Q A Answers

1 What are the seven layers of the OSI model?

Answer: The seven layers of the OSI model are as follows:

• Application

• Presentation

2 What layer of the OSI model is responsible for ensuring that IP packets are routed from one location to another?

Answer: The network layer is primarily responsible for routing IP packets from one destination to another.

3 What mechanism is used in Ethernet to guarantee packet delivery over the wire?

Answer: Carrier Sense Multiple Access/Collision Detection (CSMA/CD) is the Ethernet mechanism used to ensure that when devices detect collisions, other devices on the segment are sent a jam signal. CSMA/CD ensures that when collisions occur, other devices (such as PCs or routers) back off (do not transmit) for a specified period of time. When a device receives a jam signal, it will wait a random amount of time to retransmit. This lowers the chance of another collision. All devices that detect a jam signal can transmit up to 16 times before sending an error message to the application layer.

4 Name two physical characteristics of 10BaseT?

Answer: 10BaseT is an Ethernet physical layer standard that defines a maximum length of 100 m and a network speed of 10 Mbps.

5 What Catalyst command displays the bridging or CAM table on a Cisco 5000 series switch?

Answer: show cam dynamic

6 What are the possible states of spanning tree?

Answer: The possible states of spanning tree are as follows:

• Disabled—The port is not participating in spanning tree and is not active.

• Listening—The port has received data from the interface and will listen for frames. In this state, the bridge only receives data and does not forward any frames to the interface or to other ports.

• Learning—In this state, the bridge still discards incoming frames. The source address associated with the port is added to the CAM table. BPDUs are sent and received.

• Forwarding—The port is fully operational; frames are sent and received.

• Blocking—The port has been through the learning and listening states and, because this particular port is a dual path to the root bridge, the port is blocked to maintain a loop-free topology.

The order of spanning tree states is listening, then learning, and, finally, forwarding or blocking. Typically, each state takes around 15 seconds on Cisco Catalyst switches.

7 FastEther Channel (FEC) allows what to occur between Cisco Catalyst switches?

Answer: FEC is a Cisco method that bundles 100 MB/s fast Ethernet ports into a logical link between Cisco Catalysts switches, such as the Catalyst 5000 or 6000 series switches.

Up to four ports can be bundled together to scale bandwidth up to 800 Mbps.

8 What field in the IP packet guarantees data delivery?

Answer: The IP frame format has no settings that guarantee packet delivery, so IP is termed connectionless. The error check is only performed on the IP header fields, not the data in the packet.

9 Name some examples of connection-orientated protocols used in TCP/IP networks. Answer: Connection-orientated protocols include TCP, FTP, and Telnet.

10 Given the address,, what are the subnet and broadcast addresses? How many hosts can reside on this network?

Answer: The subnet is and the broadcast address is The number of hosts is defined by the formula ^-2=256-2=254.

11 How many hosts can reside when the subnet mask applied to the network is (or

Answer: The number of hosts is

12 Name five routing protocols that support VLSM.

Answer: Routing protocols that support VLSM include the following:

13 What is the destination port number used in a Telnet session?

Answer: The TCP port number is 23, and the source port is a random number generated by the host device.

14 What TCP/IP services are common in today's large IP networks? Answer: TCP/IP has a number of applications or services in use:

• Address Resolution protocol (ARP)

• Reverse Address Resolution protocol (RARP)

• Dynamic Host Configuration Protocol (DHCP)

• Hot Standby Router Protocol (HSRP)

• Internet Control Message Protocol (ICMP)

• File transfer protocol (FTP)

• Trivial File Transfer Protocol (TFTP)

15 What IOS command displays the IP ARP table on a Cisco IOS router?

Answer: The IOS command is show ip arp. This command displays IP ARP entries only. IOS command, show arp, displays all ARP entries for all protocols in use.

16 Cisco routers use what mechanism to determine the routing selection policy for remote networks if more than one routing protocol is running?

Answer: Cisco IOS routers use administrative distance, which defines a set number for every routing protocol in use. The lower the AD, the more trustworthy the network. For example, a static route (AD is 1) is preferred to an OSPF (AD is 110) discovered route. A static route pointing to a directly connected interface, for example, via ethernet0, has an AD set to 0, the same as a directly connected interface even though a static route is enabled.

17 What is the administrative distance for OSPF, RIP, and external EIGRP?

Answer: The AD for RIP is 120, 110 for OSPF, and 170 for external EIGRP (internal EIGRP is 90).

18 Name five characteristics of distance vector routing protocols and provide two examples of routing protocols classified as distance vector.

Answer: Distance vector characteristics and example protocols are as follows:

Periodic updates

Periodic updates are sent at a set interval; for IP RIP, this interval is 30 seconds.

Broadcast updates

Updates are sent to the broadcast address Only devices running routing algorithms will listen to these updates.

Full table updates

When an update is sent, the entire routing table is sent.

