DHCP—DHCPv6 Guard

This module describes the Dynamic Host Configuration Protocol version 6 (DHCPv6) Guard feature. This feature blocks DHCP reply and advertisement messages that originate from unauthorized DHCP servers and relay agents that forward DHCP packets from servers to clients. Client messages or messages sent by relay agents from clients to servers are not blocked. The filtering decision is determined by the device role assigned to the receiving switch port, trunk, or VLAN. In addition, to provide a finer level of filter granularity, messages can be filtered based on the address of the sending server or relay agent, or by the prefixes and addresses ranges listed in the reply message. This functionality helps to prevent traffic redirection or denial of service (DoS).

Finding Feature Information

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.

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.

Restrictions for DHCPv6 Guard

  • The DHCPv6 guard feature is not supported on Etherchannel ports.

Information About DHCPv6 Guard

DHCPv6 Guard Overview

The DHCPv6 Guard feature blocks reply and advertisement messages that come from unauthorized DHCP servers and relay agents.

Packets are classified into one of the three DHCP type messages. All client messages are always switched regardless of device role. DHCP server messages are only processed further if the device role is set to server. Further processing of server messages includes DHCP server advertisements (for source validation and server preference) and DHCP server replies (for permitted prefixes).

If the device is configured as a DHCP server, all the messages need to be switched, regardless of the device role configuration.

How to Configure DHCPv6 Guard

Configuring DHCP—DHCPv6 Guard

SUMMARY STEPS

    1.    enable

    2.    configure terminal

    3.    ipv6 access-list access-list-name

    4.    permit host address any

    5.    exit

    6.    ipv6 prefix-list list-name permit ipv6-prefix 128

    7.    ipv6 dhcp guard policy policy-name

    8.    device-role {client | server}

    9.    match server access-list ipv6-access-list-name

    10.    match reply prefix-list ipv6-prefix-list-name

    11.    preference min limit

    12.    preference max limit

    13.    trusted-port

    14.    exit

    15.    interface type number

    16.    switchport

    17.    exit

    18.    vlan configuration vlan-id

    19.    ipv6 dhcp guard [attach-policy policy-name]

    20.    exit

    21.    exit

    22.    show ipv6 dhcp guard policy [policy-name]


DETAILED STEPS
     Command or ActionPurpose
    Step 1 enable


    Example: 
    Device> enable
     

    Enables privileged EXEC mode.

    • Enter your password if prompted.

     
    Step 2 configure terminal


    Example: 
    Device# configure terminal
     

    Enters global configuration mode.

     
    Step 3 ipv6 access-list access-list-name


    Example: 
    Device(config)# ipv6 access-list acl1
     

    Defines the IPv6 access list and enters IPv6 access list configuration mode.

     
    Step 4 permit host address any


    Example: 
    Device(config-ipv6-acl)# permit host FE80::A8BB:CCFF:FE01:F700 any
     

    Sets the conditions in the named IP access list.

     
    Step 5 exit


    Example: 
    Device(config-ipv6-acl)# exit
     

    Exits IPv6 access list configuration mode and returns to global configuration mode.

     
    Step 6 ipv6 prefix-list list-name permit ipv6-prefix 128


    Example: 
    Device(config)# ipv6 prefix-list abc permit 2001:0DB8::/64 le 128
     

    Creates an entry in an IPv6 prefix list.

     
    Step 7 ipv6 dhcp guard policy policy-name


    Example: 
    Device(config)# ipv6 dhcp guard policy pol1
     

    Defines the DHCPv6 guard policy name and enters DHCP guard configuration mode.

     
    Step 8 device-role {client | server}


    Example: 
    Device(config-dhcp-guard)# device-role server
     

    Specifies the device role of the device attached to the target (interface or VLAN).

     
    Step 9 match server access-list ipv6-access-list-name


    Example: 
    Device(config-dhcp-guard)# match server access-list acl1
     

    (Optional) Enables verification of the advertised DHCP server and relay address in inspected messages from the configured authorized server access list. If not configured, this check will be bypassed. An empty access list is treated as a permit.

     
    Step 10 match reply prefix-list ipv6-prefix-list-name


    Example: 
    Device(config-dhcp-guard)# match reply prefix-list abc
     

    (Optional) Enables verification of the advertised prefixes in DHCP reply messages from the configured authorized prefix list. If not configured, this check will be bypassed. An empty prefix list is treated as a permit.

     
    Step 11 preference min limit


    Example: 
    Device(config-dhcp-guard)# preference min 0
     

    (Optional) Enables verification that the advertised preference (in preference option) is greater than the specified limit. If not specified, this check will be bypassed.

