Cabling and cable-related issues will become part of your day-to-day routine as a network engineer. You will need to know which cables plug into which devices, the industry limitations, and how to configure equipment for use with the correct cable type.
LAN Cables
Ethernet Cables
Most cable-related network problems will occur on the Local Area Network (LAN) side rather than on the Wide Area Network (WAN) side due to the sheer volume of cables and connectors, and the higher frequency of reseating (unplugging and plugging in) the cables for device moves and testing.
Ethernet cables are used to connect workstations to the switch, switch-to-switch, and switch-to-router. The specifications and speeds have been revised and improved many times in recent years, which means you can soon expect today’s standard speeds to be left behind for new and improved high-speed links right to your desktop. The current standard Ethernet cable still uses eight wires twisted into pairs to prevent electromagnetic interference (EMI), as well as crosstalk, which is a signal from one wire spilling over into a neighbouring cable.
Cable categories, as defined by ANSI/TIA/EIA-568-A, include Categories 3, 5, 5e, and 6. Each one gives standards, specifications, and achievable data throughput rates, which can be achieved if you comply with distance limitations. Category 3 cabling can carry data up to 10Mbps. Category 5 cabling is primarily used for faster Ethernet networks, such as 100BASE-TX and 1000BASE-T. Category 5e cabling uses 100-MHz-enhanced pairs of wires for running GigabitEthernet (1000Base-T). Finally, with Category 6 cabling, each pair runs 250 MHz for improved 1000Base-T performance. (“1000” refers to the speed of data in Mbps, “Base” stands for baseband, and “T” stands for twisted pair.) Table 1.6 below demonstrates some common Ethernet standards you should be familiar with:
Table 1.6. Common Ethernet Standards
|
Speed |
Name |
IEEE Name |
IEEE Standard |
Cable/Length |
|
10Mbps |
Ethernet |
10BASE-T |
802.3 |
Copper/100 m |
|
100Mbps |
FastEthernet |
100BASE-T |
802.3u |
Copper/100 m |
|
1000Mbps |
GigabitEthernet |
1000BASE-LX |
802.3z |
Fibre/5000 m |
|
1000Mbps |
GigabitEthernet |
1000BASE-T |
802.3ab |
Copper/100 m |
|
10Gbps |
TenGigabitEthernet |
10GBASE-T |
802.3an |
Copper/100 m |
Cisco like to sneak cable specification questions into the exam from time to time, so make sure you memorise the table above.
Duplex
When Ethernet networking was first used, data was able to pass on the wire in only one direction at a time. This is because of the limitations of the cables used at that time. The sending device had to wait until the wire was clear before sending data on it, without a guarantee that there wouldn’t be a collision. This is no longer an issue because a different set of wires is used for sending and receiving signals.
Half duplex means that data can pass in only one direction at a time, while full duplex means that data can pass in both directions on the wire at the same time (see Figure 1.22). This is achieved by using spare wires inside the Ethernet cable. All devices now run at full duplex unless configured otherwise.
Figure 1.22 – Duplex Topology
You will still be expected to understand and troubleshoot duplex issues in the exam; we will cover troubleshooting Layer 1 and Layer 2 issues later in this guide. You can easily check an interface’s duplex settings with the show interface X command.
Switch#show interface FastEthernet0/1
FastEthernet0/1 is down, line protocol is down (disabled)
Hardware is Lance, address is 0030.a388.8401 (bia 0030.a388.8401)
BW 100000 Kbit, DLY 1000 usec,
reliability 255/255, txload 1/255, rxload 1/255
Encapsulation ARPA, Loopback not set
Keepalive set (10 sec)
Half-duplex, 100Mb/s
If this interface was connected to a full-duplex device, you would see interface errors immediately and experience slow traffic on the link. You can also issue the show interfaces status command on a live switch, although this command may not work in the exam because a router simulator has limited commands (same for Packet Tracer). You can see possible issues with interface Fast Ethernet 1/0/2 below:
Switch#show interfaces status
Port Name Status Vlan Duplex Speed Type
Fa1/0/1 notconnect 1 auto auto 10/100BaseTX
Fa1/0/2 notconnect 1 half 10 10/100BaseTX
Fa1/0/3 notconnect 1 auto auto 10/100BaseTX
Fa1/0/4 notconnect 1 auto auto 10/100BaseTX
Fa1/0/5 notconnect 1 auto auto 10/100BaseTX
And of course you can fix this issue easily, as shown below:
Switch(config)#int f1/0/2
Switch(config-if)#duplex ?
auto Enable AUTO duplex configuration
full Force full duplex operation
half Force half-duplex operation
Switch(config-if)#duplex full
Please do try this and all the other commands on live Cisco equipment, GNS3, or at least Packet Tracer in order to remember them! We will cover the speed setting next.
Speed
You can leave the speed of the Ethernet port on your routers or switches as auto-negotiate, or you can hard set them to 10Mbps, 100Mbps, or 1000Mbps.
To set the speed manually, you would configure the router as follows:
Router#config t
Router(config)#interface GigabitEthernet 0/0
Router(config-if)#speed ?
10 Force 10 Mbps operation
100 Force 100 Mbps operation
1000 Force 1000 Mbps operation
auto Enable AUTO speed configuration
The following commands would allow you to view the router Ethernet interface settings:
Router#show interface FastEthernet0
FastEthernet0 is up, line protocol is up
Hardware is DEC21140AD, address is 00e0.1e3e.c179 (bia 00e0.1e3e.c179)
Internet address is 1.17.30.4/16
MTU 1500 bytes, BW 10000 Kbit, DLY 1000 usec, rely 255/255, load 1/255
Encapsulation ARPA, Loopback not set, keepalive set (10 sec)
Half-duplex, 10Mb/s, 100BaseTX/FX
Specifications for Ethernet cables by EIA/TIA dictate that the end of the cable presentation should be RJ45 male (see Figure 1.23; Figure 1.24 shows the female end), which will allow you to insert the cable into the Ethernet port on your router/switch/PC.
