CCNA Flash Cards

VLAN, subnet or network, broadcast domain, or data link.

Switches and Bridges

Routers are used to separate LANs and connect LANs

• Class A: 1 - 126 (24 bits for host)
• Class B: 128 - 191 (16 bits for host)
• Class C: 192 - 223 (8 bits for host)

Private IP addresses are not routable on the public Internet. By using NAT you can use these private IP addresses on your internal networks.

• Class A: 10.0.0.0 - 10.255.255.255
• Class B: 172.16.0 - 172.31.255.255
• Class C: 192.168.0.0 - 192.168.255.255

Each technology has it's own characteristics and methods for sharing routing information between routers.

• RIP and IGRP are distance vector routing protocols.
• EIGRP is a hybrid
• OSPF is a link state protocol.
• IGRP and EIGRP are Cisco proprietary, RIP and OSPF are industry standard.

• You can achieve network adds, moves, and changes by configuring a port into the appropriate VLAN.
• You can put a group of users needing high security into a VLAN so that no users outside of the VLAN can communicate with them.
• As a logical grouping of users by function, VLANs can be considered independent from their physical or geographic locations.
• VLANs can enhance network security
• VLANs increase the number of broadcast domains while decreasing their size.

refers to VLAN identification. This is what switches use to keep track of all those frames as they are traversing a switch fabric. It's how switches identify which frames belong to which VLAN.

Inter-Switch Link (ISL) is what you use to explicitly tag VLAN information onto a Ethernet Frame. This tagging information allows VLANs to be multiplexed over a trunk line through an external encapsulation method, which allows the switch to identify the VLAN membership of a frame over the link. ISL is a Cisco-proprietary frame-tagging method that can only be used on Cisco switches.

This is a non-proprietary IEEE method of frame tagging. If you are trunking between a Cisco switch link and a different brand of switch, you have to use 802.1Q for the trunk to work.

Router(config-subif)#encapsulation dot1q vlan#

• Standard access lists make decisions based on source IP address only.
• Extended access lists can look at source and destination information at Layers 3 and 4, as well as protocol type information.

Access lists are directional - you can only have one access list per direction (inbound or outbound) on an interface. The implicit deny means that any packet not matching any line of an access list will be denied; it is as if every access list ends with "deny all" function.

Cisco routers use a proprietary High-Level Data Link Control (HDLC) encapsulation on all their serial links by default.

LMI is a signaling standard between a router and a frame relay switch. The LMI is responsible for managing and maintaining status between these devices. This standard also provides transmission keepalives to ensure that the PVC does not shut down because of inactivity. (Local Management Interface)

Cisco uses 2 different Frame Relay encapsulation methods on their routers. Cisco is the the default, and it means that the router is connected to a Cisco Frame Relay switch; Internet Engineering Task Force (IETF), the second, means your router is connecting to anything but a Cisco Frame Relay switch.

• CIR is the rate, in bits per second, at which the Frame Relay switch agrees to transfer data.
• (Committed Information Rate)

The 3 Data Link layer protocols are the Network Control Protocol (NCP), which defines the Network layer protocols; the Link Control Protocol (LCP), a method of establishing, configuring, maintaining, and terminating the point-to-point connection; and the High-Level Data Link Control (HDLC), the MAC layer protocol that encapsulates the packets.

Increased number of addresses. The designers created a hierarchical addressing scheme when they created the IP address so that more addresses would be available to each network.

• 100.
• IGRP's default administrative distance is 100.
• RIP's default administrative distance is 120.

• Ensures that information about a route will not be sent back in the direction from which the original update came.
• A split horizon will not advertise a route back to the same router from which it learned the route.

• It describes when a router sets the metric for a downed link to infinity.
• A poison reverse is used to communicate to a router that the link is down and that the hop count to that network is set to infinity or unreachable.

• Bandwidth & Delay
• Bandwidth and Delay are the only parameters used by default to calculate the metric of a route. Reliability and load are legitimate parameters that may also be used in the metric calculation, but they are not used by default.

It uses the Hello protocol to establish adjacencies & It sends updates only when changes occur in the network.

• For transmission keepalives to ensure that the PVC does not shut down because of inactivity.
• The LMI provides keepalives between the router and the frame switch to verify that the link and connection are still active.

