WAN Troubleshooting Facts

WAN Troubleshooting Facts

The following list of commands may help when troubleshooting router connections.

Use...	To...
router#show interfaces	List a large set of information about each interface.
router#show interface status	View summary information about the interface status.
router#show ip interfaces	View a small set of information about each IP interface.
router#show ip interfaces brief	View a single line of information about each IP interface.
router#show ip route <ip address>	View details about the route the router would match for a packet sent to the listed IP address.
router#show running-config	View the currently running configuration file.
router#show controllers <serial interface>	View the serial interface configuration, such as the type of serial cable and which end of the cable is connected to the device (DCE or DTE).
router#ping <ip address>	Test communication with a specific interface using its IP address.

You can use the interface status to understand connectivity problems and quickly see whether the link between the router and the network is operational. The following table summarizes some possible conditions indicated by the interface status:

Line status	Protocol status	Indicates...
administratively down	down	The interface is configured with the shutdown command.
down	down	There is a hardware or network connection problem (Physical layer), such as: No cable or bad cable The device on the other end of the cable is powered off or the other interface is administratively shutdown (with the shutdown command)
up	down	There is a connection or communication problem (Data Link layer), such as: No clock rate provided by the DCE device Mismatched encapsulation Incorrect authentication parameters for PPP, including: Mismatched authentication method Missing username statements Mismatched passwords
up	up	The interface is working correctly

After verifying that the interfaces have Layer 1 and Layer 2 connectivity, proceed to troubleshoot TCP/IP connectivity including:

• Verifying that devices have unique IP addresses.
• Verifying that the same subnet mask is used on all devices on the same subnet.
• Verifying that the IP address assigned to each device is on the same subnet.
• Verifying routing table entries.

Be aware of the following when troubleshooting connectivity:

• If a problem exists at Layer 1, you must correct that problem before troubleshooting Layer 2 connectivity. If a problem exists at Layer 2, you must correct that problem before you can proceed to troubleshoot upper-layer connectivity.
• Use ping and traceroute to verify Network-layer connectivity, and use Telnet to verify Application-layer connectivity and configuration.
• A failed ping or traceroute test might indicate Layer 1, Layer 2, or Layer 3 problems. Examine the interface status to rule out Layer 1 and Layer 2 problems.
• A successful Telnet test means that ping and traceroute will also be successful. A failed Telnet test only indicates a failure at the Application layer or below. By itself, it does not tell you at which layer the problem exists.
• Because some devices do not respond to ICMP messages, you might have Network-layer connectivity between devices even if ping or traceroute fail.
• A successful ping test followed by an unsuccessful Telnet test means that Network-layer connectivity exists. Troubleshoot the upper-layer configuration.
• Even if Telnet to a router fails, the router might still be routing packets. This is because routing happens at the Network layer, while Telnet happens at the Application layer.

Serial Interface Configuration Command List

Serial Interface Configuration Command List

Use the following commands to configure the router.

Use . . .	To . . .
Router(config-if)#clock rate <rate>	Set the clock rate on the DCE serial interface. Note: In the back-to back router lab configuration, if the clock rate command is not issued on the DCE, clocking is not provided, and the interface status between the two routers will not change to up.
Router(config-if)#ip address <address> <mask>	Assign an IP address and subnet mask to the interface.
Router(config-if)#encapsulation hdlc Router(config-if)#encapsulation ppp Router(config-if)#encapsulation frame-relay	Modify the router encapsulation method. Note: HDLC is the default encapsulation method. The encapsulation method should match for both routers.
Router#sh interfaces	View all interface configurations, including serial connection encapsulation and bandwidth.
Router#sh ip int brief	View a consolidated message concerning each IP interface, including its IP address, line and protocol status, and how the address was configured (DHCP or Manual)
Router#sh run	View the clock rate and bandwidth of a serial configuration.
Router#sh controllers <serial interface>	View the serial interface configuration, such as the type of serial cable and which end of the cable is connected to the device (DCE or DTE).

Examples
The following set of commands configures the IP address 192.168.1.229 with a mask of 255.255.255.0 for the first Serial interface on the router and activates the interface.

Router(config)#int ser 0/1/0

Router(config-if)#ip address 192.168.1.229 255.255.255.0

Router(config-if)#no shutdown

The following set of commands configures the second Serial interface on the router with PPP encapsulation and activates the interface.

Router(config)#int ser 0/1/1

Router(config-if)#encapsulation ppp

Router(config-if)#no shutdown

WAN Encapsulation Facts

WAN Encapsulation Facts

WAN Physical layer protocols specify the hardware and bit signaling methods. Data Link layer protocols control some or all of the following functions:

• Error checking and correction
• Link establishment
• Frame-field composition
• Point-to-point flow control

Data Link layer protocols also describe the encapsulation method or the frame format. WAN encapsulation methods are typically called HDLC (high-level data link control). This term is both a generic name for Data Link protocols and the name of a specific protocol within a WAN protocol suite or service. Depending on the WAN service and connection method, you will select one of the following encapsulation methods.

