Tag - Telnet

Cisco Discovery Protocol

  1. Proprietary Cisco Layer 2 protocol that uses multicast to gather hardware and protocol information about directly connected devices.
  2. Network layer protocol and media independent.
  3. Enabled by default on all Cisco devices, but can be disabled globally:

Router(config)#nocdp run

or can be disabled on interface-by-interface basis:

Router(config-if)#nocdp enable

  • To learn the remote device’s Layer 3 address and IOS version

Router>showcdp neighbor detail


Router>showcdp entry *


Telnet enables a virtual terminal connection to a remote device’s IP address using the Application layer protocol called Telnet (TCP port 23 at the Transport layer).

To Telnet from IOS, enter the keywordtelnetfollowed by the IP address or hostname. If youenter only an IP address or hostname in user or privileged EXEC, IOS automatically assumes that you are Telnetting. To Telnet to a Cisco device, the vty passwords must be set, or you receive the “Password required, but none set” error. To access Privileged EXEC in a Telnet session, you must have enable password set, or you receive the “% No password set” error.

  • To suspend the Telnet session, press Ctrl+Shift+6, x.
  • To see a list of the active sessions in the originating router, use the show sessions command.
  • To resume a suspended session, press the Enter key from user EXEC or privileged EXEC mode, or enter resume followed by the session number.
  • To close a Telnet session from the device you are Telnetted into, enter exitor logout from user EXEC or privileged EXEC mode.
  • To close a Telnet session from the originating device, enter disconnect followed by the session number.
  • To see log messages in your Telnet session, use the privileged EXEC mode command terminal monitor in the device that you are Telnetted into.


Your Cisco device can act as a DHCP server and respond to DHCP requests on a segment. To configure the Cisco device as a DHCP server, you must first enable the interface that will receivethe DHCP requests and assign an IP address to it. After the interface is enabled, you define theDHCP address pool with theip dhcp pool poolname global configuration command. Indhcp – configmode, you can define the DHCP address scope with thenetwork command followed bythe IP subnet to be assigned. You can also define additional paramvcers such as the default gate-way, DNS server, domain name, and length of the IP lease. To exclude IP addresses from being assigned (such as if you have statically assigned them to specific devices), use the ip dhcp excluded -address ip-addresscommand to remove the IP(s) from the scope.

To verify the devices that have been assigned IP addresses from the DHCP address scope, use the show dhcp bindings command.


Switches have the following functions:

  • Segment LANs into multiple collision domains.
  • Learn MAC addresses by examining the source MAC address of each frame received and store them in a CAM table.
  • Base their forwarding decisions based on the destination MAC address of an Ethernet frame.
  • Flood broadcast, multicast, and unknown unicast frames out all ports except the one it was received.

A switch has three methods of forwarding frames:

Store-and-forward: Latency varying transmission method that buffers the entire frame and calculates the CRC before forwarding the frame.

Cut-through: Only looks at the destination MAC address in an Ethernet frame and forwards it.

Fragment-free: Checks the first 64 bytes for frame fragments (due to collisions) before forwarding the fame.


Duplex Connections

  • Half-duplex interfaces have one-way communication with suboptimal throughput because they operate in a collision domain in which CSMA/CD must be enabled. When connected to a hub, they must run half duplex.
  • Full-duplex interfaces simultaneously send and receive, allowing higher throughput because CSMA/CD is disabled. Connections to other switches or devices can be full duplex.

Spanning Tree Protocol IEEE 802.1d

STP is a Layer 2 protocol that is used to prevent switching loops in networks with redundant switched paths.

TABLE  STP Port States
State Function Transition Time
Disabled The interface is administratively shut downor disabled from port violation. NA
Blocking Does not forward any user data. All ports start out in this state.Does not send, but still can receive BPDUs to react to topology changes. 0 to 20 seconds
ListeningLearning Begins to transition to a forwarding state by listening and sending BPDUs.No user data sent.Begins to build MAC addresses learned on the interface. No user data sent. 15 seconds15 seconds
Forwarding User data forwarded.


