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This document describes the Dynamic Host Configuration Protocol Gleaning feature.
Ensure that the interface to be configured is a Layer 2 interface.
Ensure that global snooping is enabled.
The following sections provide information about DHCP gleaning.
Gleaning helps extract location information from Dynamic Host Configuration Protocol (DHCP) messages when messages are forwarded by a DHCP relay agent; the process is a completely passive snooping functionality that neither blocks nor modifies DHCP packets. Additionally, gleaning helps to differentiate an untrusted device port that is connected to an end user from a trusted port connected to a DHCP server.
DHCP gleaning is a read–only DHCP snooping functionality that allows components to register and glean only DHCP version 4 packets. When you enable DHCP gleaning, it does a read-only snooping on all active interfaces on which DHCP snooping is disabled. You can add a secondary VLAN to a private VLAN. When add a secondary VLAN to a private VLAN, ensure that gleaning is enabled on the secondary VLAN, even though snooping is disabled on the primary VLAN. By default, the gleaning functionality is disabled. However, when you enable a device sensor, DHCP gleaning is automatically enabled.
Dynamic Host Configuring Protocol (DHCP) snooping is a security feature that acts like a firewall between untrusted hosts and trusted DHCP servers. The DHCP snooping feature performs the following activities:
Validates DHCP messages received from untrusted sources and filters out invalid messages.
Rate-limits DHCP traffic from trusted and untrusted sources.
Builds and maintains the DHCP snooping binding database, which contains information about untrusted hosts with leased IP addresses.
Utilizes the DHCP snooping binding database to validate subsequent requests from untrusted hosts.
Other security features, such as dynamic Address Resolution Protocol (ARP) inspection (DAI), also uses information stored in the DHCP snooping binding database.
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 on a range of VLANs.
You can enable or disable DHCP gleaning on a device. You can configure an interface as a trusted or untrusted source of DHCP messages. Verify that no DHCP packets are dropped when DHCP gleaning is enabled on an untrusted interface or on a device port.
 Note |
By default, DHCP gleaning is disabled. |
You can configure DHCP trust on the following types of interfaces:
Layer 2 Ethernet interfaces
Layer 2 port-channel interfaces
Note |
By default, all interfaces are untrusted. |
| Command or Action | Purpose | |
|---|---|---|
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Step 1 |
enable Example: |
Enables privileged EXEC mode.
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Step 2 |
configure terminal Example: |
Enters global configuration mode. |
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Step 3 |
ip dhcp snooping glean Example: |
Enables DHCP gleaning on an interface. |
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Step 4 |
interface type number Example: |
Enters interface configuration mode, where type number is the Layer 2 Ethernet interface which you want to configure as trusted or untrusted for DHCP snooping. |
|
Step 5 |
[no] ip dhcp snooping trust Example: |
Configures the interface as a trusted interface for DHCP snooping. The no option configures the port as an untrusted interface. |
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Step 6 |
end Example: |
Exits interface configuration mode and returns to privileged EXEC mode. |
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Step 7 |
show ip dhcp snooping statistics Example: |
Displays packets that were dropped on the device port configured as an untrusted interface. |
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Step 8 |
show ip dhcp snooping Example: |
Displays DHCP snooping configuration information, including information about DHCP gleaning. |
This example shows how to enable Dynamic Host Configuration Protocol (DHCP) gleaning and configure an interface as a trusted interface:
Device> enable
Device# configure terminal
Device(config)# ip dhcp snooping glean
Device(config)# interface gigabitEthernet 1/0/1
Device(config-if)# ip dhcp snooping trust
Device(config-if)# end
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Related Topic |
Document Title |
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Cisco IOS commands |
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Description |
Link |
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The Cisco Support and Documentation website provides online resources to download documentation, software, and tools. Use these resources to install and configure the software and to troubleshoot and resolve technical issues with Cisco products and technologies. Access to most tools on the Cisco Support and Documentation website requires a Cisco.com user ID and password. |
The following table provides release information about the feature or features described in this module. This table lists only the software release that introduced support for a given feature in a given software release train. Unless noted otherwise, subsequent releases of that software release train also support that feature.
Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to www.cisco.com/go/cfn. An account on Cisco.com is not required.|
Feature Name |
Releases |
Feature Information |
|---|---|---|
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DHCP Gleaning |
Cisco IOS XE Fuji 16.8.1a |
DHCP gleaning is a read–only DHCP snooping functionality that allows components to register and glean only DHCP version 4 packets. |
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