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Cisco CCNA

OSI Reference / Network Protocols

– The application layer provides services directly to applications. The functions of the application layer can include identifying communication partners, determining resource availability, and synchronizing communication . Some examples of application layer implementations include TCP/IP and OSI applications such as Telnet, FTP, and SMTP, File Transfer, Access, and Management (FTAM), Virtual Terminal Protocol (VTP), and Common Management Information Protocol (CMIP).

–The presentation layer provides a variety of coding and conversion functions that are applied to application layer data. These functions ensure that information sent from the application layer of one system will be readable by the application layer of another system. Examples of presentation layer coding and conversion schemes include ASCII, EBCDIC, JPEG, GIF, TIFF, MPEG, QuickTime, various encryption methods, and other similar coding formats.

–The session layer establishes, manages, maintains, and terminates communication sessions between applications. Communication sessions consist of service requests and service responses that occur between applications located in different network devices. Some examples of session layer implementations include Remote Procedure Call (RPC), Zone Information Protocol (ZIP), and Session Control Protocol (SCP).

– The transport layer segments and reassembles data into data streams. It is also responsible for both reliable and unreliable end-to-end data transmission. Transport layer functions typically include flow control, multiplexing, virtual circuit management, and error checking and recovery. Some examples of transport layer implementations include Transmission Control Protocol (TCP), Name Binding Protocol (NBP), and OSI transport protocols.

–The network layer uses logical addressing to provide routing and related functions that allow multiple data links to be combined into an internetwork. The network layer supports both connection-oriented and connectionless service from higher-layer protocols. Network layer protocols are typically routing protocols. However, other types of protocols, such as the Internet Protocol (IP), are implemented at the network layer as well. Routers reside here at the network layer. Some common routing protocols include Border Gateway Protocol (BGP), Open Shortest Path First (OSPF), and Routing Information Protocol (RIP). Packets and datagrams are sent across this layer of the OSI model.

Data Link
– The data link layer provides reliable transmission of data across a physical medium. The data link layer specifies different network and protocol characteristics, including physical addressing, network topology, error notification, sequencing of frames, and flow control. The Data link layer is composed of two sublayers known as the Media Access Control (MAC) Layer and the Logical Link Control (LLC) layer.
This can be seen in the following diagram:

The LLC sublayer manages communications between devices over a single link of a network. LLC supports both connectionless and connection-oriented services used by higher-layer protocols. The MAC sublayer manages protocol access to the physical network medium. The IEEE MAC specification defines MAC addresses, which allow multiple devices to uniquely identify one another at the data link layer.

Data link layer implementations can be categorized as either LAN or WAN specifications. The most common LAN data link layer implementations include Ethernet/IEEE 802.3, Fast Ethernet, FDDI, and Token Ring/IEEE 802.5. The most common WAN data link layer implementations include Frame Relay, Link Access Procedure, Balanced (LAPB), Synchronous Data Link Control (SDLC), Point-to-Point Protocol (PPP), and SMDS Interface Protocol (SIP).

– The physical layer defines the electrical, mechanical, procedural, and functional specifications for activating, maintaining, and deactivating the physical link between communicating network systems.
Physical layer specifications define such characteristics as voltage levels, timing of voltage changes, physical data rates, maximum transmission distances, and the physical connectors to be used. Physical layer implementations can be categorized as either LAN or WAN specifications. Some common LAN physical layer implementations include Ethernet/IEEE 802.3, Fast Ethernet, FDDI, and Token Ring/IEEE 802.5.Some common WAN physical layer implementations include High-Speed Serial Interface (HSSI), SMDS Interface Protocol (SIP), and X.21bis.

Steps of Data Encapsulation

  1. User information is converted to data

  2. Data converted to segments

  3. Segments converted to packets or datagrams

  4. Packets and datagrams are converted to frames

  5. Frames are converted to bits

Data link addresses: Physical address. Flat addressing scheme, physical address burned into network card (MAC address)

Network address: Logical address. IP or IPX – hierarchical scheme, assigned to a machine manually or dynamically.

