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This document describes the Dynamic Host Configuration Protocol Gleaning feature.
Your software release may not support all the features documented in this module. For the latest caveats and feature information, see Bug Search Tool and the release notes for your platform and software release. To find information about the features documented in this module, and to see a list of the releases in which each feature is supported, see the feature information table at the end of this module.
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.
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:
Note | By default, all interfaces are untrusted. |
1.
enable
2.
configure
terminal
3.
ip
dhcp
snooping
glean
4.
interface
type
number
5.
[no]
ip
dhcp
snooping
trust
6.
end
7.
show
ip
dhcp
snooping
statistics
8.
show
ip
dhcp
snooping
This example shows how to enable Dynamic Host Configuration Protocol (DHCP) gleaning and configure an interface as a trusted interface:
configure terminal ip dhcp snooping glean interface gigabitEthernet 1/0/1 ip dhcp snooping trust exit
Related Topic | Document Title |
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Master Commands List |
|
DHCP Commands |
|
IP Source Guard |
IP Source Guard |
Dynamic ARP Inspection |
Configuring Dynamic ARP Inspection |
Standard/RFC | Title |
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RFC-2131 |
Dynamic Host Configuration Protocol |
RFC-4388 |
DHCP Leasequery |
MIB | MIBs Link |
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To locate and download MIBs for selected platforms, Cisco IOS releases, and feature sets, use Cisco MIB Locator found at the following URL: |
Description | Link |
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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 |
---|---|---|
DHCP Gleaning |
Cisco IOS 15.2(1)E Cisco IOS 15.2(2)E |
This document describes the DHCP Gleaning feature. In Cisco IOS Release 15.2(2)E, this feature is supported on the following platforms: The following commands were introduced or modified for this feature:ip dhcp snooping glean, show ip dhcp snooping |