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

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  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.