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CERTguide's CCNA 2.0 Study Guide
 
Boot Sequence can be changed by using the boot system command.  i.e. 
boot system tftp igs-j-l.103.7.4 24.128.102.7  will look for an IOS file named igs-l.103.7.4 on a  tftp server with the IP address of 24.128.102.7 to boot from.

Key Sequences  
TAB - finish command if it has enough characters to recognize.
CTRL A - Move to beginning of line
CTRL B -Move back one character without erasing
CTRL E - Move to end of line
CTRL F - Move forward one character without erasing
CTRL N -Move forward one command.
CTRL P -Show previous command (also up arrow)
CTRL R - Repeats previous command line
ESC  B - Moves cursor to 1st character of previous word.
ESC F - Moves cursor to 1st character of next word.
Backspace - deletes characters and moves left.

Setup
Setup Mode - Setup mode can be accessed by typing setup while in the privileged exec mode or by clearing the startup config
anything you see in square brackets when in setup mode means it is either the default setting or the current setting.

3 methods to access a router
Console
AUX
Telnet - connect to other hosts using VT100 emulation.  Routers will host 5 concurrent Telnet sessions.

Router Prompts
Router> - user exec mode
Router# - privileged exec mode
Router(config) - global configuration mode
Router(config-if) - Interface configuration mode
Router(boot)>or# - no IOS was found on boot.

Memory
There are 4 types of router memory:
Flash - this is where the OS is stored.
RAM - This is where your running config is stored.
ROM - stores bootstrap version of IOS in case it can't be found at boot up.
NVRAM - permanent memory located on memory chips that stores the startup config.

Commands
Hostname - Changes the hostname of the router.
Enable- brings you from user exec to privileged exec mode
Disable - brings you from privileged exec mode to user exec mode
Show Hosts - display all known hostnames and their IPs
Show startup-config - displays contents of the file used to load configuration info during boot process. The file is stored in NVRAM.
Show running-config - display current running configuration that is stored in RAM.  This allows you to view changes before saving to NVRAM.
Show Flash - displays names and file sizes of files stored in flash memory.  Typically the IOS file.  Displays info about available, used and free flash memory.
Show Memory - displays info regarding memory usage of the router, the process memory location and free memory pool statistics.
Show Protocols - shows which protocols are configures globally & status and protocol address on each interface.
Show Processes - display processor usage info and process ID's of active processes.
Show Version - displays IOS version, configuration registry settings.  available from any mode.
Show Interfaces - used to check the status of all interfaces including interface address, interface state, maximum transmission unit size, encapsulation type and traffic stats.
IP Address - used to set interface IP address.  Command is: ip address ip address netmask  i.e. ip address 10.1.2.3 255.255.255.0
Show IP route - displays current contents of the Internet Protocol routing table. Routers add info that they learn to their routing table.  Routing table is built dynamically and kept in RAM.  This command will also display any static routes.
Show Sessions - displays info about telnet (VTY) connections to the router including IP address and session statistics.
Message of the day (MOTD) - configured from Global config mode.  Welcome message to be shown when a Telnet or console session is started.  To set a welcome message use the following command:
banner motd %This is the message%
You can use any character for the delimiter before and after your message but you must use the same character for the beginning and ending delimiter.  i.e.&This is the message&
Exit - In privilege or user exec mode, exit will terminate your session.  In config mode, exit will bring you to privilege mode.
CTRL+SHIFT+6 followed by an X - allows you to suspend an active telnet session & return to a router prompt without disconnecting the telnet session.
CTRL-Z - Exits you from the config mode.  Config changes are put in RAM when you hit enter. You must save to NVRAM to have after reboot.

Flow Control - 3 methods (flow control prevents network congestion.)
Buffering - stores packets in memory to process later.  Packets are discarded if buffer is full
Source Quench Messaging - The receiving device sends a message saying its buffer is full and to stop.  The receiving device will then send a message to start again.
Windowing - Sender waits for acknowledgement after X number of packets.  The number of packets is the window size.

OSI Model - The Open Systems Infrastructure (OSI) model is a seven layer model used to give us a framework for how data is transmitted.  The advantages of the OSI layered model are:
It makes it easier to learn Protocol specifications.
It simplifies the development of applications
It provides better interoperability between vendors Aids in troubleshooting.

Application Layer - Establishes availability of the intended communication partner.  Resource Availability.  Synchronize sending and receiving.  Telnet, SMTP, FTP, WWW are examples of Application Layer.

Presentation Layer - Translates data into format for transmission.  Data Compression & Encryption. The Presentation Layer is responsible for keeping standards so the presentation layer on the receiving system can process the data.  ASCII, EBCDIC, GIF, JPEG, MPEG are examples.

Session Layer - System independent processor program that provides services to the end user.  Responsible for opening, maintaining and closing communication sessions.  Examples are SQL, ASP, RPC, NFS.

Transport Layer - Flow Control (amount of data transferred at once), Multiplexing (several apps can send data onto one physical connection), Virtual Circuit Management (open, maintain and close virtual circuits), Error Checking, Segment to Segment Communication.  Orders and numbers frames and reassembles them into order on the other side.  TCP (port based) and SPX (socket based) reside here as well as UDP.