Triggered updates

Also known as Flash updates, triggered updates are sent when a change occurs outside the update interval.

Split horizon

This method stops routing loop. Updates are not sent out an outgoing interface from which the route was received. This also saves bandwidth.

Maximum Hop Count limit

For RIP, the limit is 15, and for IGRP it's 255.


An example is Bellman-Ford for RIP.



19 IP RIP runs over what protocol and port number when sending packets to neighboring routers?

Answer: UDP port number 520

20 How many networks can be contained in an IP RIP update? Answer: Up to 25 networks

21 Specify three main differences between RIPv1 and RIPv2?

Answer: RIPv1 does not support VLSM, authentication, or multicast updates. RIPv2 supports VLSM, authentication, multicast updates, and unicast updates to remote routers.

22 What is an EIGRP Feasible Successor?

Answer: An EIGRP Feasible Successor is a neighboring EIGRP Cisco router with a lower AD.

23 What is the metric used by OSPF?

Answer: The metric used by OSPF is cost and is defined by the formula 10 / Bandwidth for a given interface. The cost to a remote path is the sum of all the costs that a packet will transverse to reach the remote network.

24 If OSPF is configured for one area, what area assignment should be used?

Answer: Good OSPF design defines area 0, or the backbone, as the core area, and area 0 should always be used. If the OSPF network resides in one area only, theoretically, any area assignment is possible.

25 What LSA types are not sent in a total stubby area?

Answer: Totally stubby areas block LSA types 3, 4, and 5. Although similar to a stub area, a totally stubby area blocks LSAs of type 3, as well. This solution is Cisco proprietary and is used to further reduce a topological database. The only Link State Advertisement (LSA) type permitted is a specific type 3 LSA advertising a default router only.

26 What IOS command disables an interface from participating in the election of an OSPF DR/BDR router?

Answer: To disable an interface on a Cisco router when electing a DR, the IOS command is ip ospf priority 0. The router with the highest priority (range is between 0 and 255) will be elected the DR.

27 On an Ethernet broadcast network, a DR suddenly reboots. When the router recovers and discovers neighboring OSPF routers, will it be the designated router once more?

Answer: Once the router fails, the Backup DR (BDR) assumes the functions of the DR and another OSPF router (if it exists) is elected the BDR. After the failed router recovers, neighboring OSPF hello packets will advise that a DR/BDR already exists and there is no need to assume the functions of DR or BDR until another election process is initiated.

28 What Layer 4 protocol does BGP use to guarantee routing updates, and what destination port number is used?

Answer: BGP4 uses TCP and the destination port number is 179.

29 What are ISDN BRI and PRI?

Answer: ISDN can be supplied by a carrier in two main forms: Basic Rate Interface (BRI) and Primary Rate Interface (PRI). An ISDN BRI consists of two 64-kbps services (B channels) and one 16-kbps signaling channel (D channel). An ISDN PRI consists of 23 B or 30 B channels and a 64-kbps D channel, depending on the country. In North America and Japan, a PRI service consists of 23 B channels for a total bit rate of up to 1.544 Mbps. In Asia and Australia, a PRI delivers 30 B-channels and one 64-kbps D channel, delivering a total bit rate of 2.048 Mbps.

30 What are the three phases that occur in any PPP session? Answer: The three phases that occur in any PPP session are

• Link establishment—Link Control Program (LCP) packets are sent to configure and test the link.

• Authentication (optional)—After the link is established, authentication can be used to ensure that link security is maintained.

• Network layers—In this phase, NCP packets determine which protocols will be used across the PPP link. An interesting aspect of PPP is that each protocol (IP, IPX, and so on) supported in this phase is documented in a separate RFC that discusses how it operates over PPP.

31 Define what BECN and FECN mean in a Frame Relay network?

Answer: Forward explicit congestion notification (FECN)—Bit set by a Frame Relay network device to inform DTE receiving the frame that congestion was experienced in the path from source to destination. DTE receiving frames with the FECN bit set can request that higher-level protocols take flow-control action, as appropriate.

Backward explicit congestion notification (BECN)—Bit set by a Frame Relay network device in frames traveling in the opposite direction of frames encountering a congested path. DTE receiving frames with the BECN bit set can request that higher-level protocols take flow-control action, as appropriate.

32 Frame Relay DLCI values are used for what purpose?

Answer: The data-link connection identifier (DLCI) value specifies a PVC or SVC in a Frame Relay network. DLCIs are locally significant. There are globally significant DLCIs used for LMI communication between Frame Relay switches.

33 What is the IP address range used in IP multicast networks? Answer: The range of networks is from to

34 What type of network environment typically uses an AS5300?

Answer: The AS5300, or universal Access Server (AS), is a versatile data communications platform that provides the functions of an access server, router, and digital modems in a single modular chassis. Internet Service Providers typically use AS5300 to allow clients to use ISDN or PSTN when accessing the Internet. The AS5300 also supports voice communication.

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