IP Address Classes

Class A



1 – 127

127 networks, 16M nodes

Class B



128 – 191

16K networks 65K nodes

Class C




2M networks 254 nodes

Subnetting Formulas
(count the bits only from the Node portion of the address. Therefore, for a Class B address, the total masked bits + unmasked bits = 16):

Max # of Subnets: 2(masked bits)-2

Max # of Hosts (per subnet): 2(unmasked bits)-2


To turn on

ipx routing

Then, on interface

ipx network {#} encapsulation {sap, arpa, snap, hdlc, novell-ether} {sec}

ipx network 3100 encapsulation sap sec

To monitor

sh ipx traffic

sh ipx int e0

Frame Types

802.3 – novell-ether – default

802.2 – sap

Ethernet_II – arpa

Ethernet_snap – snap

LAN Switching

All nodes on an ethernet network can transmit at the same time, so the more nodes you have the greater the possibility of collisions happening, which can slow the network down.

LAN Segmentation: breaking up the collision domains by decreasing the number of workstations per segment.

FastEthernet (100bt) – provides 10 times the bandwidth of older 10bastT Ethernet. Must have Cat5 cable, no longer than 100 meters, and FastEthernet NIC’s and Hubs/Switches

Full-Duplex Ethernet – can provide double the bandwidth of traditional ethernet, but requires a single workstation on a single switch port, and NIC must support it. Collision free because there are separate send and receive wires, and only one workstation is on the segment. Half-Duplex must provide for collision detection, therefore can only use 50% of bandwidth available

Bridges – examines MAC address, and forwards frames unless the address was local. Forwards to all other segments it is attached to. Forwards multicast packets, so broadcast storms can occur.

Routers – examines network address, and forwards using the best available route to destination network. Can have multiple active paths.

Switching – examines MAC address. Same as multiport bridge.

Store-and-Forward – copies entire frame into buffer, checks for CRC errors. Higher latency. Used by Catalyst 5000 switches

Cut-Through – reads only the destination address into buffer, and forwards immediately. Low latency

Spanning-Tree Protocol (STP) IEEE 802.1d. – developed to prevent routing loops. STA (Spanning-Tree Algorithm) is implemented by STP to calculate a loop-free network topology. In Catalyst 5000 network, BPDUs are send and received by all switches, and processed to determine the spanning-tree topology.

Virtual LAN’s – have different ports on a switch be parts of different subnetworks. Some benefits: Simplify moves, adds, changes. Reduce adminstrative costs, better control of broadcosts, tighten security, distribute load. Relocate server into secured locations.


IOS / Routing / Network Security

  • Cisco IOS (operating system) is stored in flash memory (EEPROM)

  • IOS configuration is stored in NVRAM

User Mode
– ordinary tasks – checking status, etc. Need password depending on how you’re entering (Virtual Terminal pw for telnet session, Auxiliary pw for aux port, Console pw for console port)

conf t

line vty 0 {line aux 0} {line con 0}


password letmein

Privileged Mode

conf t

enable password letmein


conf t

banner motd #


conf t

hostname MyRouter


CTRL+A – beginning of line

CTRL+E – end of line

show history

TAB completes command


Press ? after any command for a list of what comes next

Router Elements/Configuration

show startup-config

show running-config

copy running-conifg startup-config

erase startup-config



boot system {flash / tftp}

copy flash tftp

copy tftp flash

copy run tftp

copy tftp run

show proc

show mem

show buff

show flash

show cdp

Routing Protocols

Interior (within an autonomous system – AS – group of routers under the same administrative authority)

  • Distance Vector – understand the direction and distance to any network connection on the internetwork. Knows how many hops (the metric) to get there. All routers w/in the internetwork listen for messages from other routers, which are sent every 30 to 90 seconds. They pass their entire routing tables. Possible problems: Slow convergance, Routing Loops, Counting to Infinity (this is solved by maximum hop count) Solutions: Split Horizon (cannot send information back in the direction it was received) Hold-Downs (prevent regular update messages from reinstating a route that’s gone down)

