Section 10. WAN Technologies

Question 1

Q1-What is a WAN?

Answer 1

A wide-area network (WAN) is a data communications network that extends beyond the geographic scope of a LAN. WANs use service provider connections to interconnect LANs into one internetwork.

Question 2

Q2-WAN technologies operate at what layers of the OSI model?

Answer 2

WAN technologies operate at the physical and data link layers of the OSI model.

A WAN interconnects LANs that are separated by a large geographical distance not supported by typical LAN media.

The physical layer defines the electrical, mechanical, and operational connections of WANs, in addition to the interface between the data terminal equipment (DTE) and data communications equipment (DCE).

The data link layer defines the WAN Layer 2 encapsulation, such as Frame Relay, ATM, HDLC, Ethernet emulation, and PPP.

Question 3

Q3-What are three major characteristics of WANs?

Answer 3

Three major characteristics of WANs are

They connect LANs that are located over wide geographical areas.

They use service providers such as telephone companies, cable companies, satellite systems, and network providers for connections.

They use various connection types to provide access to bandwidth over large geographical areas.

Question 4

Q4-What is the largest WAN in the world?

Answer 4

The Internet is the largest WAN in the world. The Internet is the best example of a WAN. It is a collection of thousands of interconnected networks all over the world.

Question 5

Q5-What are three primary differences of WANs when compared to LANs?

Answer 5

Three primary differences of WANs when compared to LANs are

WANs span a wide geographic area. LANs are contained to a building or small geographic area.

WAN links are owned by a service provider. LAN links are owned by the organization.

WANs have recurring monthly costs for the use of the links. After they are installed, LANs do not have reoccurring monthly costs.

Question 6

Q6-What roles do routers perform in a WAN?

Answer 6

Routers perform several functions in a WAN. They provide a means to interconnect WAN links to LAN interfaces. They provide routing and perform certain functions needed for WANs such as clocking and encapsulation.

Routers are not the only devices that provide clocking for WAN connections. Often CSU/DSUs and modems serve the clocking source for a circuit.

Question 7

Q7-What are the three types of interfaces typically found on routers?

Answer 7

The three interfaces typically found on routers are

LAN interfaces: copper or fiber

WAN interfaces: copper or fiber

Management interfaces: console, Ethernet, auxiliary

Question 8

Q8-Define customer premises equipment (CPE), and give an example.

Answer 8

Typically, CPE is equipment that is located on the customer’s (or subscriber’s) premises. It is equipment owned by the customer or equipment leased by the service provider to the customer for the purpose of connecting to the service provider network. An example is a router.

Question 9

Q9-What is the demarcation point (demarc)?

Answer 9

The demarc is a point where the CPE ends and the local loop begins. It is the point between the wiring that comes in from the local service provider and the wiring installed to connect the customer’s CPE to the service provider. It is the last responsibility of the service provider and is usually a network interface device (NID) located in the customer’s telephone wiring closet. Think of the demarc as the boundary between the customer’s wiring and the service provider’s wiring.

Question 10

Q10-What is the local loop?

Answer 10

The local loop is the physical cable that extends from the demarc to the service provider’s network.

Question 11

Q11-Define the central office (CO).

Answer 11

The CO is the WAN service provider’s office where the local loop terminates and in which circuit switching occurs. The CO is normally associated to a telco.

Question 12

Q12-What are five available WAN connection types?

Answer 12

Five available WAN connection types are as follows:

Dedicated connections (leased lines)

Circuit-switching connections

Packet-switching connections

Cell-switching connections

Broadband VPN connections

Question 13

Q13-What is WAN signaling?

Answer 13

WAN signaling is the process of sending a transmission signal over a physical medium for communication. WAN transmission facilities feature standardized signaling schemes that define transmission rates and media types. For example, the signaling standard for a T1 line in North America is DS1 with a transmission rate of 1.544 Mbps.

Question 14

Q14-What are WAN data link layer protocols?

Answer 14

WAN data link layer protocols include High-Level Data Link Control (HDLC), PPP, ATM, Frame Relay, and so on.

Designed to operate over dedicated lines, multipoint services, and multiaccess-switched services such as Frame Relay, data link layer protocols provide the data link layer encapsulations associated with synchronous serial lines.

Question 15

Q15-What is a point-to-point communication link across a WAN?

Answer 15

A point-to-point communication link across a WAN provides a single established WAN connection path from the customer premises through a service provider network to a remote network.

Question 16

Q16-What is the Point-to-Point Protocol (PPP)?

Answer 16

PPP is an industry-standard protocol that provides router-to-router or router-to-host connections over synchronous and asynchronous links. It can be used to connect WAN links to other vendors’ equipment. It is protocol independent; thus it works with several network layer protocols. PPP provides optional authentication through Password Authentication Protocol (PAP), Challenge Handshake Authentication Protocol (CHAP), or Microsoft CHAP (MS-CHAP).

