Starting from the bottom-layer of the OSI model and working your way up to the top

Starting from the bottom-layer of the OSI model and working your way up to the top

Application

 

·         OSI Layer #7 - Provides services such as e-mail, web browsing, and file transfer services: determines resource availability.

 

Presentation

 

·         OSI Layer #6 - Presents data to the application layer: Multimedia, encryption, decryption, and data compression.

 

Session

 

·         OSI Layer #5 - Set up, manage, and terminate sessions or dialogues between devices, which take place over logical links: the joining of two software applications.

 

Transport

 

·         OSI Layer #4 - Break down the data from the higher layers into smaller parts (segments). Virtual circuits are set up here, which are required before devices can communicate: Flow Control, Windowing, Acknowledgements

 

Network

 

·         OSI Layer #3 - Takes segments from the Transport Layer and breaks them down into smaller units called packets: determines the best path to take from one network to another.

 

Data Link

 

·         OSI Layer #2 - Chops down packets into smaller units referred to as frames. Layer 2 switches work at this layer and use the hardware or MAC addresses, so they can switch traffic much faster because there is no need to check IP addresses and routing tables: WAN protocols work at this layer, including HDLC, ISDN, and PPP. Ethernet also works at this layer.

 

Physical

 

·         OSI Layer #1 - Frames are converted into bits for placing on the wire. These bits consist of electrical pulses which correspond to on and off pulses, or 1s and 0s in binary: hubs work at this layer, and here is where you will find cable specifications, such as RJ45.

 

Flow Control

 

·         If the receiving system is being sent more information than it can process, it will ask the sending system to stop for a short time. The packet sent telling the other device to stop is known as a source quench message.

 

Windowing

 

·         Each system agrees upon how much data is to be sent before an acknowledgement is required. This "window" opens and closes as data moves along in order to maintain a constant flow.

 

Acknowledgements

 

·         When a certain amount of segments is received, the fact that they all arrived safely and in the correct order needs to be communicated to the sending system.

 

Three-way Handshake

 

·         Where you send packets to establish the session. The first packet is called a synchronize (SYN) packet. Then the remote device responds with a synchronize-acknowledgement packet (SYN-ACK) packet. The session is established in the third phase when an acknowledgement (ACK) packet is sent. That is all done via the TCP service.

 

Bottom-up Method (Troubleshooting)

 

·         Starting from the bottom-layer of the OSI model and working your way up to the top.

 

debug ip ftp

 

·         Debugs FTP traffic

 

copy flash tftp

 

·         Store a backup copy with tftp

 

running config tftp

 

·         Back up your running configuration file with tftp

 

debug tftp

 

·         Debugs TFTP traffic

 

Telnet

 

·         The only utility that can check all seven layers of the OSI model.

 

ICMP

 

·         This protocol specifically delivers messages inside IP packets. For instance, because pings have a TTL (Time-To-Live field, they give a good indication of network latency (delay).

 

Ping

 

·         Command that provides granularity by allowing you to specify source, quantity, and size you're sending, as well as other parameters. Messages encapsulated in IP packets: includes notations on Cisco devices such as !, ., U, N, P, Q, M, ?. Because attacks with this protocol are common, this protocol is usually blocked on a network.

 

Traceroute

 

·         Follows the destination IP packets by sending UDP packets with a small maximum TTL field, and then listens for an ICMP time-exceeded message. As this protocol's packet progresses, the records are displayed hop by hop. Each hop is measured three times. An asterisk indicates that a hop has exceeded its time limit: includes notations on Cisco devices such as ..., U, H, P, N, ?, Q.

 

Proxy ARP

 

·         Enables hosts on an Ethernet network to communicate with hosts on other subnets or networks, even though they have no knowledge of routing. Source and destination addresses never change but in order for the packet to be passed to a next-hop address, the MAC address (in the frame) changes between devices.

 

GARP (Gratuitous Address Resolution Protocol)

 

·         An ARP request packet where the source MAC, the source IP, and the destination IP addresses are all set to the IP address of the machine issuing the packet, and the destination MAC address is the broadcast address FFFF:FFFF:FFFF. Ordinarily, no reply packet will occur. Has the objective of updating the Layer 2 CAM table

 

Straight-Through Cable

 

·         If each pin on one end matches the other side (RJ45)

 

Rollover/Console Cable (flat cable)

 

·         Reverse order of pins on one side of a cable to the other side (RJ45).

 

Terminal Configuration Settings

 

·         Bits per second: 9600
Data bits: 8
Parity: None
Stop bits: 1
Flow control: None

 

 

User Mode

 

·         The first mode you'll be presented with when the Cisco device boots. Useful for basic Cisco device commands, but is severely limited in which commands can be executed.

 

Privileged Mode

 

·         Useful for looking at the entire configuration of the router, the statistics about how it is performing, and even which modules you have connected to the Cisco device.

 

Global Configuration Mode

 

·         Get into this with the command config t. In this mode you can actually configure the Cisco device

 

Interface Configuration Mode

 

·         Allows you to enter commands for individual router interfaces, such as FastEthernet, Serial, etc.

 

show ip interface brief

 

·         Command that will allow you to see which interfaces are available.

 

Line Configuration Mode

 

·         You can control who has access to the Cisco Device in this configuration mode, as well as assign passwords, or enable a security feature called ACLs (Access Control Lists).

 

Router Configuration Mode

 

·         In order to configure a routing protocol onto the router so it can dynamically build a picture of the network, you'll need to be in this mode.

 

VLAN Configuration Mode

 

·         This mode actually only applies to switches, but is used to configure VLANs (Virtual Local Area Networks).

 

 

Interface Notation

 

·         Consists of an Interface type (Ethernet, FastEthernet, etc.) & an Interface slot/ module + port number.

 

Basic Router Interface Configuration

 

·         Speed, Duplex, IP address

 

Description

 

·         Command that allows you to configure a description to an interface (Interface Configuration Mode).

 

show version

 

·         Command that shows useful information that might represent a starting point in verifying most of the router operations.

 

show ip route

 

·         Command that provides deep information regarding the routing capabilities of the device. Lists all the networks the router can reach and information about the way they can be reached.