• Cisco HDLC for synchronous, point-to-point connections with other Cisco routers. This is the default encapsulation method for synchronous serial links on Cisco routers. Note: Cisco HDLC does not communicate with other vendors' implementations of HDLC.
• LAPB for X.25 networks.
• LAPD in combination with another protocol for the B channels in ISDN networks. LAPD is a Layer 2 ISDN protocol that manages flow and signaling.
• PPP for dial-up LAN access, circuit-switched WAN networks, and ISDN networks. PPP is non-proprietary, so it works in implementations that use products from multiple vendors.
• Cisco/IETF for Frame Relay networks.

Note: Routers on each side of a WAN link must use the same encapsulation method to be able to communicate.

WAN Connectors and Ports

WAN connections are provided through several different connector types. The following table illustrates the connector types and their ports:

Type	Description
DB-60 Connector Serial WIC port	WAN interface cards (WIC) with a single serial port use a DB-60 connector. The connector has 4 rows of 15 pins each.
Smart Serial Connector Smart Serial WIC ports	WICs with two serial ports use the high-density Smart Serial connector.
RJ-48 Connector Integrated T1 DSU/DSU WIC port	Integrated T1 DSU/CSU WIC ports use an RJ-48 connector. Note: The RJ-48 connector has the same size and shape as the RJ-45 used in Ethernet connections, but has a different pinout.
RJ-11 Connector DSL WIC port	DSL (Digital Subscriber Line) WIC ports use an RJ-11 connector. The RJ-11 port connects to the phone line.

WAN Connection Facts

WAN Connection Facts

Whether ordering a WAN connection from a service provider or configuring a back-to-back router lab, you should know how to connect the devices. There are several different types of cables used in WAN connections and choosing the correct cable largely depends upon identifying the following:

Component	Details
Interface operating mode	When a device communicates over a serial interface, the interface operates in one of the following modes: The interface/device providing clocking is known as the DCE (Data Communications Equipment). The interface/device not providing clocking is known as the DTE (Data Terminal Equipment). When you configure a router to connect to a network through a serial interface, the router must be connected to a device (such as a CSU/DSU or another router) that provides clocking signals. When you configure two routers in a back-to-back configuration through their serial ports, one router interface must be configured to provide the clocking signals for the connection.
Service	The following WAN service options will also affect which type of cable is needed for a WAN connection: X.25 Frame Relay Asynchronous Transfer Mode (ATM) Integrated Services Digital Network (ISDN) DSL

Be aware of the following WAN connection details:

• When ordering a WAN service:
  • The network engineer specifies how fast the circuit should run, such as 1.544 Mbps (T1) or 56 Kbps (POTS or DS0).
  • The Telco installs the circuit running at the specified speed.
  • The network engineer purchases a CSU/DSU to connect to the end of the circuit, and then connects the router to the CSU/DSU.
  • The CSU/DSU clock rate must then be configured to match the circuit speed. Because the router is the DTE, the clock rate is received by the CSU/DSU and does not require configuration.
• When connecting a back-to-back router lab configuration, one router should be chosen as the DCE and the other router as the DTE. The cable connecting the two back-to-back routers has both a DCE and a DTE end. Connect the DCE end of the cable to the interface you want to be the DCE device.

WAN Services Facts

WAN Services Facts

Listed below are the most common WAN transmission media.

Carrier	Speed	Description
Plain Old Telephone Service (POTS)	56 Kbps	Existing wires use only one twisted pair Analog signals are used through the local loop A modem is required to convert digital signals to analog
T1 (a.k.a. DS1)	1.544 Mbps	T-Carrier is a digital standard widely deployed in North America. T1 lines usually run over two-pairs of unshielded twisted pair (UTP) cabling, although they can also run over other media such as coaxial, fiber-optic, and satellite. A T1 line has 24 channels (also known as DS0's) that each run at 64 Kbps. T3 lines usually run over fiber-optic cable. A T3 line has 672 channels that each run at 64 Kbps. A T1/T3 connection requires a CSU/DSU.
T3 (a.k.a. DS3)	44.736 Mbps
E1	2.048 Mbps	E-Carrier is a digital standard very similar to T-Carrier, but it is widely deployed in Europe. An E1 line has 32 channels (also known as DS0's) that run at 64 Kbps. An E3 line transmits 16 E1 signals at the same time. E1/E3 connections also require a CSU/DSU.
E3	34.368 Mbps
J1	1.544 Mbps	J-Carrier is a digital standard very similar to T-Carrier, but it is widely deployed in Japan. A J1 line is virtually identical to a T1 line. A J3 line has 480 channels that run at 32 Mbps. J1/J3 connections also require a CSU/DSU.
J3	32.064 Mbps

Note: WAN services also use fiber optic, wireless, satellite, and other transmission media. However, the use of these media to the local loop is not common at this time.

