Configuring DHCP Snooping

This chapter contains the following sections:

Information About DHCP Snooping

DHCP snooping acts like a firewall between untrusted hosts and trusted DHCP servers. DHCP snooping performs the following activities:

  • Validates DHCP messages received from untrusted sources and filters out invalid messages.

  • Builds and maintains the DHCP snooping binding database, which contains information about untrusted hosts with leased IP addresses.

  • Uses the DHCP snooping binding database to validate subsequent requests from untrusted hosts.

DHCP snooping is enabled on a per-VLAN basis. By default, the feature is inactive on all VLANs. You can enable the feature on a single VLAN or a range of VLANs.

Feature Enabled and Globally Enabled

When you are configuring DHCP snooping, it is important that you understand the difference between enabling the DHCP snooping feature and globally enabling DHCP snooping.

Feature Enablement

The DHCP snooping feature is disabled by default. When the DHCP snooping feature is disabled, you cannot configure it or any of the features that depend on DHCP snooping. The commands to configure DHCP snooping and its dependent features are unavailable when DHCP snooping is disabled.

When you enable the DHCP snooping feature, the switch begins building and maintaining the DHCP snooping binding database. Features dependent on the DHCP snooping binding database can now make use of it and can therefore also be configured.

Enabling the DHCP snooping feature does not globally enable it. You must separately enable DHCP snooping globally.

Disabling the DHCP snooping feature removes all DHCP snooping configuration from the switch. If you want to disable DHCP snooping and preserve the configuration, globally disable DHCP snooping but do not disable the DHCP snooping feature.

Global Enablement

After DHCP snooping is enabled, DHCP snooping is globally disabled by default. Global enablement is a second level of enablement that allows you to have separate control of whether the switch is actively performing DHCP snooping that is independent from enabling the DHCP snooping binding database.

When you globally enable DHCP snooping, on each untrusted interface of VLANs that have DHCP snooping enabled, the switch begins validating DHCP messages that are received and used the DHCP snooping binding database to validate subsequent requests from untrusted hosts.

When you globally disable DHCP snooping, the switch stops validating DHCP messages and validating subsequent requests from untrusted hosts. It also removes the DHCP snooping binding database. Globally disabling DHCP snooping does not remove any DHCP snooping configuration or the configuration of other features that are dependent upon the DHCP snooping feature.

Trusted and Untrusted Sources

You can configure whether DHCP snooping trusts traffic sources. An untrusted source might initiate traffic attacks or other hostile actions. To prevent such attacks, DHCP snooping filters messages from untrusted sources.

In an enterprise network, a trusted source is a switch that is under your administrative control. These switches include the switches, routers, and servers in the network. Any switch beyond the firewall or outside the network is an untrusted source. Generally, host ports are treated as untrusted sources.

In a service provider environment, any switch that is not in the service provider network is an untrusted source (such as a customer switch). Host ports are untrusted sources.

In a Cisco Nexus device, you indicate that a source is trusted by configuring the trust state of its connecting interface.

The default trust state of all interfaces is untrusted. You must configure DHCP server interfaces as trusted. You can also configure other interfaces as trusted if they connect to switches (such as switches or routers) inside your network. You usually do not configure host port interfaces as trusted.


Note

For DHCP snooping to function properly, you must connect all DHCP servers to the switch through trusted interfaces.

DHCP Snooping Binding Database

Using information extracted from intercepted DHCP messages, DHCP snooping dynamically builds and maintains a database. The database contains an entry for each untrusted host with a leased IP address if the host is associated with a VLAN that has DHCP snooping enabled. The database does not contain entries for hosts that are connected through trusted interfaces.


Note
The DHCP snooping binding database is also referred to as the DHCP snooping binding table.

DHCP snooping updates the database when the switch receives specific DHCP messages. For example, the feature adds an entry to the database when the switch receives a DHCPACK message from the server. The feature removes the entry in the database when the IP address lease expires or the switch receives a DHCPRELEASE message from the host.

Each entry in the DHCP snooping binding database includes the MAC address of the host, the leased IP address, the lease time, the binding type, and the VLAN number and interface information associated with the host.

You can remove entries from the binding database by using the clear ip dhcp snooping binding command.

Guidelines and Limitations for DHCP Snooping

Consider the following guidelines and limitations when configuring DHCP snooping:

  • The DHCP snooping database can store 2000 bindings.

