This Cram Sheet contains the distilled, key facts about Exams 640-821, 640-811, and 640-801. Review this information as the last thing you do before you enter the testing center, paying special attention to those areas where you feel that you need the most review. You can transfer any of these facts from your head onto a blank sheet of paper immediately before you begin the exam.
The primary advantage of bridging is increased bandwidth available on a segment because of the decreased number of devices in the collision domain.
Switches are high-speed, multiport bridges. Switches support the same functionality as bridges but usually have a greater port density. Each switch port is a separate collision domain, and each port provides dedicated bandwidth.
Virtual local area networks (VLANs) can be used to make a group of switch ports into a separate, isolated LAN. Routing is required for communication between VLANs.
VLANs can function across multiple switches when they are connected by a trunk connection. Inter-switch linking (ISL) is used to create a trunk connection between Fast Ethernet ports on Cisco switches.
Switches make it possible to run Ethernet devices in full-duplex mode. In full-duplex mode, two devices share the Ethernet wire simultaneously and exclusively, enabling faster throughput because no collisions are possible.
Store-and-forward switching reads the entire frame before making a forwarding decision; cut-through switching reads only the first six bytes the destination media access control (MAC) address to make a forwarding decision. Store-and-forward switching performs error checking; cut-through switching does not.
The primary advantages of routers are
They allow you to connect dissimilar LANs.
They provide multiple paths to a destination network.
They allow the interconnection of large and complex networks.
Connection-oriented communication uses a nonpermanent path for data transfer. It involves three steps: establish the connection, transfer the data, and terminate the connection. A practical example of a connection-oriented communication would be a walkie-talkie conversation in which the connection has to be established each time to the receiver by pressing a button, and after you are finished talking, you release the button until you want to talk again. Connectionless communication uses a permanently established link. An example of a connection-oriented protocol is TCP, and an example of connectionless communication protocol is UDP. Again, a practical example would be that of a telephone conversation in which a connection is made and maintained throughout the duration of the call.
The layers of the OSI model are as follows:
7. Application
6. Presentation
5. Session
4. Transport
3. Network
2. Data Link
1. Physical
Encapsulation, or tunneling, takes frames from one network system and places them inside frames from another network system.
The Presentation layer concerns itself with data representation, data encryption, and data compression. It supports different protocols for text, data, sound, video, graphics, and images, such as ASCII, MIDI, MPEG, GIF, and JPEG.
The Session layer establishes, manages, and terminates sessions between applications. Network file system (NFS), structured query language (SQL), and remote procedure calls (RPCs) are examples of Session layer protocols.
The Transport layer sits between the upper and lower layers of the OSI model. It performs flow control by buffering, multiplexing, and parallelization. It provides end-to-end data transport services by segmenting upper-layer applications, establishing an end-to-end connection, sending segments from one end host to another, and ensuring reliable data transport.
The primary functions of the Network layer of the OSI model are path determination and logical addressing. In addition, remember that the Network layer is the domain of routing.
The primary functions of the Data-Link layer of the OSI model are
It allows the upper layers of the OSI model to work independently of the physical media.
It performs physical hardware addressing.
It provides optional flow control.
It generates error notification.
Convergence occurs when all routers in an internetwork agree on optimal routes. A routing loop occurs when a packet bounces back and forth between two or more routers.
Distance vector routing protocols send their entire routing tables to their neighbors. Link state protocols send the state of their own interfaces to every router in the internetwork.
Counting to infinity is a problem for distance vector routing protocols. This problem can be eliminated or mitigated by using the following techniques: maximum hop count, split horizon, route poisoning, and hold-down timers.
TCP provides a connection-oriented and reliable service to the applications that use its services with the use of acknowledgments, sequence number checking, error and duplication checking, and the TCP three-way handshake. User Datagram Protocol (UDP) provides a connectionless and best-effort service to the applications that use its services.
Well-known port numbers include
File Transfer Protocol (FTP) 21
Telnet 23
Simple Mail Transfer Protocol (SMTP) 25
Domain Name System (DNS) 53
TFTP 69
Simple Network Management Protocol (SNMP) 161, 162
HTTP 80
Address Resolution Protocol (ARP) maps a known IP address to a physical address. Reverse Address Resolution Protocol (RARP) maps a known physical address to a logical address.