Question 17

Q17-How do you enable a point-to-point link on a router interface using Ethernet emulation?

Answer 17

Enabling a point-to-point link with Ethernet emulation is the same as configuring an IP address on an Ethernet interface and enabling the interface. The following example configures a router’s Fast Ethernet interface 0/0 for Ethernet emulation using a /30 subnet mask:
RouterB(config-if)# interface f0/0
RouterB(config-if)# ip address 192.168.1.254 255.255.255.252
RouterB(config-if)# no shutdown

Question 18

Q18-After configuring an interface for point-to-point WAN connectivity through a service provider network, how do you test end-to-end connectivity with the remote network?

Answer 18

To test end-to-end connectivity, use ping, tracert, or traceroute to verify communications with the remote network.

Question 19

Q19-What are the purposes of routing protocols?

Answer 19

The primary purposes of routing protocols are to

Discover remote networks

Maintain up-to-date routing information

Choose the best path to a destination network when multiple networks are learned from the same routing process

Find a new best path if the current path is no longer available

Question 20

Q20-What is administrative distance?

Answer 20

Administrative distance (AD) is an integer from 0 to 255 that rates the trustworthiness of the source of the IP routing information. It is only important when a router learns about a destination route from more than one source. The path with the lowest AD is the one given priority.

Question 21

Q21-What is the default administrative distance for each of the following?

Directly connected interfaces
Static route
EIGRP
OSPF
IS-IS
BGP
RIP
External EIGRP
Unknown

Answer 21

The default ADs are as follows:

Directly connected interface: 0

Static route: 1

EIGRP: 90

OSPF: 110

IS-IS: 115

RIP: 120

External EIGRP: 170

Unknown: 255

Question 22

Q22-What is an autonomous system (AS)?

Answer 22

An AS is a collection of networks under common administrative control that share a common routing strategy.

Question 23

Q23-What is the difference between interior gateway protocols (IGP) and exterior gateway protocols (EGP)?

Answer 23

IGPs exchange routing topology within an autonomous system (AS). EGPs exchange routing topology between autonomous systems. Examples of IGPs are RIPv2, EIGRP, IS-IS, and OSPF. BGP is an example of an EGP.

Question 24

Q24-What is a routing protocol metric?

Answer 24

A routing protocol metric is a factor that determines the desirability of a route that has been sent within routing protocol data. A router uses the metric to determine the best or optimal path to which a same destination has been learned multiple times through the same routing protocol process. This value is evaluated after the administrative distance when building the routing table. Each routing protocol uses a unique metric.

Question 25

Q25-What are the most common metrics used by routing protocol algorithms?

Answer 25

The most common metrics used by routing protocol algorithms are as follows:

Bandwidth: The data capacity of a link

Delay: The length of time required to move a packet from source to destination

Hop count: The number of routers a packet must take to reach its destination

Cost: A value assigned by the network administrator, usually based on bandwidth on the link

Load: The amount of activity on the link or network resource

Reliability: A reference to the error rate on each network link

Also, bandwidth and delay are not actual values but are configured values.

Question 26

Q26-How do distance vector routing protocols function?

Answer 26

Distance vector routing protocols pass complete routing tables to neighboring routers. Neighboring routers then combine the received routing table with their own routing tables. Each router receives a routing table from its directly connected neighbor. RIPv2 is the most common distance vector protocol used in today’s internetworks.

Question 27

Q27-How do distance vector routing protocols keep track of changes to the internetwork?

Answer 27

Distance vector routing protocols keep track of changes to the internetwork by periodically broadcasting updates out all active interfaces.

These broadcasts contain the entire routing table. This method is often called “routing by rumor.”

Updates are not sent out interfaces where the router originally learned the update.

Question 28

Q28-What are link-state routing protocols? List two common link-state routing protocols.

Answer 28

Link-state routing protocols create a complete picture of the internetwork by determining the status of each interface (link) in the internetwork. When the interface goes down, link-state protocols send link-state advertisements (LSA) out all other interfaces, informing other routers of the downed link. Thus LSAs are only sent when there is a link change. Link-state protocols find the best paths to destinations by applying Dijkstra’s algorithm against the link-state database.

OSPF and IS-IS are the most common link-state protocols used.

Question 29

Q29-Which of the following is an advanced distance vector routing protocol?

RIPv2
EIGRP
DECnet
OSPF

Answer 29

EIGRP is an advanced distance vector routing protocol.

EIGRP was developed by Cisco and is considered an advanced distance vector routing protocol because it combines the aspects of distance vector and link-state routing protocols.