Exam CRAM CCNA-200-125

 OSI Model

7. Application Layer

Provides Services to lower layers. Enables program to program communication and

determines if sufficient resources exist for communication. Examples are e-mail gateways (SMTP), TFTP, FTP and SNMP.

6. Presentation Layer

Presents information to the Application layer. Compression, data conversion, encryption

and standard formatting occur here. Contains data formats JPEG, MPEG, MIDI, TIFF.

5. Session Layer

Establishes and maintains communication ‘sessions’ between applications (dialogue

control). Sessions can be simplex (one direction only), half-duplex (one direction at a

time) or full duplex (both ways simultaneously). Session layer keeps different

applications data separate from other applications. Protocols include NFS, SQL, X

Window, RPC, ASP, and NetBios Names.

4. Transport Layer

Responsible for end to end integrity of data transmissions and establishes a logical

connection between sending and receiving hosts via ‘virtual circuits’. Windowing works

at this level to control how much information is transferred before acknowledgement is

required. Data is segmented and reassembled at this layer. Port numbers are used to

keep track of different conversations crossing the network at the same time. Supports

TCP. UDP, SPX, NBP, Segmentation works here (Segments) and error correction (not

detection).

3. Network Layer

Routes data from one node to another and determines the best path to take. Routers

operate at this level. Network addresses are used here which are used for routing

(Packets). Routing tables, subnetting and control of network congestion occur here.

Routing protocols regardless of which protocol the run over reside here. RIP, IP, IPX,

ARP, IGRP, Appletalk.

2. Data Link Layer

Sometimes referred to as the LAN layer. Responsible for the physical transmission of

data from one node to another. Error detection occurs here. Packets are translated into

frames here and hardware address is added. Bridges and switches operate at this layer.

Logical Link Control sub layer (LLC) 802.2 :- manages communications between devices

over a single link on a network. Uses Service Access Points (SAPs) to help lower layers

talk to the Network Layer.

Media Access Control (MAC) 802.3 :- builds frames from the 1’s and 0’s that the

Physical Layer (address = 6-byte/48 bit) picks up from the wire as a digital signal and

runs a Cyclic Redundancy Check (CRC) to assure no bits were lost or corrupted.

1. Physical Layer

Puts data onto the wire and takes it off, physical layer specifications such as the

connectors, voltage, physical data rates and DTE/DCE interfaces. Some common

implementations include Ethernet/IEEE 802.3, Fast Ethernet, and Token Ring/IEEE

802.5.

Cisco Hierarchical Model

Core Layer – purpose is to switch traffic as quickly as possible. Fast transport to

enterprise services (internet etc). No packet manipulation, VLANs, access-lists. High

speed access required such as FDDI, ATM.

Distribution Layer – time sensitive manipulation such as routing, filtering and wan

access. Broadcast/Multicast, media translations, security.

Access Layer – switches and routers, segmentation occurs here and workgroup access.

Static (not dynamic) routing.

TCP/IP

Port Numbers

These are used to connect to various services and applications and piggy back onto IP

addresses. Common port numbers are:

20 - File Transfer Protocol – Data (TCP)

21 - File Transfer Protocol – Control (TCP) (Listens on this port)

22 - SSH (TCP)

23 - Telnet (TCP)

25 - Simple Mail Transfer Protocol (TCP)

53 - Domain Name Service (TCP/UDP)

69 - Trivial File Transfer Protocol (UDP)

80 - HTTP/WWW (TCP)

110 - Post Office Protocol 3 (TCP)

119 - Network News Transfer Protocol (TCP)

123 - Network Time Protocol (UDP)

161/162 - Simple Network Management Protocol (UDP)

443 - HTTP over Secure Sockets Layer (TCP)

TCP – (protocol 6) reliable, sequenced connection-oriented delivery, 20-byte header.

UDP – (protocol 17) connectionless, unsequenced best effort delivery, 8-byte header.

Sends data but does not check to see if it is received.

Telnet – used to connect to a remote device (TCP). A password and username is required

to connect. Telnet tests all seven layers of the OSI model.

FTP – connection orientated (TCP) protocol used to transfer large files.

TFTP – connectionless (UDP) protocol used for file transfer.

SNMP – allows remote management of network devices.