You should understand the basic concepts of IP addressing. Dotted-decimal notation is the decimal representation of a 32-bit IP address. The dotted-decimal notation represents the four octets of bits by performing binary-to-decimal conversion for each octet and providing a decimal value for each octet.
You should know the decimal representation of classes A, B, and C addresses as well as the number of networks and nodes each supports as follows:
Class A: 1 through 126
Class B: 128 through 191
Class C: 192 through 223
You should be able to recognize the default mask for each class of IP address as follows:
Class A: 255.0.0.0
Class B: 255.255.0.0
Class C: 255.255.255.0
The network number and broadcast address for a given subnet are the first and last IP addresses, respectively. The range of usable IP addresses is all addresses between the network number and broadcast address. In binary format, the network number has all of the host bits of the address set to 0. The broadcast address has all of the host bits set to 1.
You should know how to do subnetting tasks very quickly. This capability will save you valuable time in the end.
The interface between the customer network and the WAN provider network occurs between the data terminal equipment (DTE) and the data communication equipment (DCE). DTE devices are usually routers. DCE devices are usually modems, channel service units/data service units (CSUs/DSUs), and terminal adapter/network terminations 1 (TA/NT1s).
Frame relay is a high-speed, packet-switching WAN protocol that operates at the Data-Link layer. It runs on nearly any type of serial interface, uses frame check sequence (FCS) as its error-checking mechanism, and relies on a discard eligibility bit for congestion management. A virtual circuit must connect two DTE devices within a frame relay network. Permanent virtual circuits (PVCs) are more widely used than switched virtual circuits (SVCs) in frame relay networks.
Data link connection identifier (DLCI) serves as the addressing scheme within a frame relay network. Local Management Information (LMI) is a set of enhancements to frame relay that was developed by Cisco, StrataCom, Northern Telecom, and DEC. Cisco routers support LMI variations for American National Standards Institute (ANSI), Q933a, and Cisco.
DLCIs are mapped to network layer addresses through inverse ARP or by using the frame-relay map command.
Committed Information Rate (CIR) is the rate, in bits per second, at which data is transferred across the frame relay network.
Password Authentication Protocol (PAP) uses a two-way handshake to authenticate Point-to-Point Protocol (PPP) connections and transmits username/password information in clear text. Challenge Handshake Authentication Protocol (CHAP) uses a three-way handshake and relies on secret, encrypted passwords and unique IDs to authenticate PPP.
Integrated Services Digital Network (ISDN) can be ordered as either basic rate interface (BRI) or primary rate interface (PRI). ISDN functions represent devices or hardware functions within ISDN. Reference points describe the logical interfaces between functions.
ISDN can be used to add bandwidth for telecommuting, improve Internet response time, carry multiple network layer protocols, and encapsulate other WAN services.
Dial-on-demand routing (DDR) works with ISDN to establish and terminate connections. It uses access lists to look for interesting traffic.
EXEC includes the following:
Context-sensitive help for syntax checking, command prompting, and keyword completion. Use the question mark (?) to activate context-sensitive help.
Command history that provides a record of recent commands. Use the up- and down-arrow keys to scroll through the history list. Tab completes a partially entered command.
Enhanced editing that enables commands retrieved from the command history to be changed quickly and then reexecuted. The terminal editing and terminal no editing commands enable and disable enhanced editing.
Use the Tab key to allow the router to complete commands after you get a %incomplete command% message.
Examine the status of a router with the following commands: show version, show memory, show protocols, show running-config (or write terminal), show startup-config (or show configuration), show interfaces, and show flash.
The Cisco Discovery Protocol (CDP) displays summary information about directly connected devices and operates at the Data Link layer. The show cdp neighbors command displays ID, local and remote port, holdtime, platform, and capability information. The show cdp entry <device id> command displays information about a specific device, including all Layer 3 addresses and Internetwork Operating System (IOS) versions.
The command to back up a router configuration file (copy a configuration file from a router to a Trivial File Transfer Protocol [TFTP] server) is copy running-config tftp. The command to restore a configuration file (copy a configuration file from a TFTP server to a router) is copy tftp running-config.
