Hold-down timers prevent temporary routing loops while convergence takes place. Hold-down timers defeat the counting-to-infinity problem when a router has multiple paths to many subnets.

Poison reverse updates are sent to remove a route and place it in hold-down. When a route to a subnet goes down, the router advertises an infinite metric for the failed route out of all interfaces including the interfaces previously prevented by split horizon.

Split horizon prevents routing updates from being propagated back out of the interface on which they were learned. Split horizon solves the counting-to-infinity problem when topology changes force convergence.

2. Answer C is correct. From privileged exec mode, "configure terminal" brings you into global configuration mode, where nearly all router configuration changes are made.

Step two, "router rip" enables Routing Information Protocol (RIP) routing and brings you into router configuration mode. The prompt looks like this:

router(config-router)#

Finally, entering the "network" command followed by the network number to advertise selects network(s) that participate in RIP. This sequence of commands properly enables RIP from the privileged exec mode.

"Config t" is an abbreviated form of "configure terminal."

"Interface interface_number" enters you into interface configuration mode.

"Ip-rip routing" and "enable ip rip" use incorrect syntax.

3. Answer B is correct. The command "show interface s 3/1" is an abbreviated form of the Cisco IOS router software command "show interface serial 3/1". This command displays interface information for a router with serial port 1 in slot 3. Routers have expansion slots to add additional features and capabilities after initial purchase. Slots are numbered from the left to right starting with slot 0. If an expansion card in a slot has more than one interface on it, each interface is numbered starting at zero from left to right. Serial 3/1 is the serial card in the fourth slot from the left, and the second port from the left on that particular card.

4. Answer D is correct. Integrated Services Digital Network (ISDN) Basic Rate Interface (BRI) consists of three channels, referred to as "two Bs and a D" that can provide up to 128 kilobits per second (kbps) of data throughput.

The two Bearer Channels (B Channels) are capable of speeds up to 64kbps. These two B Channels are "bonded" or combined through Multi-Link PPP to provide an aggregate bandwidth of 128kbps. The single D-Channel is rated at 16kbps and is used for control.

In some areas of the country, the B-Channels are limited to 56kbps each. This causes the aggregate bandwidth to be:

2 times 56kbps B-Channels + 1 times 16kbps D-Channel = 128kbps total

Other areas of the country provide B Channels that are rated at 64kbps each. This causes the aggregate bandwidth to be:

2 times 64kbps B-Channels + 1 times 16kbps D-Channel = 144kbps total

Faced with these disparities, it is best to think of ISDN as 128kbps capable.

The 19.2kbps and 33.6kbps values are standard analog modem transfer rates.

5. Answer D is correct. After enabling Internet Protocol (IP) routing on Cisco routers, you must manually configure a routing protocol. This is done with the "network" command, which is used to advertise the networks to be included in routing updates.

The command "router(config)# router <protocol name>" enables routing for the protocol specified.

Enabling Internet Packet Exchange (IPX) routing on Cisco routers automatically enables IPX RIP.

Enabling AppleTalk routing on Cisco routers automatically enables RTMP.

Interior Gateway Routing Protocol (IGRP) and Routing Information Protocol (RIP) are distance vector protocols. Distance vector protocols converge slowly and learn about routes to distant networks from routing updates from other routers. Routing updates are transmitted even if no changes in the network occur.

Open Shortest Path First (OSPF) is a standard routing protocol that is link-state based. This means that the routers communicate changes in the paths between routers only when they change. The "best" path is the fastest path as opposed to the path between the fewest routers, as in RIP.

6. Answer A is correct. When you connect switches together for the purpose of extending your network, it would be catastrophic if traffic from your VLANs were jumbled all together making it unreadable when it exited the far side of the switch link. This is where the Interswitch Link (ISL) Protocol comes into the picture. ISL places a tag on each frame that shows from which VLAN each frame originated. This is also known as Frame Tagging.

VLAN Frame Tagging is the process of adding VLAN information to frames in order to identify which VLAN a frame is associated as it travels from one switch, across a trunk, to another switch.

Subnetwork Access Protocol (SNAP) is part of a Layer 2 framing protocol.