ICMP – supports packets containing error, control and informational messages. Ping

uses ICMP to test network connectivity.

ARP – used to map an IP address to a physical (MAC) address. A host wishing to obtain a physical address broadcasts an ARP request onto the TCP/IP network. The  host replies with its physical address.

DNS – resolves hostnames to IP addresses (not the other way around).

To configure the router to use a host on the network use the command

ROUTER(config)#ip nameserver 4.2.2.2 and to configure DNS the command

ip name-server is usually already turned on for the router config by default. If you want hosts on the network to use the router as a proxy DNS server put the command ROUTER(config)#ip dnsserver onto the router.

DHCP – involves a central server or devices which relays TCP information to hosts on a

network. You can configure a router to be a DHCP server with the below config. You

must have hosts on the same LAN as the router interface:

Router(config)#ip dhcp pool E00_DHCP_Pool

Router(dhcp-config)#network 10.10.10.0 255.255.255.0

Router(dhcp-config)#dns-server 24.196.64.39 24.196.64.40

Router(dhcp-config)#domain-name mydomain.com

Router(dhcp-config)#default-router 10.10.10.254

Router(dhcp-config)#lease 1

Cisco IOS

Six modes

User EXEC:- Router>

Privileged EXEC:- Router#

Global Configuration:- Router(config)#

ROM Monitor:- > or rommon>

Setup:- series of questions

RXBoot:- Router

Editing Commands

Ctrl+W - Erases a word

Ctrl+U - Erases a line

Ctrl+A - Moves cursor to beginning of line

Ctrl+E - Moves cursor to end of line

Ctrl+F - (or right arrow) – Move forward one character

Ctrl+B - (or left arrow) – Move back one character

Ctrl+P - (or up arrow) – Recalls previous commands from buffer

Ctrl+N - (or down arrow) – Return to more recent commands in buffer

Esc+B - Move back one word

Esc+F - Move forward one word

Tab - completes a command you have started

Router# copy ru _ press tab key after the ‘u’

Router# copy running-configuration

? gives you the command options

Router#copy ?

flash: Copy from flash: file system

ftp: Copy from ftp: file system

nvram: Copy from nvram: file system

running-config Copy from current system configuration

startup-config Copy from startup configuration

system: Copy from system: file system

tftp: Copy from tftp: file system (truncated to save space)

or the commands beginning with the letters you have typed:

Router#a?

access-enable access-profile access-template

Router Elements

DRAM – working area for router. Contains routing tables, ARP cache, packet buffers,

IOS and running config. Some routers run the IOS from DRAM.

show version – shows information about IOS in RAM and displays how much physical

memory is installed. Also shows the config register setting.

show process – shows info about programs running in DRAM.

show running-configuration – shows active configuration in DRAM.

show memory/stacks/buffers – to view tables and buffers

NVRAM – stores routers start up configuration. Does not lose data when powered off

due to a battery power source.

show startup-configuration

erase startup-configuration

copy running-configuration startup-configuration (copy run start)

Config register 0x2142 skips start up config file in NVRAM (for password recovery)

Config register 0x2102 loads start up config files from NVRAM

Flash – (EEPROM or PCMCIA card) holds the compressed operating system image

(IOS). This is where software upgrades are stored.

show flash

dir flash:

ROM – contains power on diagnostics, a bootstrap program and a mini IOS (rommon).

You can specify which file the routers boots from if you have more than one in flash

memory.

Router(config)#boot system flash {IOS filename}

Or that it boots from a TFTP server if for example the image is too large to fit in flash.

Router(config)#boot system tftp {IOS filename}{tftp address)

You can also back up the flash image for emergency use.

Router(config)#copy flash tftp

Router Management

Console port: a PC connected to the console port via a rollover cable. Used for initial

configuration or disaster recovery.

Virtual Terminals: normally accessed by telnetting to the router. Five lines available

numbered 0-4

Auxiliary port: normally a modem connected to this port.

TFTP server: the router can get its configs or IOS from a server (PC for example)

running TFTP software and holding the necessary files.

NMS: network management station. Uses SNMP to manage the router normally via a

web style interface.

CDP

Cisco Discovery Protocol runs only on Cisco devices (proprietary), it allows you to

gather information about other routers and switches. It is enabled by default.