STP elects root bridge/switch by dvcermining which switch has the lowest Bridge ID in the topology learned from sending and receiving BPDUs. Bridge ID is a combination of Priority and MAC address.

All nonroot switches dvcermine root port based on the fastest (lowest cumulative cost) path back to root switch. If a tie occurs, the Bridge ID followed by port priority and port number are the tie breakers.

On each segment, the switch advertising the fastest way back to the root switch is the desig­nated port for that segment.

If port is not a root or a designated port, it is blocking.


Port Cost Values
Interface Cost
10Gbps 2
1Gbps 4
100Mbps 19
10Mbps 100

IOS Terminal Access Methodologies

To gain access to an EXEC session to an IOS for configuration and administration, you can use the following methods:

  • Console: Out-of-band CLI access via a rollover cable connected to the COM port of your terminal PC.
  • Auxiliary: Out-of-band CLI access via rollover cable connected to external modem for remote access.
  • Telnet: In-band CLI access to an active IP address on the device’s vty lines using the Telnet protocol. Requires configuration.
  • SSH: Secure encrypted in-band CLI access to an active IP address using the SSH pro­tocol. Requires configuration.
  • HTTP/HTTPS:In-band GUI access to an active IP address using the HTTP or HTTPS protocol. Requires configuration.

IOS Terminal

IOS Boot Processes

To solidify the startup process, the following is a recap of the stages of the bootup, any fallback procedures, and the memory locations involved:

  1. POST located in ROM tests hardware.
  2. Bootstrap located in ROM looks at boot field in configuration register to locate IOS. 0x2 100 boots to ROMmon located in ROM.
  3. 0x2 101 to 0x210F prompt bootstrap to parse startup-config in NVRAM for any boot system commands. If there are any commands, do what they say.
  4. If no boot system commands, load first file in flash. If no file in flash, TFTP boot. If no IOS file found from TFTP, go to ROMmon mode.
  5. After IOS is loaded, check configuration register. If 0x2 142, ignore startup-config in NVRAM. If 0x2 102, load startup-config in NVRAM. If no startup-config, TFTP autoinstall. If no TFTP autoinstall configuration found, enter Setup Mode.


 IOS Navigation Modes  
Mode Prompt Description
User EXEC Router> Basic troubleshooting and verification
Privileged EXEC Router# All available commands, including delvce,

clear, erase, configure, copy, andreload

Global configuration Router(config)# Configurations that apply to the entire device
Line configuration Router(config-line)# Configurations that apply to the terminal lines

into a device

Interface configuration Router(config-if)# Configurations that apply to interfaces
Subinterface configuration Router(config-subif)# Configurations that apply to logical extensions of

the physical interface

Router configuration Router(config-router)# Configurations that apply to routing protocols
VLAN configuration Switch(vlan)#              VLAN-specific configurations in switches


Terminal Editing Keys

Cisco IOS Terminal Editing Keystrokes

Keystroke                               Function

Ctrl+A                                          Moves the cursor to the beginning of the command line.

Ctrl+E                                          Moves the cursor to the end of the command line.

Ctrl+B                                          Moves the cursor back one character.

Ctrl+F                                          Moves the cursor forward one character.

Esc+B                                          Moves the cursor back one word.

Esc+F                                          Moves the cursor forward one word.

CCNA Labs Scenario

Scenario Labs For CCNA

  1. Setting up a Serial Interface 
  2. CDP
  3. IP Addressing
  4. Static Routes
  5. Default Routes
  6. RIP Routes
  7. IGRP Routes
  8. Using Loopback Interfaces
  9. RIP v2 Routes
  10. CHAP and RIP
  11. Standard Access-Lists with RIP
  12. Extended Access-Lists with RIP
  13. EIGRP Routes
  14. OSPF Routes
  15. Static NAT
  16. Many to One NAT
  17. NAT Pool
  18. Telnet
  19. 2950 IP Addresses
  20. 2950 Trunk
  21. 2950 Trunk (Dynamic)
  22. 2950 VLANs
  23. 2950 Deleting VLANs
  24. 2950 VTP
  25. 2950 VTP w/ client
  26. 2950 Telnet

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