• Global Configuration Mode
• enable
• config t
• Router RIP

• You must use classful addresses. e.g:
• network 10.0.0.0

You must configure IRGP using the Autonomous System (AS)

• router igrp 10
• network 10.0.0.0

this example uses AS 10 meaning the router will only accept updates from AS 10.

Basic configuration for EIGRP is similar to IGRP; however, EIGRP has several additional features such as passive interface and summarization.

• e.g.:
• router eigrp 20
• passive-interface serial 0/1

• a minimal single area configuration involves only two commands:
• Router OSPF process-id
• network x.x.x.x y.y.y.y area z

Understand how IP addressing and subnetting work. First, determine your block size by using the 256-subnet mask math. Then county your subnets and determine the broadcast address of each subnet- it is always the number right befor the next subnet. Your valid hosts are the numbers between the subnet address and the broadcast address.

The block sizes are always 4, 8, 16, 32, 64, 128 and so on. You can determine your block size by using the 256-subnet mask math.

• You check the current configuration register setting by using the
• SHOW VERSION
• command.

The 0x2102 setting is the default on all Cisco routers and tells the router to look in NVRAM for the boot sequence. 0x2101 tells the router to boot from ROM, and 0x2142 tells the router not to load the startup-config in NVRAM to provide password recovery.

If you plug a new host into a switch, then you must verify the VLAN membership of that port. If the membership is different than what is needed for that host, the host will not be able to reach the needed network services.

• show vlan
• show vlan brief

Trunk links must be considered and configured, as well as VTP configurations on individual switches.

• sample commands:
• R(config)#vtp mode client
• R(config)#vtp domain domainname
• R(config)#int fa0/2
• R(config-if)#switchport access VLAN2

The 3 types of cables that can be created from an Ethernet cable are

• straight-through (to connect a PC or a router's Ethernet Interface to a hub or switch)
• crossover (to connect hub to hub, hub to switch, switch to switch, or PC to PC)
• rolled (for a console connection from a PC to a router or switch)

• Take a rolled cable and connect it from the COM port of the host to the console port of a router.
• Start HyperTerminal and set the bits per second (bps) to 9600 and flow control to None.

Assuming the priority is equal, the switch with the lowest MAC address will become the root bridge.

By using the priviledged-mode command

copy flash tftfp

you can back up a file from a flash memory to a TFTP (network) host.

By using the priviledged-mode command

copy tftp flash

you can restore or upgrade a file from a TFTP (network) server to flash memory.

In order to back up an IOS image to a network server, you must first make sure you can access the network server, ensure the network server has adequate space for the code image, and verify the file naming and path requirements.

There are a couple of ways to save the configuration of a router, but the most common, as well as the most tested, method is

copy running-config startup-config

To erase the configuration of a router, type the priviledged-mode command

erase startup-config

and reload the router.

When you first bring up a Cisco router, it will run a power-on self test (POST), and if that passes, it will then look for and load the Cisco IOS from Flash memory, if a file is present. The IOS then proceeds to load and look for a valid configuration in NVRAM called the startup-config. If no file is present in NVRAM, the router will go into setup mode.

Setup mode automatically starts if a router boots and no startup-config is in NVRAM. You can also bring up setup mode by typing setup from the priviledged mode. Setup provides a minimum amount of configuration in an easy format for someone who does not understand how to configure a Cisco router from the command line.

• User mode provides command-line interface with very few available commands by default. User mode does not allow the configuration to be viewed or changed.
• Privileged mode allows a user to both view and change the configuration of a router. You can enter privileged mode by typing the command enable and entering the enable password or enable secret password, if set.

The show version command provides basic configuration for the system hardware as well as the software version, the names and sources of configuration filess, and the boot images.

The command sequence to set a hostname is as follows:

• enable
• config t
• hostname hostname

Both of these passwords are used to gain access to priviledged mode; however, the enable secret is newer and encrypted by default. Also, if you set the enable password and then set the enable secret, only the enable secret will be used.

To set the enable secret, you use the command enable secret. Do not use enable secret password password, or you will set your password to "password password". Commands are:

• enable
• config terminal
• enable secret secretpassword

To set the console password on a router, use the following commands:

• enable
• configure terminal
• line console 0
• login
• password consolepassword

To set the Telnet password on a router, use the following commands:

• enable
• config t
• line vty 0 4
• login
• password telnetpassword

If you type show interface serial 0 and see that it is "down, line protocol is down", this will be considered a Physical layer problem. If you see it as "up, line protocol is down", then you have a Data Link layer problem.