Router#show cdp neighbors (note: Cisco uses US spelling conventions)

This command displays the neighbouring router or switches hostname, hardware

platform, port identifier and capabilities list.

Router#show cdp neighbors detail

This command displays more detail than the previous one. You can view IP address, IOS

release and duplex setting.

To turn CDP off an interface use the command:

Router(config-if)#no cdp enable

To turn CDP off on your entire router or switch use the command:

Router(config)#no cdp run

LAN Switching

A LAN switch has three primary functions:

1. Address Learning – maintains a table (CAM – Content Addressable Memory) table of

addresses and which port they can be reached on.

2. Forward/filter decision – forwards frames only out of the relevant port.

3. Loop avoidance - STP

Broadcast frames are forwarded out of all ports. Because ethernet hosts can all transmit

at the same time this can lead to collisions thus slowing down the network considerably.

Transmitting Frames Through a Switch

Store-and-Forward – switch copies the entire frame into its buffer and computes the

CRC. Frame is discarded if there is an error. High latency.

Cut-through – reads only the destination address (first 6 bytes after preamble), looks up

address and forwards frame. Lower latency.

Fragment free – switch reads first 64 bytes before forwarding the frame. Collisions

normally occur within the first 64 bytes.

Spanning Tree Protocol (STP) IEEE 802.1d

STP is a link management protocol that provides path redundancy whilst preventing

undesirable loops in the network. For communication to work correctly on an ethernet

network there can only be one path between two destinations. STP uses Bridge Protocol

Data Units (BPDU) received by all switches to determine the spanning-tree topology. A

port on a switch is either in forwarding or blocking state. Forwarding ports provide the

lowest cost path to the root bridge, a port will remain in blocking state from start up if

spanning tree determines there is a better path.

Rapid Spanning Tree Protocol (RSTP) IEEE 802.1w

Spanning tree takes up to 50 seconds to converge to a stable network whereas RSTP takes 2 seconds. RSTP port roles are root port, designated port, backup port, alternate port and disabled. Most implementations of RSTP use PVST+, Per VLAN Spanning Tree+, here multiple instances of Spanning Tree are running so the load on the CPU is higher but we can load share over the links.

To enable RSTP for each VLAN in our switched network we use the following

command:

Switch(config)#spanning-tree mode rapid-pvst

Bridging / Switching

Bridges are primarily software based and have one spanning-tree instance per bridge.

Normally 16 ports per bridge. LAN Switches are primarily hardware based. Many

spanning-tree instances per switch and up to 100 ports.

Virtual LAN (VLAN)

A VLAN is a switched network that consists of logically segmented communities without

regard to physical location. Each port on a switch can belong to a VLAN. VLAN ports

share broadcasts. A router is needed to route traffic between VLANs because layer 2

devices do not use IP addresses. Reduces admin costs, tighter security and better control of broadcasts

Subnetting

Max # of Subnets = 2(to the power of masked bits) (– 2 if subnet zero not allowed)

Max # of Hosts (per subnet) = 2(to the power of unmasked bits) – 2

Easy Subnetting

What network is host 172.16.5.68 255.255.255.240 on?

256-240 = 16 so you have the subnets going up in increments of 16 starting with zero (if

subnet zero is permitted in the exam). Each subnet will need to have a subnet and a

broadcast number so this leaves 14 hosts per subnet. The subnets start at 0,16,32,48, 64,

80….224, 240 (the 0 and 240 are only valid if subnet zero is allowed).

IPV6

An IPv6 address consist of 128 bits represented in hexadecimal format separated into

eight parts e.g. EEDE:AC89:4323:5445:FE32:BB78:7856:2022. There are no broadcast

packets, only anycast – multicast – unicast.

The two methods of migrating from IPv4 to IPv6 are dual stack and tunnelling. Cisco

IOS support IPv6 commands in version 12.2(2)T and later.

IP Routing

Routers must have some means of learning networks that are not directly connected.