  • DHCP snooping is not active until you enable the feature, enable DHCP snooping globally, and enable DHCP snooping on at least one VLAN.

  • Before globally enabling DHCP snooping on the switch, make sure that the switches that act as the DHCP server and the DHCP relay agent are configured and enabled.

  • If a VLAN ACL (VACL) is configured on a VLAN that you are configuring with DHCP snooping, ensure that the VACL permits DHCP traffic between DHCP servers and DHCP hosts.

  • DHCP snooping does not work with DHCP relay configured on the same nexus device.

  • When you configure DHCPv6 server addresses on an interface, a destination interface cannot be used with global IPv6 addresses.

Default Settings for DHCP Snooping

This table lists the default settings for DHCP snooping parameters.

Table 1. Default DHCP Snooping Parameters

Parameters

Default

DHCP snooping feature

Disabled

DHCP snooping globally enabled

No

DHCP snooping VLAN

None

DHCP snooping Option 82 support

Disabled

DHCP snooping trust

Untrusted

VRF support for the DHCP relay agent

Disabled

VRF support for the DHCPv6 relay agent

Disabled

DHCP relay agent

Disabled

DHCPv6 relay agent

Disabled

DHCPv6 relay option type cisco

Disabled

Configuring DHCP Snooping

Minimum DHCP Snooping Configuration

  1. Enable the DHCP snooping feature.

Procedure

  Command or Action Purpose

Step 1

Enable the DHCP snooping feature.

When the DHCP snooping feature is disabled, you cannot configure DHCP snooping.

For details, see Enabling or Disabling the DHCP Snooping Feature.

Step 2

Enable DHCP snooping globally.

For details, see Enabling or Disabling DHCP Snooping Globally.

Step 3

Enable DHCP snooping on at least one VLAN.

By default, DHCP snooping is disabled on all VLANs.

For details, see Enabling or Disabling DHCP Snooping on a VLAN.

Step 4

Ensure that the DHCP server is connected to the switch using a trusted interface.

For details, see Configuring an Interface as Trusted or Untrusted.

Enabling or Disabling the DHCP Snooping Feature

You can enable or disable the DHCP snooping feature on the switch. By default, DHCP snooping is disabled.

Before you begin

If you disable the DHCP snooping feature, all DHCP snooping configuration is lost. If you want to turn off DHCP snooping and preserve the DHCP snooping configuration, disable DHCP globally.

Procedure

  Command or Action Purpose

Step 1

configure terminal

Example:

switch# configure terminal
switch(config)#

Enters global configuration mode.

Step 2

[no] feature dhcp

Example:

switch(config)# feature dhcp

Enables the DHCP snooping feature. The no option disables the DHCP snooping feature and erases all DHCP snooping configuration.

Step 3

(Optional) show running-config dhcp

Example:

switch(config)# show running-config dhcp
 (Optional)

Shows the DHCP snooping configuration.

Step 4

(Optional) copy running-config startup-config

Example:

switch(config)# copy running-config startup-config
 (Optional)

Copies the running configuration to the startup configuration.

Enabling or Disabling DHCP Snooping Globally

You can enable or disable the DHCP snooping globally on the switch. Globally disabling DHCP snooping stops the switch from performing any DHCP snooping but preserves DCHP snooping configuration.

Before you begin

Ensure that you have enabled the DHCP snooping feature. By default, DHCP snooping is globally disabled.

Procedure

  Command or Action Purpose

Step 1

configure terminal

Example:

switch# configure terminal
switch(config)#

Enters global configuration mode.

Step 2

[no] ip dhcp snooping

Example:

switch(config)# ip dhcp snooping

Enables DHCP snooping globally. The no option disables DHCP snooping.

Step 3

(Optional) show running-config dhcp

Example:

switch(config)# show running-config dhcp
 (Optional)

Shows the DHCP snooping configuration.

Step 4

(Optional) copy running-config startup-config

Example:

switch(config)# copy running-config startup-config
 (Optional)

Copies the running configuration to the startup configuration.

Enabling or Disabling DHCP Snooping on a VLAN

You can enable or disable DHCP snooping on one or more VLANs.

Before you begin

By default, DHCP snooping is disabled on all VLANs.

Ensure that DHCP snooping is enabled.


Note

If a VACL is configured on a VLAN that you are configuring with DHCP snooping, ensure that the VACL permits DHCP traffic between DHCP servers and DHCP hosts.