• Router#config t
• Router(config)#ip address 172.16.10.15 255.255.255.0
• Router(config)#ip default-gateway 172.16.10.1

• R#config t
• R(config)#int vlan1
• R(config-if)#ip address 172.16.10.17 255.255.255.0
• R(config-if)#no shut
• R(config-if)#exit
• R(config)#ip default-gateway 172.16.10.1

R#delete nvram

R#erase startup-config

• To configure a standard IP access list, use the access-list numbers 1-99 or 1300-1999 in global configuration mode.
• Choose permit or deny, then choose the source IP address you want to filter on using either (host, network, or wildcard masking)

• To configure an extended IP access list, use the access-list numbers 100-199 or 2000-2699 in global configuration mode.
• Choose permit or deny, the Network layer protocol, the source IP address you want to filter on, the destination address you want to filter on, and finally the Transport layer protocol (if selected).

You can use the encapsulation PPP command on a serial interface to change from HDLC to PPP encapsulation. Also, you can further configure authentication using the ppp authentication command.

You can use the encapsulation frame-relay command on a serial interface to change the encapsulation to Frame Relay. Additionally, you will need to configure DLCI information, LMI and encapsulation types if they will differ from the default, and potentially subinterfaces if you will have multiple PVCs per physical interface.

• 1. Dialer
• 2. Map
• 3. Protocol
• 4. Next hop
• 5. Dial string

router eigrp as

show controllers Serial 0

PAP and CHAP are valid authentication methods available to PPP authentication.

OSI is a LAYERED approach. Functions are divided into layers, and the layers are bound together. This allows layers to operate transparently to each other, that is changes in one layer should not impact other layers.

User information is encapsulated to data, data to segments, segments to packets or datagrams, packets or datagrams to frames, and frames to bits.

Use the show vlan command on the Catalyst 1900 to see the configured VLANs, or use the same command on the Catalyst 2950 to examine the contents of the VLAN database.

Changes to VLAN configurations can propogate between switches; your VTP mode must be transparent if you do not want your switches to participate in VTP.

show ip route provides you with the contents of the routing table. An R on the left side of the table indicates a RIP found route. The debug ip rip command shows you RIP updates being sent and received on your router. If you see a route with a metric of 16, that route is considered down.

show ip route shows you the routing table, and an I on the left side of the table indicates and IGRP found route. The [100/123456] indicates the administrative distance - 100 is for IGRP, the composite metric. The composite metric is determined by bandwidth and delay of the line, by default.

• You can verify EIGRP operation using the commands
• show ip eigrp neighbor and show eigrp topology

• There are many show commands that provide useful detail on OSPF. You should be completely familiar with the output of each of the following:
• show ip ospf, show ip ospf database, show ip ospf interface, show ip ospf neighbor, and show ip protocols

Packet Internet Groper (PING) uses ICMP echo requests and ICMP echo replies to verify an active IP address on a network.

You can ping an IP address from a router's user mode or priviledged mode, but NOT from configuration mode.

Cisco Discovery Protocol can be used to help you document and troubleshoot your network.

show cdp neighbors command provides the following information: device ID, local interface, holdtime, capability, platform, and port ID

• If you want to keep your connection to a remote device but still come back to your original router console, you can press the CTRL+SHIFT+6 key combination, release it, and then press X.
• If you telnet to a router or swith, you can end the connection by typing exit at any time.

show sessions will provide you with all the sessions your router has to other routers.

• By using the global configuration mode command
• ip host host_name ip_address
• you can build a static host table on your router.

• You can verify the host table with
• show hosts
• command

• To see whether an access list is set on an interface and in which direction it is filtering, use the show ip interface command.
• This command will not show you the contents of the access list, merely which access lists are applied on the interface.

To see the configured access lists on your router, use the show access-list command. This command will not show you which interfaces have an access list set.

The show frame relay lmi command will give you the LMI traffic statistics exchanged between the local router and the Frame Relay switch. The show frame pvc command will list all configured PVCs and DLCI numbers.

In addition to standard show interface and IP troubleshooting tools, there are debug and show commands specific to each technology.