Network Layer - Determines best route, Logical Addressing (IP address, AppleTalk Address). End to End delivery of packets.  IP and IPX

Data Link Layer -  Physical Addressing (MAC address),  CRC (Cyclic Redundancy check)  is added at this layer.  Bridging and Switching.  Data link address types are Unicast (host to host communication), Multicast (host to group communication), and Broadcast (host to entire segment)

    MAC SubLayer - 802.3, 802.4, 802.5

    LLC SubLayer - 802.2
 
Physical Layer - Sends and Receives bits.  The physical media and hardware.  NIC cards, cables, hubs, repeaters.
 
Connection-Oriented Communication - uses error checking and flow control.  guaranteed delivery.  Examples are TCP, SPX.

Connectionless Communication -  Packets are sent without error checking or flow control. There is no guaranteed delivery.  Deemed unreliable.  Examples are IP, IPX, UDP

Data Encapsulation 5 steps
A good pneumonic for remembering this is:  Do some people fry bacon? (Data, Segments, Packets, Frames, Bits)

User info is converted to data (Application Layer)
Data is converted to Segments (Transport Layer)
Segments are converted to packets (Network Layer)
Packets are converted to frames (Data Link Layer)
Frames are converted to Bits and put on media (Physical Layer) 

Basic steps of routing
1) Data is sent from source node to router closest to source
2) Data travels to router closest to destination address
3) Data is sent to destination address

Routing Protocols

Distance-Vector routing protocol - uses metrics (hop count) to find best route.  sends update broadcast every 30 seconds (sends routing table to neighbors). Limited to 15 hops then packet is dropped.

RIP- RIP is a distance-vector protocol.  RIP is good for small-medium networks.  Low overhead and simple to implement. To start RIP, type Router RIP from Config mode. Next add the network.  Router(config-router)# network1.2.3.4
The routing invalid timer is set to 90 seconds by default. (amount of time before a route is marked invalid)
The route flush timer is set to 240 seconds by default. (amount of time after a route is marked invalid before it is removed from the routing table)

IGRP (Interior Gateway Routing Protocol) - IGRP is a distance-vector routing protocol.  Maximum hop count is 255.  IGRP is capable of multipath routing (if two equal routes are available, the packets are sent to one, then the other to utilize available bandwidth)  Sends broadcast updates every 90 seconds to neighbors.  Default holddown timer is 3 times the update timer plus 10 seconds.  Default is 280 seconds (the holddown timer prevents routing loops while routers converge)

Link State Routing Protocol - Uses metric/bandwidth combo to find best route.  builds routing table & topology map of network.  Updates broadcast every 2 hours (sends routing table to neighbors)  Makes more accurate routing choices.  Requires more processor and RAM than Distance-Vector.

Balanced Hybrid Routing Protocol - combination of distance-vector and link-state.  Better decisions than distance-vector with less overhead than link-state

OSPF (open shortest path first) - OSPF is a link-state protocol.

IP Addressing

These are some of the well known ports that you should be aware of:
21 - FTP
23 - Telnet
25 - SMTP
69 - TFTP
80 - HTTP
161 - SNMP
 

Subnetting - you will need to know how to subnet an IP address.  Please read the subnetting section in our TCP/IP tutorial.  http://www.CERTguide.com/tcpip.htm

Switching Methods

Store & Forward - wait for entire frame before sending.

Cut Through - starts forwarding as soon as destination arrives.  Doesn't wait for entire frame.

Fragment Free - After receiving 64 bytes it assumes that the rest is good and starts sending.

Network Hardware

Bridges (transparent) - Doesn't change packet, just directs it.  Splits network into segments and forwards or filters packets.  Works at Layer 2. Uses Spanning Tree Protocol to prevent loops.  Uses store and forward switching method which increases latency.

Switches - work at Layer 2 of the OSI model.  Broadcast and Multicast are forwarded.  Uses store and forward switching method which increases latency.

Routers - Creates broadcast and collision domain as a router (unlike hubs, switches and bridges) will not forward broadcasts.  Routers require address planning as they work at Layer 3 using Network addressing.  Receives entire packet before transmitting which increases latency.

Spanning Tree Protocol

Used by switches and bridges to prevent loops.  Has redundant paths for failover although only one can be active at a time.  The four states are:
Blocking
Listening
Learning
Forwarding

Ethernet

Access Lists

There are two types of access lists.  You can have one access list per interface.

Standard - based on source IP.

Extended - based on source IP, destination IP, Port #, Protocol type.

The format for issuing the access list command  for extended IP is as follows:

access-list (access list #) (permit or deny) (protocol) (source) (destination) (port name or #)

access-list 115 deny tcp host 24.128.188.23 host 24.128.102.7 eq 80

This will prevent the host 24.128.188.23 from connecting to host 24.128.102.7 with http traffic (port 80)

The format for issuing the access list command for extended IPX is as follows.

access-list (access list #) (permit or deny) (protocol) (source network) (destination network)

access-list 913 deny -1 26 46

This will prevent all traffic (-1 designates all protocols) from passing from network 26 to network46.

Wide Area Networks (WAN)

Frame Relay

show frame-relay PVC displays the status of frame relay connections and shows traffic stats for the connection.

show frame-relay LMI displays traffic generated by the LMI (local management interface) such as the number of messages between the frame switch and the local router.

show frame-relay map lists the DLCI commands.

show frame-relay interfaces serial shows info on DLCI's and LMI's but does not include traffic info.

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