Procedure

  Command or Action Purpose

Step 1

configure terminal

Example:

switch# configure terminal
switch(config)#

Enters global configuration mode.

Step 2

[no] ip dhcp snooping vlan vlan-list

Example:

switch(config)# ip dhcp snooping vlan 100,200,250-252

Enables DHCP snooping on the VLANs specified by vlan-list . The no option disables DHCP snooping on the VLANs specified.

Step 3

(Optional) show running-config dhcp

Example:

switch(config)# show running-config dhcp
 (Optional)

Shows the DHCP snooping configuration.

Step 4

(Optional) copy running-config startup-config

Example:

switch(config)# copy running-config startup-config
 (Optional)

Copies the running configuration to the startup configuration.

Enabling or Disabling Strict DHCP Packet Validation

You can enable or disable the strict validation of DHCP packets by the DHCP snooping feature. By default, strict validation of DHCP packets is disabled.

Procedure

  Command or Action Purpose

Step 1

configure terminal

Example:

switch# configure terminal
switch(config)#

Enters global configuration mode.

Step 2

[no] ip dhcp packet strict-validation

Example:

switch(config)# ip dhcp packet strict-validation

Enables the strict validation of DHCP packets by the DHCP snooping feature. The no option disables strict DHCP packet validation.

Step 3

(Optional) show running-config dhcp

Example:

switch(config)# show running-config dhcp
 (Optional)

Shows the DHCP snooping configuration.

Step 4

(Optional) copy running-config startup-config

Example:

switch(config)# copy running-config startup-config
 (Optional)

Copies the running configuration to the startup configuration.

Configuring an Interface as Trusted or Untrusted

You can configure whether an interface is a trusted or untrusted source of DHCP messages. You can configure DHCP trust on the following types of interfaces:

  • Layer 2 Ethernet interfaces

  • Layer 2 port-channel interfaces

Before you begin

By default, all interfaces are untrusted.

Ensure that DHCP snooping is enabled.

Procedure

  Command or Action Purpose

Step 1

configure terminal

Example:

switch# configure terminal
switch(config)#

Enters global configuration mode.

Step 2

Enter one of the following commands:

  • interface ethernet port/slot
  • interface port-channel channel-number

Example:

switch(config)# interface ethernet 2/1
switch(config-if)#

  • Enters interface configuration mode, where port / slot is the Layer 2 Ethernet interface that you want to configure as trusted or untrusted for DHCP snooping.

  • Enters interface configuration mode, where port / slot is the Layer 2 port-channel interface that you want to configure as trusted or untrusted for DHCP snooping.

Step 3

[no] ip dhcp snooping trust

Example:

switch(config-if)# ip dhcp snooping trust

Configures the interface as a trusted interface for DHCP snooping. The no option configures the port as an untrusted interface.

Step 4

(Optional) show running-config dhcp

Example:

switch(config-if)# show running-config dhcp
 (Optional)

Shows the DHCP snooping configuration.

Step 5

(Optional) copy running-config startup-config

Example:

switch(config-if)# copy running-config startup-config
 (Optional)

Copies the running configuration to the startup configuration.

Enabling or Disabling the DHCP Relay Agent

You can enable or disable the DHCP relay agent. By default, the DHCP relay agent is enabled.

Before you begin

Ensure that the DHCP feature is enabled.

Procedure

  Command or Action Purpose

Step 1

config t

Example:

switch# config t
switch(config)#

Enters global configuration mode.

Step 2

[no] ip dhcp relay

Example:

switch(config)# ip dhcp relay

Enables the DHCP relay agent. The no option disables the relay agent.

Step 3

(Optional) show ip dhcp relay

Example:

switch(config)# show ip dhcp relay
 (Optional)

Displays the DHCP relay configuration.

Step 4

(Optional) show running-config dhcp

Example:

switch(config)# show running-config dhcp
 (Optional)

Displays the DHCP configuration.

Step 5

(Optional) copy running-config startup-config

Example:

switch(config)# copy running-config startup-config
 (Optional)

Copies the running configuration to the startup configuration.

Creating a DHCP Static Binding

You can create a static DHCP source binding to a Layer 2 interface.

Before you begin

Ensure that you have enabled the DHCP snooping feature.

Procedure

  Command or Action Purpose

Step 1

configure terminal

Example:

switch# configure terminal
switch(config)#

Enters global configuration mode.