Static routing:

Router(config)#ip route {destination network}{mask}{next hop address}

e.g ip route 172.16.5.2 255.255.255.0 172.16.12.8

Dynamic addressing is done by using a routing protocol:

for RIP v2

Router(config)#router rip

Router(config-router)#version 2

Router(config-router)#network 172.16.0.0

Router(config-router)#no auto-summary _ optional

for EIGRP

Router(config)# router eigrp 20

Router(config-router)#network 172.16.0.0

Router(config-router)#no auto-summary _ optional

for OSPF

Router(config)#router ospf 20

Router(config-router)#network 172.16.0.0 0.0.255.255 area 0

Facts

RIP v2

Uses UDP port 520

Classless

Max hop count 15

Multicasts route updates to 224.0.0.9

Supports authentication

Update timer 30 seconds

Invalid 90 seconds

Hold down 180 seconds

Flush 270 seconds

EIGRP

Uses IP protocol 88

Classless

Hybrid of distance vector and link state

Multicasts updates to 224.0.0.10

Uses feasible successors to determine alternative routes to networks.

The feasible successor is a backup route based upon the topology table.

OSPF

Uses IP protocol 89

Classless

Uses Dijkstras shortest path algorithm (SFP)

Router ID is the highest IP address but loopback address used if present

Backbone area is area 0

All non backbone areas must connect directly to area 0

Areas can be numbered from 0 to 65535

Multicasts on 224.0.0.5

OSPF uses cost as a metric (see below - * indicates the most common)

Interface                                                                               Cost (108/Bandwidth)

ATM, Fast Ethernet, Gigabit Ethernet, FDDI (> 100 Mbps)   1

HSSI (45Mbps)                                                                         2

16 Mbps Token Ring                                                                  6

10 Mbps Ethernet                                                                      10

4 Mbps Token Ring                                                                   25

T1 (1.544 Mbps)*                                                                     64

DS-0 (64k)*                                                                              1562

56k                                                                                            1785

Distance Vector

Distance Vector protocols understand the direction and distance to any given network

connections. Algorithms calculate the cost to reach the connection and pass this

information to every neighbour router. Examples are RIP and IGRP. Problems with

distance vector protocols include routing loops and counting to infinity.

To overcome these problems the following can be implemented:

Defining a maximum number of hops, 15 for RIP and 255 for IGRP

Split Horizon – if the router learns a route on an interface do not advertise it out of the

same interface.

Route Poisoning – Information passed out of an interface is marked as unreachable by

setting the hop count to 16 (for RIP).

Hold Down Timers – ignores new routing updates until a determined time has passed.

Triggered Updates – instead of routing updates being sent at the default intervals; a

triggered update is sent every time to indicate a change in the routing table.

Link State

These have a picture of the entire network from link state advertisements (LSA) and link

state packets (LSP). Once these have all been passed only changes to the network are

sent out reducing network traffic.

Link state protocols do require a lot of CPU time and bandwidth when LSAs are flooded

out. Examples are OSPF and ISIS.

Routers use administrative distances to determine how believable the route learned is

depending upon the protocol it learns the router from.

Source                                         Default Distance

Directly Connected Interface               0

Static hop to next router                      1

EIGRP Summary                                5

External BGP                                     20

EIGRP (Internal)                                90

OSPF                                                110

IS-IS                                                 115

RIP                                                   120

Exterior Gateway Protocol (EGP)      140

External EIGRP                                 170

Internal BGP                                     200

Unknown                                          255

An administrative distance of 0 is most preferred. So a router running RIP and OSPF will prefer the OSPF routes most and install these in the routing table.

Routing protocols maintain a table of hosts and which interface they can be reached by.

Examples RIP, OSPF

BGP is an exterior gateway protocol. It is used to connect autonomous systems together.

Routed protocols are used to transport traffic from source to destination. Examples: IP,

IPX, Appletalk.

When a packet traverses the network from device to device (hop to hop) the IP address

remains constant, the hardware (MAC) address changes.

NAT

Network Address Translation will convert and address from the inside of your network to

another address on the outside of your network and vice versa. It is most commonly used to convert a non-routable address to a routable address.

For all configs, you must specify which interfaces are internal for NAT and which are

external:

Router(config-if)#ip nat inside/outside

Static NAT – maps one address to one address such as 192.168.1.1 to 200.1.1.1

Router(config)#ip nat inside source static 192.168.1.1 200.1.1.1

Dynamic NAT – maps a number of internal addresses to a pool of external addresses. The below config creates a pool of 10 addresses with a mask (prefix length) of 255.255.255.0

and the name ‘ad_team.’ The hosts to be NATted are on the 192.168.1.0 network. The

access list (source list) tells the router which addresses to NAT.