Figure 1.23 – RJ45 Male End
Figure 1.24 – RJ45 Female End
Straight Cables
Each Ethernet cable contains eight wires and each wire connects to a pin at the end. The position of these wires when they meet the pin determines what the cable can be used for. If each pin on one end matches the other side, then this is known as a straight-through cable. These cables can be used to connect an end device to an Ethernet port on a switch, and a switch to a router. You can easily check whether the wires match by comparing one side of the cable to the other, as shown in Figures 1.25 and 1.26 below:
Figure 1.25 – Comparing Cable Ends
Figure 1.26 – Cable Ends Match
Crossover Cables
By swapping two of the wires on the cable, it can now be used to connect a PC to a PC (without the use of a switch or a hub, although Auto-MDIX ports on newer network interfaces detect whether the connection requires a crossover, and automatically chooses the MDI or MDIX configuration to properly match the other end of the link) or a switch to a switch. The wire on pin 1 on one end needs to connect to pin 3 on the other end, and pin 2 needs to connect to pin 6 on the other end (see Figure 1.27). I have created my own colour scheme for the cables purely to illustrate my point – red, yellow, blue, green, cyan, pink, magenta, and lilac.
Figure 1.27 – Pin 1 to Pin 3 and Pin 2 to Pin 6
Rollover/Console Cables
All Cisco routers and switches have physical ports to connect to for initial set up and disaster recovery or access. These ports are referred to as console ports and you will regularly use these as a Cisco engineer. In order to connect to this port, you need a special type of cable called a rollover or console cable (see Figure 1.28). It can sometimes be referred to as a flat cable because, as opposed to most round-bodied Ethernet cables, it is often flat along its body.
A rollover cable swaps all pins (see Figure 1.29), so pin 1 on one end goes to pin 8 on the other end, pin 2 goes to pin 7, and so on.
Figure 1.28 – A Typical Rollover Cable
Figure 1.29 – All Pins Swapped
Rollover cables usually have an RJ45 connection on one end and a 9-pin D-shaped connection on the other end, which is designed to connect to the COM port on a PC or laptop. The trouble is that devices no longer come with these ports, as they were so rarely used. You can now buy a DB9-to-USB converter cable (see Figure 1.30) from many electrical stores or online. They come with software drivers which allow you to connect a logical COM port on your PC via a terminal programme, such as Putty or HyperTerminal.
Cisco have started to put mini-USB ports (in addition to RJ45 ports) on their devices to allow for console port connectivity using the USB Type A to 5-pin mini-Type B cable. If both console cables are plugged in at the same time, the mini-USB cable takes precedence and becomes active. Figures 1.31 and 1.32 below show the different connection types.
Figure 1.30 – A COM-to-USB Converter Cable
Figure 1.31 – Connecting the Cable to the COM Port
Figure 1.32 – Connecting the Cable to the Router or Switch Console Port
WAN Cables
Used for Wide Area Network connections, serial cables can come in several shapes, sizes, and specifications, depending upon the interface on the router and connection type. ISDN uses different cables than Frame Relay or ADSL do, for example.
One common type of WAN cable you will use, especially if you have a home network to practise on, is a DB60 (see Figure 1.33). For this type of cable, you will have a data terminal equipment (DTE) end, which plugs in to the customer equipment, and a data communication equipment (DCE) end, which determines the speed of the connection from the ISP. Figure 1.34 below shows a DB60 serial interface on a WIC-1T card.
Figure 1.33 – A DB60 Cable
Figure 1.34 – The DB60 Serial Interface on a WIC-1T Card
There is another common presentation type for Cisco WAN Interface Cards (WICs) known as a smart serial cable:
Figure 1.35 – Smart Serial Cable
Of course, you will need the correct interface card in the router if you use this cable type, as shown below in Figure 1.36:
Figure 1.36 WIC-2T Smart Serial Card
The smart serial WIC card gives you two connections for one slot on the router, as opposed to only one connection you would get with a standard WIC-1T card. Each serial connection can use a different encapsulation type, such as PPP for one and Frame Relay for the other.
The most important thing to remember about DCE and DTE cables is that you need to apply a clock rate to the DCE end in order for the line to come up. Normally, your ISP would do this because they own the DCE end, but on a home lab or live rack, you own the DCE end, which makes you the customer on one router and the ISP on the other router. The command you would enter is clock rate 64000 (or whatever speed you like from the available options in bits per second). You can type clock rate ? to see your options.
Please ensure that you understand the following commands before typing them out on a router. Firstly, to establish which router has the DCE cable attached, you need to type the show controllers command, followed by the interface number. This is a very useful command to know for troubleshooting problems in the actual exam (and in the real world). You can see which interfaces you have on your router with the show ip interface brief command.
You can actually shorten most Cisco IOS commands, which is demonstrated in the output below. The shortened versions may not work in the exam, though, because the exam uses a router simulator (i.e., not a live router).
Router#sh ip int brie
Interface IP-Address OK? Method Status Protocol
FastEthernet0/0 unassigned YES unset administratively down down
FastEthernet0/1 unassigned YES unset administratively down down
Serial0/1/0 unassigned YES unset administratively down down
Vlan1 unassigned YES unset administratively down down
Router#show controllers s0/1/0
Interface Serial0/1/0
Hardware is PowerQUICC MPC860
DCE V.35, no clock
Router(config-if)#clock rate ?
Speed (bits per second)
1200
2400
4800
9600
19200
38400
56000
64000
…
[Truncated Output]