Enabling DHCP Snooping (CLI Procedure)

 

DHCP snooping enables the switch to monitor and control DHCP messages received from untrusted devices connected to the switch. The switch builds and maintains a database of valid bindings between IP address and MAC addresses (IP-MAC bindings) called the DHCP snooping database.

Note

If you configure DHCP snooping for all VLANs and you enable a different port security feature on a specific VLAN, you must also explicitly enable DHCP snooping on that VLAN. Otherwise, the default value of no DHCP snooping applies to that VLAN.

This topic describes:

Enabling DHCP Snooping

You configure DHCP snooping per VLAN, not per interface (port). By default, DHCP snooping is disabled for all VLANs. You can enable DHCP snooping on all VLANs or on specific VLANs.

To enable DHCP snooping:

  • On a specific VLAN:

    [edit ethernet-switching-options secure-access port]

    user@switch# set vlan vlan-name examine-dhcp
  • On all VLANs:

    [edit ethernet-switching-options secure-access port]

    user@switch# set vlan all examine-dhcp


To enable DHCPv6 snooping:

  • On a specific VLAN:

    [edit ethernet-switching-options secure-access port]

    user@switch# set vlan vlan-name examine-dhcpv6
  • On all VLANs:

    [edit ethernet-switching-options secure-access port]

    user@switch# set vlan all examine-dhcpv6


Tip

By default, the IP-MAC bindings are lost when the switch is rebooted and DHCP clients (the network devices, or hosts) must reacquire bindings. However, you can configure the bindings to persist by setting the switch to store the database file either locally or remotely. See Making IP-MAC Bindings in the DHCP Snooping Database Persistent (CLI Procedure).

Tip

For private VLANs (PVLANs), enable DHCP snooping on the primary VLAN. If you enable DHCP snooping only on a community VLAN, DHCP messages coming from PVLAN trunk ports are not snooped.

Applying CoS Forwarding Classes to Prioritize Snooped Packets

On EX Series switches you might need to use class of service (CoS) to protect packets from critical applications from being dropped during periods of network congestion and delay, and might also need to configure the port security features of DHCP snooping on the ports through which those packets enter or leave.

Note

Prioritizing snooped packets by using CoS forwarding classes is not supported on the QFX Series switch.

To apply CoS forwarding classes and queues to snooped packets:

  1. Create a user-defined forwarding class to be used for prioritizing snooped packets:
    [edit class-of-service]

    user@switch# set forwarding-classes class class-name queue-num queue-number
  2. Enable DHCP snooping on a specific VLAN or on all VLANs and apply the required forwarding class on the snooped packets:
    • On a specific VLAN:

      [edit ethernet-switching-options secure-access port]

      user@switch# set vlan vlan-name examine-dhcp forwarding-class class-name
    • On all VLANs:

      [edit ethernet-switching-options secure-access port]

      user@switch# set vlan all examine-dhcp forwarding-class class-name
    Note

    Replace examine-dhcp with examine-dhcpv6 to enable DHCPv6 snooping.