Router(config)#ip nat pool ad_team 10.0.0.1 10.0.0.10 prefix-length 24

Router(config)#ip nat inside source list 1 pool ad_team out

Router(config)#access-list 1 permit 192.168.1.0 0.0.0.255

Overload NAT – (or PAT) maps private internal addresses to one or more external

addresses using port numbers. The below config creates a pool of ten addresses (it could be more) and the command ‘overload’ tells the router to use port address translation.

Router(config)#ip nat pool ad_team 10.0.0.1 10.0.0.10 prefix-length 24

Router(config)#ip nat inside source list 1 pool ad_team out overload

Router(config)#access-list 1 permit 192.168.1.0 0.0.0.255

Wireless Networking

Wireless Basics

Wireless clients connect to access points. The two wireless modes are ad-hoc and

infrastructure. Ad hoc is similar to peer-to-peer networking where nodes connect directly

to each other. They must have the same SSID and channel for this to work. In

infrastructure mode the clients connect to the access point. They can be via basic service set (BSS – 1 access point and multiple clients) or extended service set (ESS – 2 or more BSS’).

Wireless Security

The two methods for wireless authentication are open system and shared key. In open

system the host sends an association request to the wireless access point and it will be

sent a success or failure message. With shared key, a key or pass phrase is configured on both the host and access point.

There are three types of shared key authentication WEP,

WPA and WPA2.

WEP is an encryption algorithm built in the 802.11 standard. It uses RC4 40bit or 104 bit

keys and a 24bit initialization vector.

WPA uses dynamic key management, adds a stronger encryption cipher and is built on

the EAP/802.1X mechanism. It uses TKIP, Temporal Key Integrity Protocol and the

Initialization Vector is increased to 48bit (more then 500 trillion key combinations). It is

used with RADIUS in the enterprise.

WPA2 is the next generation in wireless security. It uses even stronger encryption than

WPA and this is achieved by using AES, Advanced Encryption Standard. Also WPA2

creates a new key for every new association this has a benefit over WPA that the client's

keys are unique and specific to that client.

Network Security

Access Lists

Access lists are a set of conditions that permit or deny access to or through a routers

interface.

Range                     Usage

1-99                        IP Standard

1300-1999              IP Standard (Expanded Range)

100-199                  IP Extended

2000-2699              IP Extended (Expanded Range)

Standard Access Lists

Standard IP access lists check only the source address of the packet and permits or denies

the entire TCP/IP suite. You cannot choose a particular port or application to block.

Cisco recommends that they are placed as close to the destination as possible.

Router(config)#access-list{number 1-99}{permit/deny}{source address}

access-list 10 permit 172.16.5.2 _ address can be a host or network

Extended Access Lists

These allow for a lot more granularity when filtering IP traffic. They can filter packets

based upon source or destination, a particular IP protocol and port number. Cisco

recommends that they are placed as close to the source as possible.

Router(config)#access-list {number 100-99}{permit/deny}{protocol}

{source}{destination}{port}

access-list 112 permit tcp host 172.16.5.2 host 172.16.10.2 eq www

Named Access Lists

Router(config)#ip access-list {standard/extended} name

Router(config)#ip access-list extended no_ftp

Access lists applied to inbound interfaces save the router having to process the packet,

denied packets will be dropped at the interface. Outbound access lists will be processed

by the router and then dropped at the outbound interface if they match the access list.

Access lists can be applied to multiple interfaces but there can only be one access list per

protocol per direction per interface.

Wildcard masks tell the router which parts of the address to look at and which to

disregard.

access-list 12 permit 172.16.5.0 0.0.0.255

This would permit any host on network 172.16.5.x

Access lists are applied to interfaces:

Router(config)#access-list 1 permit 172.16.5.2

Router(config)#interface e0

Router(config-if)#ip access-group 1 in

Use the term ‘access-class’ if applying to console/aux/vty lines

show ip access-lists

show access-list 1

Packets are processed by the access list and then routed.

Passwords (command ‘service password-encryption’ encrypts all passwords)

Enable: used to get from user exec to privileged exec. Not encrypted.

