MDIX on an interface, the interface speed and duplex must be set to auto so that the feature operates correctly.

What can you do if the interface is up, but issues with connectivity are still present?

Using the show interfaces command, check for indications of excessive noise. Indications may include an increase in the counters for runts, giants, and CRC errors. If there is excessive noise, first find and remove the source of the noise, if possible. Also, verify that the cable does not exceed the maximum cable length and check the type of cable that is used. If noise is not an issue, check for excessive collisions. If there are collisions or late collisions, verify the duplex settings on both ends of the connection. Much like the speed setting, the duplex setting is usually autonegotiated. If there does appear to be a duplex mismatch, manually set the duplex to full on both ends of the connection.

What command can you use to determine the duplex for an interface?

You can use duplex followed by full for example: duplex full

What command do you use to examine the auto-MDIX settings for a specific interface?

show controllers ethernet-controller and command with the phy keyword then limit the output to lines referencing auto-MDIX this will show on or off for the feature for example this is how the command would looked like: S1# show controllers ethernet-controller fa0/1 phy | include MDIX which would result in: Auto-MDIX : On [AmindState=1 Flags=0x00052248]

What command determines the speed of an interface?

speed for example to make an interface be 100Mbps you would write: speed 100