If your organization needs WAN connectivity, you can choose from the following service options:

Service	Bandwidth (Max.)	Line Type	Signaling Method	Characteristics
Public Switched Telephone Network (PSTN)	56 Kbps	POTS	Analog	Dialup over regular telephone lines
Leased lines	56 Kbps	POTS	Analog	Dedicated line with consistent line quality
X.25	64 Kbps	POTS	Analog	Dedicated line Variable packet sizes (frames) Ideal for low-quality lines
Frame Relay	1.54 Mbps	POTS T-1 T-3	Digital	Variable packet sizes (frames)
Asynchronous Transfer Mode (ATM)	1.2 Gbps	Coaxial, twisted pair, fiber-optic	Digital	Fixed-size cells (53-byte) High-quality, high-speed lines
Integrated Services Digital Network (ISDN)	144 Kbps (BRI) 4 Mbps (PRI)	POTS T-1	Digital	Basic rate operates over regular telephone lines and is a dialup service Primary rate operates over T-carriers
DSL	6.1 Mbps (1.544 or lower is more common)	POTS	Digital	Operates using digital signals over regular telephone lines DSL comes in many different flavors (such as ADSL and HDSL)

There is no clear distinction between WAN services such as Frame Relay and ISDN. For example, you can use Frame Relay protocol over ISDN lines. Once a device connects to the WAN cloud, internal protocols can convert data traffic into the necessary formats, then convert the data again at the other end.

WAN Structure Facts

WAN Structure Facts

A typical WAN structure includes the following components.

Component	Description
Consumer Premises Equipment (CPE)	Devices physically located on the subscriber's premises. CPE includes the telephone wire, telephone, modem, and other equipment, both the devices the subscriber owns and the ones leased from the WAN provider. The wiring typically includes UTP cable with RJ-11 or RJ-45 connectors. CPE is sometimes used synonymously with DTE.
Data Terminal Equipment (DTE)	A device on the network side of a WAN link that sends and receives data. The DTE resides on the subscriber's premises, and marks the point of entry between the LAN and the WAN. DTEs are usually routers, but computers and multiplexers can also act as DTEs. Broadly, DTEs are any equipment at the customer's site, and can include all computers. In a narrow sense, the DTE is the device that communicates with the DCE at the other end.
Channel Service Unit/Data Service Unit (CSU/DSU)	The CSU/DSU is a device that connects a physical circuit installed by the telco to some CPE device, adapting between the voltages, current, framing, and connectors used in the circuit to the physical interface supported by the DTE.
Demarcation point (demarc)	The point where the telephone company's telephone wiring connects to the subscriber's wiring. The demarc can also be called the network interface or point of presence. Typically, the customer is responsible for all equipment on one side of the demarc. The phone company is responsible for all equipment on the other side of the demarc.
Local loop	Cable that extends from the demarc to the central telephone office. The demarc media is owned and maintained by the telephone company. Typically, it is UTP, but it can also be one or a combination of UTP, fiber optic, or other media. Fiber optic cable to the demarc is rare.
Central Office (CO)	The switching facility closest to the subscriber, and the nearest point of presence for the WAN provider. It provides WAN-cloud entry and exit points for incoming and outgoing calls, and acts as a switching point to forward data to other central offices. A CO provides services such as switching incoming telephone signals to outgoing trunk lines. It also provides reliable DC power to the local loop to establish an electric circuit. COs use long-distance, or toll, carriers to provide connections to almost anywhere in the world. Long-distance carriers are usually owned and operated by companies such as AT&T or MCI.
Data Communication Equipment (DCE)	A device that communicates with both DTEs and the WAN cloud. DCEs are typically routers at the service provider that relay messages between the customer and the WAN cloud. In a strict sense, a DCE is any device that supplies clocking signals to DTEs. Thus, a modem or CSU/DSU at the customer site is often classified as a DCE. DCEs may be devices similar to DTEs (such as routers), except that each device plays a different role.
WAN cloud	The hierarchy of trunks, switches, and central offices that make up the network of telephone lines. It is represented as a cloud because the physical structure varies, and different networks with common connection points may overlap. Few people thoroughly understand where data goes as it is switched through the "cloud." What is important is that data goes in, travels through the line, and arrives at its destination.
Packet-Switching Exchange (PSE)	A switch on a carrier's packet-switched network. PSEs are the intermediary points in the WAN cloud.

WAN Type Facts

WAN Type Facts

WAN types include the following:

Method	Description
Point-to-Point	A point-to-point connection is a single, pre-established path from the customer's network through a carrier network, such as a telco, to a customer's remote network. A point-to-point line is usually leased from a carrier and thus is often called a leased line.
Circuit Switching	A circuit switching network allows data connections that can be initiated when needed and terminated when communication is complete, working much like a telephone line for voice communication. A circuit switched network uses a dedicated connection between sites. It is ideal for transmitting data that must arrive quickly in the order it is sent, as is the case with real-time audio and video.
Packet Switching	A packet switched network allows data to be broken up into packets and sent across the shared resources. Packets are transmitted along the most efficient route to the destination. Packet switching is ideal for transmitting data that can handle transmission delays, as is often the case with Web pages and e-mail.