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Cisco Hierarchical Model:
For more information about this, please read our separate tutorial titled "The Cisco Hierarchical Model".

OSI Model:
The OSI model is a layered model and a conceptual standard used for defining standards to promote multi-vendor integration as well as maintain constant interfaces and isolate changes of implementation to a single layer. It is NOT application or protocol specific. In order to pass any Cisco exam, you need to know the OSI model inside and out.

The OSI Model consists of 7 layers as follows:

Layer	Description	Device	Protocol
Application	Provides network access for applications, flow control and error recovery. Provides communications services to applications by identifying and establishing the availability of other computers as well as to determine if sufficient resources exist for communication purposes.	Gateway	NCP, SMB, SMTP, FTP, SNMP, Telnet, Appletalk
Presentation	Performs protocol conversion, encryption and data compression	Gateway and redirectors	NCP, AFP, TDI
Session	Allows 2 applications to communicate over a network by opening a session and synchronizing the involved computers. Handles connection establishment, data transfer and connection release	Gateway	NetBios
Transport	Repackages messages into smaller formats, provides error free delivery and error handling functions	Gateway	NetBEUI, TCP, SPX, and NWLink
Network	Handles addressing, translates logical addresses and names to physical addresses, routing and traffic management.	Router and brouter	IP, IPX, NWLink, NetBEUI
**Data Link	Packages raw bits into frames making it transmitable across a network link and includes a cyclical redundancy check(CRC). It consists of the LLC sublayer and the MAC sublayer. The MAC sublayer is important to remember, as it is responsible for appending the MAC address of the next hop to the frame header. On the contrary, LLC sublayer uses Destination Service Access Points and Source Service Access Points to create links for the MAC sublayers.	Switch, bridge and brouter	None
Physical	Physical layer works with the physical media for transmitting and receiving data bits via certain encoding schemes. It also includes specifications for certain mechanical connection features, such as the adaptor connector.	Multiplexer and repeater	None

Here is an easy way to memorize the order of the layers:
All People Seem To Need Data Processing. The first letter of each word corresponds to the first letter of one of the layers. It is a little corny, but it works.

Class	Range	Explanation
A	1-126	IP addresses can be class A, B or C. Class A addresses are for networks with a large number of hosts. The first octet is the netid and the 3 remaining octets are the hostid. Class B addresses are used in medium to large networks with the first 2 octets making up the netid and the remaining 2 are the hostid. A class C is for smaller networks with the first 3 octets making up the netid and the last octet comprising the hostid. The later two classes aren’t used for networks.
B	128-191
C	192-223
D	224-239 (Multicasting)
E	240-255 (Experimental)

A subnet mask blocks out a portion of an IP address and is used to differentiate between the hostid and netid. The default subnet masks are as follows:

Class	Default Subnet	# of Subnets	# of Hosts Per Subnet
Class A	255.0.0.0	126	16,777,214
Class B	255.255.0.0	16,384	65,534
Class C	255.255.255.0	2,097,152	254

In these cases, the part of the IP address blocked out by 255 is the Net ID.

3COM’s IP addressing tutorial is just superior. It covers basic IP addressing options as well as subnetting and VLSM/CIDR.

IPX/SPX:
IPX will also be an important issue to consider in network management given the fact there many companies still use Netware servers. There are two parts to every IPX Network address - the Network ID and the Host ID. The first 8 hex digits represent the network ID, while the remaining hex digits represent the host ID, which is most likely the same as the MAC address, meaning we do not need to manually assign node addresses. Note that valid hexadecimal digits range from 0 through 9, and hexadecimal letters range from A through F. FFFFFFFF in hexadecimal notation = 4292967295 in decimal.

Sequenced Packet Exchange(SPX) belongs to the Transport layer, and is connection-oriented. It creates virtual circuits between hosts, and that each host is given a connection ID in the SPX header for identifying the connection. Service Advertisement Protocol(SAP) is used by NetWare servers to advertise network services via broadcast at an interval of every 60 minutes by default.

Cisco Discovery Protocol

Understanding How CDP Works

CDP is a media and protocol independent protocol that runs on all Cisco-manufactured equipment including routers, bridges, access and communication servers, and switches. Using CDP, you can view information about all the Cisco devices directly attached to the switch. In addition, CDP detects native VLAN and port duplex mismatches.

Network management applications can retrieve the device type and SNMP-agent address of neighboring Cisco devices using CDP. This enables applications to send SNMP queries to neighboring devices. CDP allows network management applications to discover Cisco devices that are neighbors of already known devices, in particular, neighbors running lower-layer, transparent protocols.

CDP runs on all media that support Subnetwork Access Protocol (SNAP), including LAN and Frame Relay. CDP runs over the Data Link layer only. Cisco devices never forward CDP packets. When new CDP information is received, Cisco devices discard old information.

**CDP Default Configuration**
Feature	Default Value
CDP global enable state	Enabled
CDP port enable state	Enabled on all ports
CDP message interval	60 seconds
CDP holdtime	180 seconds

Getting CDP Timer and Holdtime Information

The following command shows the CDP timer which is how often CDP packets are sent and the CDP holdtime which is the amount of time that the device will hold packets from neighbor devices.

   Router_2#sho cdp
   Global CDP information:
           Sending CDP packets every 60 seconds
           Sending a holdtime value of 180 seconds

Setting the Holdtime and Timer

Use the following commands to set CDP timer and holdtime values.

   Router_2#conf t
   Enter configuration commands, one per line.  End with CNTL/Z.
   Router_2(config)#cdp timer 90
   Router_2(config)#cdp holdtime 360

CDP can be disabled with the no cdp run command in global configuration mode (conf t).