Bridge Protocol Data Unit (BPDU) is a message sent by bridges and switches to create a spanning tree in redundant switch networks.

User Datagram Protocol (UDP) is the connectionless protocol at the Transport layer used for best effort data delivery.

7. Answer B is correct. The Integrated Services Digital Network (ISDN) protocols, as outlined by the International Telecommunication Union-Telecommunication Standardization Sector (ITU-T), are grouped by function and/or subject as seen below:

The E-Series: (deals with Public Switched Telephone Network (PSTN) and ISDN)

• E.163 - International Telephone Numbering Plan
• E.164 - International ISDN Addressing

The I-Series: (deals with ISDN concepts, aspects, and interfaces)

• I.100 - concepts, structures, and terminology

The Q-Series: (deals with switching and signaling)

• Q.921 - LAPD (Link Access Procedure on the D-Channel encapsulation)
• Q.931 - ISDN network layer between terminal and switch

The J-series is not related to ISDN.

8. Answers B and C are correct. The first command in this question, "access-list 166 deny udp any any eq 69" is using access list 166 to deny UDP traffic on port 69 from any source to any destination.

The next command, "access-list 166 deny tcp any any eq 23," is using access list 166 to deny TCP traffic on port 23 from any source to any destination.

The last command, "access-list 166 permit ip any any," says if the packet does not match either of the first two commands then permit IP packets from any source to any destination.

IP access lists are either standard or extended. They are assigned a number that indicates what kind of access list it is. Standard access lists are numbered 1 to 99. Extended access lists are number 100 to 199. Since the access list in the question is numbered 166, it is an extended access list, which means that you can block traffic based on source IP address, destination IP address, specific protocols, and TCP/UDP port numbers.

IP Layer 4 (TCP and UDP) uses port numbers to identify upper layer applications.

IP extended access lists 100-199 allow you to control traffic based on these port numbers.

In this question, denied ports are 69 and 23; therefore, TFTP (port 69) and Telnet (port 23) are denied.

The others, DNS (port 53), HTTP (port 80), and FTP (ports 20 and 21) remain useable.

9. Answer D is correct. You can configure a list of "host" names for the common locations to which the router connects. Host names are a kind of alias for IP Addresses. When pinging a remote router, you could type "ping corp" instead of its IP address.

You configure the hosts file by entering the command:

router(config)#ip host ?

WORD Name of host

router(config)#ip host corp ?

<0-65535> Default telnet port number

A.B.C.D Host IP address (maximum of 8)

Use the "show hosts" command to display the list of host names with their corresponding IP addresses. The "show hosts" command displays host names acquired by configuration statements or learned dynamically from a DNS server.

The "show alias" command displays the shorthand versions of administration interface commands.

The other answers use incorrect syntax.

10. Answer B is correct. The Cisco IOS offers two types of encapsulation, cisco and ietf. The ietf encapsulation conforms to the specifications of RFC 1490, and the cisco encapsulation does not. The cisco and ietf encapsulation types were designed to compensate for the lack of a protocol type field in the frame relay header.

The Link Access Procedure Frame Bearer Services (LAPF) specification defines the header and trailer used for a frame relay connected router to encapsulate a Layer 3 packet before it is sent over the frame relay network.

Frame relay switches do not care about the encapsulation type, nor do they care about the IP addressing. The service provider must define and configure the LMI type used and set up the virtual circuit (DLCI). A frame relay switch will announce its DLCI, and many switches detect the LMI used by the router.

Frame relay is based on the concept that the core of the network is managed by the service provider. Another name for the network core is the frame cloud. The cloud is the common entry point for all frame relay subscribers' data.

The customer premises equipment (CPE) connects to the frame cloud via either a Switched Virtual Circuit (SVC), which is a connect-on-demand circuit, or a Permanent Virtual Circuit (PVC), which is always active. Each virtual circuit uses a data-link connection identifier (DLCI) to uniquely identify a path between two routers.

Frame relay is connection-oriented and runs between a customer's router and the provider's switch. It may run inside the provider's network as well, or it may just run at the edge of the provider's network.

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