RIP – 15 hop count max

IGRP – 255 hop count max, uses reliability factor (255 optimal), and bandwidth

  • Link State – Understands the entire network, and does not use secondhand information. Routers exchange LSP’s (hello packets). Each router builds a topographical view of the network, then uses SPF (shortest path first) algorithm to determine the best route. Changes in topology can be sent out immediately, so convergance can be quicker

OSPF – decisions based on cost of route (metric limit of 65,535)

EIGRP – hybrid protocol, Cisco proprietary


  • EGP (Exterior Gateway Protocol)

  • BGP (Border Gateway Protocol)


Manual Routing

ip route {destination network} {mask} {port, on remote side, to get there}

ip route

Dynamic Routing

router rip


router igrp {autonomous system #}


sh ip route {rip / igrp}


Network Security / Access Lists

Standard IP access list

access-list {number} {permit / deny} {source address}

access-list 10 permit

Extended IP access list

access-list {number} {permit / deny} {protocol} {source} {destination} {port}

access-list 110 permit tcp host host eq 8080

Wildcard masks
– use masks to identify insignificant bits, eg

access-list 11 permit

(permits anybody with 172.16.30.x)

note: you can use as the mask to limit to that specific host, or perfix it with ‘host’

Applying the list to an interface
(use access-group on the interface)

int e0

ip access-group 110 out

IPX Access lists

Standard: access-list {number} {permit/deny} {source} {destination}

Extended: access-list {number} {permit/deny} {protocol} {source} {socket} {destination} {socket}

access-list 810 permit 30 10

int e0

ipx access-group 810 out

IPX SAP Filters

access-list {number} {permit/deny} {source} {service type}

To apply – on interface: ixp input-sap-filter {number}

access-list 1010 permit 11.0000.0000.0001 0

int e0

ipx input-sap-filter 1010


Access list Numbers allowed


IP Standard


IP Extended


IPX Standard


IPX Extended



To Monitor Access Lists

Show access-list


WAN Protocols

SDLC – developed by IBM in 70’s – Data link layer protocol that transports SNA over WAN’s

HDLC – modified sdlc by ISO, default on Cisco routers

X.25 – Sessions – DTE to DTE communication

Full duplex, uses virtual circuits (PVC and SVC)

Protocol Suite maps to Physical through Network

PPP – runs on async (dial-up) or sync (ISDN) lines. Supports multi-protocols.

Uses PAP or CHAP authentication.

Int s0, encapsulation PPP

Frame Relay
– shared bandwidth over public network. Virtual circuits are identified by DLCI’s.

(Data Link Connection identifiers). LMI, co-developed in 1990 by Cisco, provides message information about current DLCI values (global or local significance), and the status of virtual circutis. Subinterfaces allow you to have multiple virtual circutis on a single serial interface. You must map an IP device to the DLCI (using the frame-relay map command or the inverse-arp function)

int s0

encapsulation frame-relay {ietf}

note: if you don’s specify ietf, it uses cisco by default

frame-relay interface-dlci {#}

frame-relay lmi-type {cisco, ansi, q933a}


int s0.x {multipoint / point-to-point}


int s0

inverse-arp or

frame-relay map ip x.x.x.x #


show frame {pvc / ip / lmi / traffic / etc.}

- digital service that runs over existing telephone networks

Normally used to support applications requiring high-speed voice, video, and data communications for home users, remote offices, etc.

ISDN Terminal equipment types

TE1 – understand ISDN standards

TE2 – predate ISDN standards, require a TA (terminal adaptor)

Reference Points describe the point between

R – non-ISDN and TA

S – user terminals and NT2

T – NT1 and NT2 devices

U – NT1 and line termination

ISDN Protocols

E – on existing telephone network

I – concepts, terminology, and services

Q – switching and signaling

ISDN BRI: 2 64K B channels, plus 1 16K D channel


23 64K B channels, plus 1 64K D channel (North America & Japan)
30 64K B channels, plus 1 64K D channel (Europe & Australia)

Configuration example

config t

isdn switch-type basic-dms100

int bri0

encap ppp

isdn spid1 775154572

isdn spid2 455145664

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