CCNA Notes

200-301 Exam Prep

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1Networking Devices2Interfaces And Cables3OSI Model and TCP/IP Suite4Intro to the CLI5Ethernet LAN Switching - Part 16Ethernet LAN Switching - Part 27IPv4 Addressing - Part 18IPv4 Addressing - Part 29Switch Interfaces10The IPv4 Header11Routing Fundamentals - Part 111Static Routing - Part 212Life of a Packet13Subnetting - Part 114Subnetting - Part 215Subnetting (VLSM) - Part 316VLANs - Part 117VLANs - Part 218VLANs - Part 319DTP / VTP20Spanning Tree Protocol - Part 121Spanning Tree Protocol - Part 222Rapid Spanning Tree Protocol23Etherchannel24Dynamic Routing25RIP / EIGRP26OSPF - Part 127OSPF - Part 228OSPF - Part 329First Hop Redundancy Protocol30TCP and UDP31IPv6 - Part 132IPv6 - Part 233IPv6 - Part 334Standard Access Control List35Extended Access Control List36CDP and LLDP37NTP - Network Time Protocol38DNS - Domain Name System39DHCP - Dynamic Host Configuration Protocol40SNMP - Simple Network Management Protocol41SYSLOG42SSH - Secure Shell43FTP and TFTP44NAT (Static) - Part 145NAT (Dynamic) - Part 246QoS (Voice VLAN) - Part 147QoS (Quality of Service) - Part 248Security Fundamentals49Port Security50DHCP Snooping51Dynamic ARP Inspection52LAN Architectures53WAN Architectures54Virtualization and Cloud - Part 154Virtualization (Containers) - Part 254Virtualization (VRF) - Part 355Wireless Fundamentals56Wireless Architectures57Wireless Security58Wireless Configuration59Introduction to Network Automation60JSON, XML, and YAML61REST APIs62Software Defined Networking63Ansible, Puppet, and Chef67CCNA Complete Cheatsheet
/DHCP - Dynamic Host Configuration Protocol
Topic 393 min read

DHCP - Dynamic Host Configuration Protocol

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39. DHCP (Dynamic Host Configuration Protocol)

THE PURPOSE OF DHCP

  • DHCP is used to allow HOSTS to automatically / dynamically learn various aspects of their NETWORK configuration; without MANUAL / STATIC configuration
  • It is an ESSENTIAL part of modern NETWORKS
    • When you connect a phone / laptop to WiFi, do you ask your NETWORK admin which IP ADDRESS, SUBNET MASK, DEFAULT GATEWAY, etc the phone / laptop should use ?
  • Typically used for CLIENT devices (workstations, phones, etc)
  • DEVICES (such as ROUTERS, SERVERS, etc) are usually MANUALLY configured
  • In small NETWORKS (such as Home NETWORKS), the ROUTER typically acts as the DHCP SERVER for HOSTS in the LAN
  • In LARGE NETWORKS, the DHCP SERVER is usually a Windows / Linux SERVER

BASIC FUNCTIONS OF DHCP

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Note: ALL the IPs are the same because this is Jeremy’s Home ROUTER (it provides all these services)

Command ipconfig /release

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Wireshark capture of the ipconfig /release mechanism

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Command ipconfig /renew

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Renewing Process has FOUR messages:

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  1. DHCP DISCOVER
  • Are there any DHCP Servers in this NETWORK? I need an IP ADDRESS ?

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NOTE the use of DHCP Reserved Ports 67 and 68

  1. DHCP OFFER:
  • How about THIS IP ADDRESS ?

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  • The DHCP OFFER message can be either BROADCAST or UNICAST
  • NOTE OPTIONS at the bottom : Message Type, Server ID, Lease Time, Subnet, etc.
  1. DHCP REQUEST
  • I want to use the IP ADDRESS that was offered

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  1. DHCP ACK
  • Okay! You may use THAT ADDRESS

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DHCP RENEW PROCESS SUMMARY

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DHCP RELAY

  • Some NETWORK engineers might choose to configure each ROUTER to act as the DHCP SERVER for its connected LANS
  • However, large enterprises often choose to use a CENTRALIZED DHCP SERVER
  • If the SERVER is centralized, it won’t receive the DHCP CLIENTS’ Broadcast DHCP messages
  • To FIX this, you can configure a ROUTER to act as a DHCP RELAY AGENT
  • The ROUTER will forward the clients’ Broadcast DHCP messages to the remote DHCP SERVER as a Unicast messages

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CONFIGURING DHCP IN CISCO IOS

Commands for configuring DHCP SERVERS in Cisco IOS

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Command show ip dhcp binding

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DHCP RELAY AGENT CONFIGURATION IN IOS

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RELAY AGENT MUST HAVE CONNECTIVITY WITH DHCP SERVER

DHCP CLIENT CONFIGURATION IN IOS

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COMMANDS SUMMARY

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