Getting Neighbor Information

Because the router stores the CDP information in its cache memory, you can view it with a show command. It will only show information about directly connected devices since CDP packet aren't passed through the device.

   Router_2#sho cdp neighbors

   Capability Codes: R - Router, T - Trans Bridge, B - Source Route Bridge
                     S - Switch, H - Host, I - IGMP, r - Repeater

   Device ID        Local Intrfce     Holdtme    Capability  Platform  Port ID
   Router3             Ser 1          120           R        2500      Ser 0
   Router1             Eth 1          180           R        2500      Eth 0
   Switch1             Eth 0          240           S        1900      2

CDP Neighbor Information includes

Neighbor's device ID
Local port type and number
Holdtime value (in seconds)
Neighbor's network device capability
Neighbor's hardware platform
Neighbor's remote port type and number

Show CDP Entry

The sho cdp entry [device id] command shows more information about the specified neighbor.

   Router_2#sho cdp entry Router1
   -------------------------

   Device ID: Router1
   Entry address(es): 
    IP address: 192.168.1.2
   Platform: cisco 2500, Capabilities: Router
   Interface: Ethernet1,  Port ID (outgoing port): Ethernet0
   Holdtime : 180 sec

   Version:
   Cisco Internetwork Operating System Software
   IOS (tm) 2500 Software (2500-JS-L), Version 11.2(15)
   RELEASED SOFTWARE (fcl)
   Copyright (c) 1986-1998 by Cisco Systems, Inc.
   Compiled Mon 06-Jul-98 22:22 by tmullins

Show CDP Neighbors Detail

The following is a sample output for one neighbor from the show cdp neighbors detail command. Additional detail is shown about neighbors, including network address, enabled protocols, and software version.

   router#show cdp neighbors detail
 
   Device ID: 008024 1EEB00 (milan-sw-1-cat9k)
   Entry address(es):
      IP address: 1.15.28.10
   Platform: CAT5000, Capabilities: Switch
   Interface: Ethernet1/0, Port ID (outgoing port): 2/7
   Holdtime : 162 sec
 
   Version :
   Cisco Catalyst 5000
   Duplex Mode: full
   Native VLAN: 42
   VTP Management Domain: `Accounting Group'

`show cdp neighbors detail` Field Descriptions
Field	Definition
Device ID	The name of the neighbor device and either the MAC address or the serial number of this device.
Entry address(es)	A list of network addresses of neighbor devices.
[network protocol] address	The network address of the neighbor device. The address can be in IP, IPX, AppleTalk, DECnet, or CLNS protocol conventions.
Platform	The product name and number of the neighbor device.
Capabilities	The device type of the neighbor. This device can be a router, a bridge, a transparent bridge, a source-routing bridge, a switch, a host, an IGMP device, or a repeater.
Interface	The protocol and port number of the port on the current device.
Holdtime	The remaining amount of time, in seconds, the current device will hold the CDP advertisement from a transmitting router before discarding it.
Version	The software version of the neighbor device.
Duplex Mode	The duplex state of connection between the current device and the neighbor device.
Native VLAN	The ID number of the VLAN on the neighbor device.
VTP Management Domain	A string that is the name of the collective group of VLANs associated with the neighbor device.

Show CDP Traffic

The following example specifies information associated with the show cdp traffic command:

   router# show cdp traffic
 
   Total packets output: 543, Input: 333
   Hdr syntax: 0, Chksum error: 0, Encaps failed: 0
   No memory: 0, Invalid: 0, Fragmented: 0
   CDP version 1 advertisements output: 191, Input: 187
   CDP version 2 advertisements output: 352, Input: 146

`show cdp traffic` Fields
Field	Definition
Total packets output	The number of CDP advertisements transmitted by the local device. Note this value is the sum of the CDP Version-1 advertisements output and CDP Version-2 advertisements output fields.
Input	The number of CDP advertisements received by the local device. Note this value is the sum of the CDP Version-1 advertisements input and CDP Version-2 advertisements input fields.
Hdr syntax	The number of CDP advertisements with bad headers, received by the local device.
Chksum error	The number of times the checksum (verifying) operation failed on incoming CDP advertisements.
Encaps failed	The number of times CDP failed to transmit advertisements on an interface because of a failure caused by the local device's bridge port.
No memory	The number of times the local device did not have enough memory to store the CDP advertisements in the advertisement cache table when the device was attempting to assemble advertisement packets for transmission and parse them when receiving them.
Invalid	The number of invalid CDP advertisements received and transmitted by the local device.
Fragmented	The number of times fragments or portions of a single CDP advertisement were received by the local device instead of the complete advertisement.
CDP version 1 advertisements output	The number of CDP Version-1 advertisements transmitted by the local device.
Input	The number of CDP Version-1 advertisements received by the local device.
CDP version 2 advertisements output	The number of CDP Version-2 advertisements transmitted by the local device.
Input	The number of CDP Version-2 advertisements received by the local device.

Show CDP Interface

To display information about the interfaces on which CDP is enabled, use the show cdp interface privileged EXEC command.

Router#sh cdp interface [type number]

type
(Optional) Type of interface about which you want information.

number
(Optional) Number of the interface about which you want information.

   Router_2#sho cdp interface
   Ethernet0 is up, line protocol is up
      Encapsulation ARPA
      Sending CDP packets every 60 seconds
      Holdtime is 180 seconds
   Serial0 is up, line protocol is up
      Encapsulation HDLC
      Sending CDP packets every 60 seconds
      Holdtime is 180 seconds
   Serial1 is up, line protocol is up
      Encapsulation HDLC
      Sending CDP packets every 60 seconds
      Holdtime is 180 seconds

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