Step 2

ip source binding IP-address MAC-address vlan vlan-id {interface ethernet slot/port | port-channel channel-no}

Example:

switch(config)# ip source binding 10.5.22.7 001f.28bd.0013 vlan 100 interface ethernet 2/3

Binds the static source address to the Layer 2 Ethernet interface.

Step 3

(Optional) show ip dhcp snooping binding

Example:

switch(config)#  ip dhcp snooping binding
 (Optional)

Shows the DHCP snooping static and dynamic bindings.

Step 4

(Optional) show ip dhcp snooping binding dynamic

Example:

switch(config)#  ip dhcp snooping binding dynamic
 (Optional)

Shows the DHCP snooping dynamic bindings.

Step 5

(Optional) copy running-config startup-config

Example:

switch(config)# copy running-config startup-config
 (Optional)

Copies the running configuration to the startup configuration.

Example

The following example shows how to create a static IP source entry associated with VLAN 100 on Ethernet interface 2/3:

switch# configure terminal
switch(config)# ip source binding 10.5.22.7 001f.28bd.0013 vlan 100 interface ethernet 2/3
switch(config)#

Verifying the DHCP Snooping Configuration

To display DHCP snooping configuration information, perform one of the following tasks. For detailed information about the fields in the output from these commands, see the System Management Configuration Guide for your Cisco Nexus device.

Command

Purpose

show running-config dhcp

Displays the DHCP snooping configuration.

show ip dhcp relay

Displays the DHCP relay configuration.

show ipv6 dhcp relay [interface interface]

Displays the DHCPv6 relay global or interface-level configuration.

show ip dhcp snooping

Displays general information about DHCP snooping.

Displaying DHCP Bindings

Use the show ip dhcp snooping binding command to display the DHCP static and dynamic binding table. Use the show ip dhcp snooping binding dynamic to display the DHCP dynamic binding table.

For detailed information about the fields in the output from this command, see the System Management Configuration Guide for your Cisco Nexus device.

This example shows how to create a static DHCP binding and then verify the binding using the show ip dhcp snooping binding command. 

switch# configuration terminal
switch(config)# ip source binding 10.20.30.40 0000.1111.2222 vlan 400 interface port-channel 500

switch(config)# show ip dhcp snooping binding
MacAddress         IpAddress        LeaseSec  Type        VLAN  Interface
-----------------  ---------------  --------  ----------  ----  -------------
00:00:11:11:22:22  10.20.30.40      infinite  static      400   port-channel500

Clearing the DHCP Snooping Binding Database

You can remove entries from the DHCP snooping binding database, including a single entry, all entries associated with an interface, or all entries in the database.

Before you begin

Ensure that DHCP snooping is enabled.

Procedure

  Command or Action Purpose

Step 1

(Optional) clear ip dhcp snooping binding

Example:

switch# clear ip dhcp snooping binding
 (Optional)

Clears all entries from the DHCP snooping binding database.

Step 2

(Optional) clear ip dhcp snooping binding interface ethernet slot/port[.subinterface-number]

Example:

switch# clear ip dhcp snooping binding interface ethernet 1/4
 (Optional)

Clears entries associated with a specific Ethernet interface from the DHCP snooping binding database.

Step 3

(Optional) clear ip dhcp snooping binding interface port-channel channel-number[.subchannel-number]

Example:

switch# clear ip dhcp snooping binding interface port-channel 72
 (Optional)

Clears entries associated with a specific port-channel interface from the DHCP snooping binding database.

Step 4

(Optional) clear ip dhcp snooping binding vlan vlan-id mac mac-address ip ip-address interface {ethernet slot/port[.subinterface-number | port-channel channel-number[.subchannel-number] }

Example:

switch# clear ip dhcp snooping binding vlan 23 mac 0060.3aeb.54f0 ip 10.34.54.9 interface 
ethernet 2/11
 (Optional)

Clears a single, specific entry from the DHCP snooping binding database.

Step 5

(Optional) show ip dhcp snooping binding

Example:

switch# show ip dhcp snooping binding
 (Optional)

Displays the DHCP snooping binding database.

Configuration Examples for DHCP Snooping

The following example shows how to enable DHCP snooping on two VLANs, with Option 82 support enabled and Ethernet interface 2/5 trusted because the DHCP server is connected to that interface: 

feature dhcp 
ip dhcp snooping 
ip dhcp snooping info option

interface Ethernet 2/5
  ip dhcp snooping trust 
ip dhcp snooping vlan 1 
ip dhcp snooping vlan 50