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DHCP Overview

  • The Dynamic Host Control Protocol is used to allocate IPv4 addresses to end devices in your network dynamically.

  • DHCP can also be used for a variety of different reasons within the enterprise including PXE, TFTP services, SCCM, DNS allocation, and lease times.

  • DHCP can be configured within the Cisco iOS or as a service on a Windows Server.

Custom Trepa Tech DHCP image
  • Custom trepa tech dhcp image
  • DHCP uses a process called "DORA" to communicate with endpoints to delegate IP addressing or other "DHCP Options"

    • D - Discover (broadcast | UDP)

    • O - Offer (Unicast | UDP)

    • R - Request (Broadcast | UDP)

    • A - Acknowledgement (Unicast | UDP)

    Custom Trepa Tech DORA image
  • DHCPv4 messages are encapsulated within the UDP transport protocol.

  • DHCPv4 messages sent from the client use UDP source port 68 and destination port 67.

  • DHCPv4 messages sent from the server to the client use UDP source port 67 and destination port 68.

DHCP Relay

  • In networking, sometimes the best design for your campus/enterprise network is to have the DHCP server be a single service running on a VM or a cluster of VMs.

  • For endpoints logically segmented by VLANs and in their broadcast domains, the default-gateway interface will need to be configured with an "IP Helper-address" in order for endpoints to reach the DHCP server, IF the DHCP server is not within the same subnet/broadcast domain.

  • The IP helper-address will redirect "broadcast" messages into "unicast" messages to be sent to the IP Helper-addresses destination IP address.

VRF Support

  • DHCP Pools can also be configured on Cisco iOS to support users in a Virtual Routing Forwarding Instance.

  • VRFs create a separate logical routing table that is not a part of the global routing table. (much like a VM and the host OS)

DHCP Exclusions

  • In the enterprise, some endpoints you do not want to assign IP addresses dynamically.

  • Any endpoint device that has a URL/DNS record associated with it should be static.

  • Printers, share drives, and VTCs should also be considered for static IP addresses.

  • In Cisco iOS and Windows DHCP Service, you can set "DHCP Exclusions" that will not allow an IP address in the subnet to be dynamically allocated.

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