The commands to set the enable secret, console, and auxiliary passwords on a router are as follows:
Router(config)#enable password Router(config)#enable secret password Router(config)#line aux 0 Router(config-line)#login Router(config-line)#password password Router(config)#line con 0 Router(config-line)#login Router(config-line)#password password Router(config)#line vty 0 4 Router(config-line)#login Router(config-line)#password password
To create a banner for a router and a description for an interface, use the banner motd (message of the day) and description commands.
Router resource usage, bandwidth consumption, and update synchronization are problems for link state routing protocols. They can be eliminated or reduced by using the following techniques:
Lengthening update frequency
Exchanging route summaries
Using time stamps or sequence numbers
Routing Information Protocol (RIP) can be configured on a router with the following commands:
Router (config)# router rip Router (config-router)# network <network>
Enhanced Interior Gateway Routing Protocol (EIGRP) can be configured on a router with the following commands:
Router (config)# router eigrp <autonomous system number> Router (config-router)# network <network>
The most important basic commands used to monitor IP with Cisco routers are show ip interface, show ip protocol, and show ip route.
A list of the important access list numeric identifiers follows:
1 through 99: IP standard access list
100 through 199: IP extended access list
800 through 899: IPX standard access list
900 through 999: IPX extended access list
1000 through 1099: Service Advertisement Protocols (SAP) access list
Two rules for applying a wildcard mask to an IP address are
A 1 bit in the wildcard mask indicates that the corresponding bit in the IP address can be ignored. Thus, the IP address bit can be either 1 or 0.
A 0 in the wildcard mask indicates that the corresponding bit in the IP address must be strictly followed. Thus, the value must be exactly the same as specified in the IP address.
The difference in the capabilities of IP-extended access lists in comparison with IP standard access lists is that standard access lists filter IP traffic based on source IP address or address range. IP extended access lists filter traffic based on source and destination addresses, ports, and many other fields.
Know that the last line of any access list is deny any any (implicit).
A standard access list can be configured on a router using the following command:
Router(config)#access-list <1-99> <permit/deny> <source IP> <wildcard mask>
An extended access list can be configured on a router using the following command:
Router(config)#access-list <100-199> <permit/deny> <protocol> <source IP> <wildcard mask> 
<destination IP> <wildcard mask> eq <port number>
Network Address Translation (NAT) allows internal IP addresses to be translated to external IP addresses.
The three forms of NAT include
Static NAT
Dynamic NAT
NAT Overload
The most popular form of NAT is the NAT Overload configuration, which translates many internal IP addresses to a single external IP address. The commands to configure NAT Overload are as follows:
Router(config-if)#ip nat inside Router(config-if)#ip nat outside Router(config)#ip nat inside source list <access list identifying inside addresses> interface <external interface> overload
A single physical interface can be configured with several virtual subinterfaces. Each subinterface can be configured with different addressing information. Subinterfaces can be created and accessed using the serial interface number followed by a period and a number (such as serial 0.78).
The commands to configure frame relay on a router are
Router (config)# encapsulation frame-relay cisco Router (config)# frame-relay lmi-type cisco Router (config)# interface serial 0 Router (config-if)# frame-relay interface-dlci <dlci number>
The basic commands to monitor frame relay activity on a router are show frame-relay pvc, show frame-relay lmi, show frame-relay map, and debug frame-relay lmi.
The commands to configure PPP on a router are
Router (config)# username <name> password <password> Router (config)# interface serial 0 Router (config-if)# encapsulation ppp Router (config-if)# ppp authentication chap
The basic commands to monitor PPP activity on a router are show interface and debug ppp chap.
The commands to configure ISDN on a router are
Router (config)# isdn switch-type <switch-type> Router (config)# dialer-list <dialer-group> protocol <protocol-name> permit Router (config-if)# interface bri 0 Router (config-if)# encapsulation PPP Router (config-if)# dialer-group <number> Router (config-if)# dialer map <protocol> <next-hop address> name <hostname> speed <number> <dial-string> Router (config-if) dialer idle-timeout <seconds>
The basic commands to monitor ISDN and DDR activity on a router are show controller bri, show interface bri, and show dialer.