Packets

• Transport layer.
• The transport layer receives large data streams from the upper layers and breaks them up into smaller pieces called segments.

• show hosts
• The command to see the host table, which resolves hostnames to IP addresses, is show host or show hosts.

The show ip interface command tells you which ports have access lists applied.

show frame-relay pvc command will show the PVCs configured and the associated DLCI numbers.

Core, Distribution, & Access layers.

• APPLICATION - Application services
• PRESENTATION - encryption, compression, translation services.
• SESSION - dialog control
• TRANSPORT - end-to-end connection
• NETWORK - routing
• DATA LINK - framing
• PHYSICAL - physical topology

Transport Layer provides segmentation, sequencing, and virtual circuits.

The Data Link layer provides framing and places data on the network medium.

Connection-oriented uses acknowledgements and flow control to create a reliable session. More overhead is used than in a connectionless network service. Connectionless services are used to send data with no acknowledgements or flow control. This is considered unreliable.

• The purpose of BLOCKING state is to prevent the use of looped paths.
• A port in LISTENING state prepares to forward data frames without populating the MAC address table.
• A port in LEARNING state populates the MAC address table but doesn't forward data frames.
• The FORWARDING port sends and receives all data frames on the bridged port. Lastly a port in the DISABLED state is virtually disabled.

the main purpose of Spanning Tree Protocol (STP) is to prevent switching loops in a network with redundant switched paths.

• Ethernet_II: has a type field.
• 802.3: has a length field
• 802.2: has DSAPs and SSAPs
• SNAP: has it's own protocol field.

When the switch is in cut-through mode, it waits only for the destination hardware address to be received before it looks up the address in the MAC filter table.

• Cut-Through, Fragment Free, & Store-and-Forward.
• store-and-forward offers error checking; cut through and Fragment Free offer lower latency.

The Fragment Free LAN switch method checks the first 64 bytes of a frame before forwarding it for fragmentation.

Store-and-Forward first receives the complete data frame on the switch's buttfer, then a CRC is run, and then the switch looks up the destination address in the MAC filter table.

The frame changes at each hop, but the packet is never changed or manipulated in any way until it reaches the destination device.

Telnet is a terminal emulation protocol and it allows you to log into a remote host and run programs. FTP is a connection-oriented service that allows you to transfer files. TFTP is a connectionless file transfer program. SMTP is a send mail program.

Transmission Control Protocol (TCP) is a connection-oriented protocol that provides reliable network service by using acknowledgements and flow control. User Datagram Protocol (UDP) is a connectionless protocol that provides low overhead and is considered unreliable.

Internet Protocol (IP) is a connectionless protocol that provides network address and routing through an internetwork. Address Resolution Protocol (ARP) finds a hardware address from a known IP address. Reverse ARP (RARP) finds an IP address from a known hardware address. ICMP provides diagnostics and unreachable messages.

• RAM - Random Access Memory. Used to hold ARP cache, routing tables, and running-config.
• ROM - used to start and maintain the router.
• NVRAM - used to hold the router and switch configuration. NVRAM is not erased when the router or switch is reloaded.
• FLASH - holds the Cisco IOS. Not erased when the router is reloaded. It is an EEPROM created by Intel.

Used to control how the router boots up. This value can be seen with the show version command and typically is 0x2102, which tells the router to load the IOS from flash memory

Access lists can be used to filter or drop packets in order to control who has access to resources such as VTY, or to determine what networks are advertised by dynamic routing protocols.

At the end of every access list is an implicit deny. What this means is that if a packet does not match any of the lines in the access list, then it will be discarded. Also, if you have nothing but deny statements in your list, then the list will not permit any packets.

A leased line is a dedicated connection, a circuit-switched connection is like a phone call and can be on or off, and packet switching is essentially a connection that looks like a leased line but is priced more like a circuit-switched connection.

• The 3 primary WAN protocols are HDLC, Frame Relay, and PPP. Others include LAPB (Link Access Procedure Balanced) & ATM (Asynchronous Transfer Mode).
• HDLC (High-Level Data Link Control) is the default encapsulation on Cisco routers.
• PPP (Point-to-Point Protocol) provides an industry-standard way of encapsulating multiple routed protocols across a link and must be used when connecting equipment from different vendors.
• Frame Relay is a packet-switched technology that can offer cost advantages over leased lines but has more complex configuration options.