Router(config)# enable password {password}

Enable Secret: Encrypts password (only use enable or enable secret not both)

Router(config)# enable secret {password}

VTY: needed if telnet access is required.

Router(config)#line vty 0 4

Router(config-line)#password cisco

Router(config-line)#login

Auxiliary: allows modem access to the aux port.

Router(config)#line aux 0

Router(config-line)#password cisco

Router(config-line)#login

Console: used to allow console access

Router(config)#line console 0

Router(config-line)#password cisco

Router(config-line)#login

Protecting the Network

Firewalls divide your network into three zones – trusted, semi-trusted and un-trusted.

A VPN allows information to be send securely over an insecure medium (e.g. the

internet). A VPN can be site to site (e.g. WAN) or access (e.g. home worker).

Security Device Manager (SDM)

SDM is a GUI web based tool which will allow you to configure and manage your Cisco

routers. It can be installed on your router or your PC. To install and configure SDM you

will need to refer to www.howtonetwork.net or the CCNA theory guide because there are

a huge amount of parameters and screens to navigate.

Wan Protocols and Services

HDLC – Cisco default on serial WAN connections. No authentication available.

PPP – data link. Uses PAP (clear text) and CHAP (secure hash) authentication.

Authentication is optional. Use PPP if connecting a Cisco router to a non-cisco router.

Router(config)#hostname paul password cisco _ case sensitive

Router(config)#interface serial 0

Router(config-if)#encapsulation ppp

Router(config-if)# ppp authentication chap

Frame Relay

Based upon x.25 protocol but with less error checking so is quicker. Normally 56k to

2mb so ideal for SMEs. Works at the physical & data link layers. DLCI’s are used to

identify the circuit. Each router uses LMIs for keepalives on the line between the router

and frame relay switch. LMI type is Cisco by default. You must use another type such

as ansi if connecting to a non-cisco router.

Router(config-if)#encapsulation frame-relay

Router(config-if)#frame-relay map ip 2.2.2.2 100

Here the router is told to get to ip address 2.2.2.2 use dlci 100.

Frame Relay Problems include:

Incorrect LMI setting

Incorrect DCLI

Split horizon preventing routing updates leaving interface

Use frame relay sub-interfaces if point-to-point or multipoint connection is needed. IP

address applied to sub-interfaces for these and NOT the main interface.

Frame relay uses backwards explicit congestion notification (BECN) on returning frames

to warn of congestion and forward explicit congestion notification (FECN) is set by the

DCE end to warn of congestion from the sending end.

Troubleshooting

Always use a systematic and methodical approach to troubleshooting.

The first command to issue is ‘show ip interface brief’ to establish if the interfaces

are down or up. There are only a handful of ways to break any network in the exam.

Layer 1

Ensure that there is a clock rate on the DCE interface (use the ‘show controllers

serial X’ command to check what type of cable is attached – where X is the serial

interface number).

Ensure that the ‘no shut’ command has been applied to the interface.

Layer 2

Ensure that the correct encapsulation type is on the interface i.e. HDLC, PPP etc (use the

‘show interface serial X’ command to check).

If it is not then go into interface configuration mode and change it.

Layer 3

Ensure that the correct IP address AND subnet mask is applied to the interface.

Ensure that the correct networks are being advertised by the routing protocol (‘show ip

protocols’).

Always ensure that you can ping across directly connected router interfaces BEFORE

applying routing protocols and access lists. You have been warned.

Characteristic	OSPF	RIPv2	RIPv1
Type of protocol	Link state	Distance vector	Distance vector
Classless support	Yes	Yes	No
VLSM support	Yes	Yes	No
Auto-summarization	No	Yes	Yes
Manual summarization	Yes	No	No
Discontiguous support	Yes	Yes	No
Route propagation	Multicast on change	Periodic multicast	Periodic broadcast
Path metric	Bandwidth	Hops	Hops
Hop count limit	None	15	15
Convergence	Fast	Slow	Slow
Peer authentication	Yes	Yes	No
Hierarchical network	Yes (using areas)	No (flat only)	No (flat only)
Updates	Event triggered	Route table updates	Route table updates
Route computation	Dijkstra	Bellman-Ford	Bellman-Ford