     
    Step 12 preference max limit


    Example: 
    Device(config-dhcp-guard)# preference max 255
     

    (Optional) Enables verification that the advertised preference (in preference option) is less than the specified limit. If not specified, this check will be bypassed.

     
    Step 13 trusted-port


    Example: 
    Device(config-dhcp-guard)# trusted-port
     

    (Optional) Specifies that this policy is being applied to trusted ports. All DHCP guard policing will be disabled.

     
    Step 14 exit


    Example: 
    Device(config-dhcp-guard)# exit
     

    Exits DHCP guard configuration mode and returns to global configuration mode.

     
    Step 15 interface type number


    Example: 
    Device(config)# interface GigabitEthernet 0/2/0
     

    Specifies an interface and enters interface configuration mode.

     
    Step 16 switchport


    Example: 
    Device(config-if)# switchport
     

    Puts an interface that is in Layer 3 mode into Layer 2 mode for Layer 2 configuration.

     
    Step 17 exit


    Example: 
    Device(config-if)# exit
     

    Exits interface configuration mode and returns to global configuration mode.

     
    Step 18 vlan configuration vlan-id


    Example: 
    Device(config)# vlan configuration 1
     

    Specifies a VLAN and enters VLAN configuration mode.

     
    Step 19 ipv6 dhcp guard [attach-policy policy-name]


    Example: 
    Device(config-vlan-config)# ipv6 dhcp guard attach-policy pol1
     

    Attaches a DHCPv6 guard policy to a VLAN.

     
    Step 20 exit


    Example: 
    Device(config-vlan-config)# exit
     

    Exits VLAN configuration mode and returns to global configuration mode.

     
    Step 21 exit


    Example: 
    Device(config)# exit
     

    Exits global configuration mode and returns to privileged EXEC mode.

     
    Step 22 show ipv6 dhcp guard policy [policy-name]


    Example: 
    Device# show ipv6 dhcp policy guard pol1
     

    (Optional) Displays the policy configuration as well as all the interfaces where the policy is applied.

     

    Configuration Examples for DHCPv6 Guard

    Example: Configuring DHCP—DHCPv6 Guard

    The following example displays a sample configuration for DHCPv6 Guard: 

    
enable
configure terminal
ipv6 access-list acl1
 permit host FE80::A8BB:CCFF:FE01:F700 any
ipv6 prefix-list abc permit 2001:0DB8::/64 le 128	
ipv6 dhcp guard policy pol1
 device-role server
 match server access-list acl1
 match reply prefix-list abc
 preference min 0
 preference max 255
 trusted-port
interface GigabitEthernet 0/2/0
 switchport
 ipv6 dhcp guard attach-policy pol1 
 vlan configuration 1
  ipv6 dhcp guard attach-policy pol1
show ipv6 dhcp guard policy pol1

    Additional References

    Related Documents

    Related Topic

    Document Title

    Cisco IOS commands

    Cisco IOS Master Commands List, All Releases

    DHCP commands: complete command syntax, command modes, command history, defaults, usage guidelines, and examples

    Cisco IOS IP Addressing Services Command Reference

    DHCP conceptual and configuration information

    Cisco IOS IP Addressing Services Configuration Guide

    Standards/RFCs

    Standard

    Title

    No new or modified standards/RFCs are supported by this feature.

    —

    MIBs

    MIB

    MIBs Link

    No new or modified MIBs are supported by this feature.

    To locate and download MIBs for selected platforms, Cisco IOS releases, and feature sets, use Cisco MIB Locator found at the following URL:

    http:/​/​www.cisco.com/​go/​mibs

    Technical Assistance

    Description

    Link

    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.

    http:/​/​www.cisco.com/​cisco/​web/​support/​index.html

    Feature Information for DHCP—DHCPv6 Guard

    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 . An account on Cisco.com is not required.
    Table 1 Feature Information for DHCP—DHCPv6 Guard

    Feature Name

    Releases

    Feature Information

    DHCP—DHCPv6 Guard

    Cisco IOS XE Release 3.5E

    Cisco IOS XE 3.5 SE

    Cisco IOS XE Release 3.6E

    The DHCP—DHCPv6 Guard feature blocks DHCP reply and advertisement messages that originate from unauthorized DHCP servers and relay agents that forward DHCP packets from servers to clients. Client messages or messages sent by relay agents from clients to servers are not blocked.

    In Cisco IOS XE Release 3.6E, this feature is supported on Cisco Catalyst 3850 Series Switches

    The following commands were introduced or modified: device-role , ipv6 dhcp guard attach-policy (DHCPv6 Guard), ipv6 dhcp guard policy, match reply prefix-list, match server access-list, preference (DHCPv6 Guard), show ipv6 dhcp guard policy, trusted-port (DHCPv6 Guard).