NET 330 W3 Notes - Oliver-Mustoe/Oliver-Mustoe-Tech-Journal GitHub Wiki

NOTE: Information on this page is intended as notes and does contain text copy and pasted from readings/online sources. I do not claim to own this information, I have simply captured it for my studying needs.
Below are notes taken during/after class DATE:

Filename: D3 -- 9/12/22

Dynamic Host Configuration Protocol (DHCP)

Computers receive IP's either:

  • Statically - Manually entered by admin

  • Dynamically - Automatically assigned by the network

DHCP is used to automatically assign an IP address to a host

Fields used:

  • IP

  • Subnet Mask

  • Default Gateway

  • Lease time

  • DNS

Broadcast goes on VLAN, Any ports on VLAN

DHCP Server not on VLAN (routers):

  • Relay agent, DHCP relay, DHCP helper(same thing)

  • Router in VLAN needs to know IP of DHCP, so it can send discover forwarded  in unicast to it (sends it to next router/next hop)

  • To send back, uses default gateway listed in unicast (default gateway for VLAN)

  • DHCP server knows VLAN to assign because of source IP from router

Create pools correctly, make sure relays are setup correctly

DHCP employs a connectionless service model over UDP

Two known UDP ports numbers:

  • 67 - Server

  • 68 - Client

DHCP has two primary Operation Phases:

  • Initialization

  • Renewal

DORA:

  • Discover

  • Offer

  • Request

  • Acknowledgement

Need RA to prevent some DHCP flooding, tying up pools

Also to make sure client gets one offer

DHCP Renewal

DHCP Renewal: Process for client to request continued use(renewal) of its lease

  • By default, the begins 50% of the way through the current IP lease

  • The client sends DHCP Request packets directly to the server

  • If the server responds with a DHCP Acknowledgement, the IP lease is renewed and its time clock restarts.

DHCP Rebinding: If the server does not respond to the clients renewal requests, we eventually reach the rebingin phase:

  • By default, this begins 87.5% of the way through the current IP lease

  • The client begins sending its DHCP Request packets as broadcasts to see if any DHCP server will allow them to continue using their IP.

  • If a server responds, the lease is renewed and the timer restarts

DHCP Expiration: If no server responds before the lease ends, the lease expires and the IP is released

  • Renewing is wanting to keep using IP, unicast, if good DHCP sends back ACK, unicast

  • Rebinding is when renewing doesn't work

DHCP Relay

Unconfigured clients have no IP

  • They know nothing about the subnet gateway, etc

  • All they can do is a broadcast into the wild

DHCP Relay Helper on Cisco

Cisco IOS uses the "ip helper-address" command

Can be assigned to a physical or VLAN interface

If configured, grabs DHCP broadcasts seen on that interface and sends to DHCP server

Interface vlan 100

Ip helper address 10.16.1.50

(NEEDS TO BE DONE PER VLAN)

Routers act as the default gateway for VLANS