Cisco NX-OS/IOS DHCP Relay Comparison

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Objective

This tech note outlines the main differences in Dynamic Host Configuration Protocol (DHCP) Relay support between Cisco® NX-OS Software and Cisco IOS® Software. Sample configurations are included for Cisco NX-OS and Cisco IOS Software to demonstrate the similarities and differences. Please refer to the NX-OS documentation on Cisco.com for a complete list of supported features.


DHCP Relay Overview

The DHCP Relay feature was designed to forward DHCP broadcast requests as unicast packets to a configured DHCP server or servers for redundancy.


Important Cisco NX-OS and Cisco IOS Software Differences

In Cisco NX-OS:

  • DHCP command-line interface (CLI) configuration and verification commands are not available until you enable the DCHP feature with the feature dhcp command.
  • The DHCP service is not enabled by default, whereas it is enabled by default in Cisco IOS Software.
  • The DHCP-Relay command ip dchp relay address is equivalent to the ip helper-address command in Cisco IOS Software.
  • Only packets destined to User Datagram (UDP) port 67 (Bootps) and 68 (Bootpc) are forwarded by the relay, whereas Cisco IOS Software forwards additional protocols (Trivial File Transfer Protocol [TFTP], Domain Name System [DNS], Time, NetBios, and Neighbor Discovery).
  • The Cisco NX-OS cannot act as a DHCP server.
  • VRF-Aware (Inter-VRF) DHCP-Relay can be configured to forward DHCP requests to DHCP servers located in a specific VRF instance that is different than the interface VRF instance. The Cisco IOS Software does not support VRF-Aware DHCP-Relay.


Things You Should Know

The following list provides some additional facts about Cisco NX-OS that should be helpful when designing, configuring, and maintaining networks with the DHCP-Relay feature.

  • If you remove the feature dhcp command, all relevant DHCP configuration information is also removed.
  • Prior to NX-OS 4.2(1), the service dhcp command enabled the DHCP Relay feature. In NX-OS 4.2(1) the command was changed to ip dhcp relay.
  • Sixteen DHCP Relay addresses can be configured per interface.
  • Assign a DHCP Relay to every interface that may have a client, even if the server resides in the same Layer-2 broadcast domain (VLAN). - This has been fixed in 4.2(1) software.
  • DHCP Option 82 information can be configured with the ip dhcp relay information option global command.
  • The DHCP Relay configuration can be verified with the show ip dhcp relay command.


Configuration Comparison

The following sample code shows configuration similarities and differences between the Cisco NX-OS and Cisco IOS Software CLIs. There are two significant differences: in Cisco NX-OS, the DHCP feature must be enabled, and the DHCP relay service is not enabled by default.


Cisco IOS CLI Cisco NX-OS CLI
Enabling the DHCP Feature
Cisco IOS Software does not have the ability to enable or disable DHCP. feature dhcp
Enabling the DHCP Service
Cisco IOS Software enables service dhcp by default. ip dhcp relay
Configuring Option 82 Information
ip dhcp relay information option ip dhcp relay information option
Configuring DHCP Relay on an Interface
interface gigabitethernet 1/1

ip address 192.168.10.1 255.255.255.0

ip helper-address 1.1.1.1

interface ethernet 1/1

ip address 192.168.10.1/24

ip dhcp relay address 1.1.1.1

Configuring Inter-VRF DHCP-Relay on an Interface
The Catalyst 6500 IOS Software does not support VRF-Aware DHCP-Relay


ip dhcp relay information option vpn


interface gigabitethernet 1/1

ip address 192.168.10.1 255.255.255.0

ip helper-address vrf common-services 1.1.1.1

ip dhcp relay information option

ip dhcp relay information option vpn


interface etherent 1/1

ip address 192.168.10.1/24

ip dhcp relay address 10.10.10.10 use-vrf common-services


Verification Command Comparison

The following table compares some useful show commands for verifying and troubleshooting the DHCP-Relay feature.


Cisco NX-OS DHCP-Relay Cisco IOS Software DHCP-Relay Command Description
show ip dhcp relay - Displays all DHCP-Relay configuration information
show ip dhcp relay address - Displays a list of DHCP-Relay(s) configured for all interfaces
show ip dhcp relay address interface - Displays the DHCP-Relay(s) configured for a specific interface

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