Configuring DHCP Snooping

This chapter describes how to configure Dynamic Host Configuration Protocol (DHCP) snooping on a Cisco NX-OS device.

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 and do not disable the DHCP snooping feature.

Global Enablement

After DHCP snooping is feature 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 received and using 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 may 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 switchs 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 the Cisco Nexus 3000 Series switch, 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, all DHCP servers must be connected 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 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.

Information About the DHCP Relay Agent

DHCP Relay Agent

You can configure the device to run a DHCP relay agent, which forwards DHCP packets between clients and servers. This feature is useful when clients and servers are not on the same physical subnet. Relay agent forwarding is distinct from the normal forwarding of an IP router, where IP datagrams are switched between networks somewhat transparently. By contrast, relay agents receive DHCP messages and then generate a new DHCP message to send out on another interface. The relay agent sets the gateway address (giaddr field of the DHCP packet) and, if configured, adds the relay agent information option (Option 82) in the packet and forwards it to the DHCP server. The reply from the server is forwarded back to the client after removing Option 82.


Note

When the device relays a DHCP request that already includes Option 82 information, the device forwards the request with the original Option 82 information without altering it.

VRF Support for the DHCP Relay Agent

You can configure the DHCP relay agent to forward DHCP broadcast messages from clients in a virtual routing and forwarding instance (VRF) to DHCP servers in a different VRF. By using a single DHCP server to provide DHCP support to clients in multiple VRFs, you can conserve IP addresses by using a single IP address pool rather than one for each VRF.

Enabling VRF support for the DHCP relay agent requires that you enable Option 82 for the DHCP relay agent.

If a DHCP request arrives on an interface that you have configured with a DHCP relay address and VRF information and the address of the DCHP server belongs to a network on an interface that is a member of a different VRF, the device inserts Option 82 information in the request and forwards it to the DHCP server in the server VRF. The Option 82 information that the devices adds to a DHCP request relayed to a different VRF includes the following:

VPN identifier

Contains the name of the VRF that the interface that receives the DHCP request is a member of.

Link selection

Contains the subnet address of the interface that receives the DHCP request.

Server identifier override

Contains the IP address of the interface that receives the DHCP request.


Note

The DHCP server must support the VPN identifier, link selection, and server identifier override options.

When the device receives the DHCP response message, it strips off the Option 82 information and forwards the response to the DHCP client in the client VRF.

DHCP Relay Binding Database

A relay binding is an entity that associates a DHCP or BOOTP client with a relay agent address and its subnet. Each relay binding stores the client MAC address, active relay agent address, active relay agent address mask, logical and physical interfaces to which the client is connected, giaddr retry count, and total retry count. The giaddr retry count is the number of request packets transmitted with that relay agent address, and the total retry count is the total number of request packets transmitted by the relay agent. One relay binding entry is maintained for each DHCP or BOOTP client.

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 acting 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.

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

Configuring DHCP Snooping

Minimum DHCP Snooping Configuration

      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.

    SUMMARY STEPS

      1.    config t

      2.    [no] feature dhcp

      3.    show running-config dhcp

      4.    (Optional) copy running-config startup-config


    DETAILED STEPS
        Command or Action Purpose
      Step 1 config t


      Example: 
      switch# config t
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 show running-config dhcp


      Example: 
      switch(config)# show running-config dhcp
       

      Shows the DHCP snooping configuration.

       
      Step 4 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. It preserves DCHP snooping configuration.

      Before You Begin

      Ensure that you have enabled the DHCP snooping feature.

      By default, DHCP snooping is globally disabled.

      SUMMARY STEPS

        1.    config t

        2.    [no] ip dhcp snooping

        3.    show running-config dhcp

        4.    (Optional) copy running-config startup-config


      DETAILED STEPS
          Command or Action Purpose
        Step 1 config t


        Example: 
        switch# config t
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 show running-config dhcp


        Example: 
        switch(config)# show running-config dhcp
         

        Shows the DHCP snooping configuration.

         
        Step 4 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.
        SUMMARY STEPS

          1.    config t

          2.    [no] ip dhcp snooping vlan vlan-list

          3.    show running-config dhcp

          4.    (Optional) copy running-config startup-config


        DETAILED STEPS
            Command or Action Purpose
          Step 1 config t


          Example: 
          switch# config t
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 show running-config dhcp


          Example: 
          switch(config)# show running-config dhcp
           

          Shows the DHCP snooping configuration.

           
          Step 4 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.

          SUMMARY STEPS

            1.    config t

            2.    [no] ip dhcp packet strict-validation

            3.    show running-config dhcp

            4.    (Optional) copy running-config startup-config


          DETAILED STEPS
              Command or Action Purpose
            Step 1 config t


            Example: 
            switch# config t
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 show running-config dhcp


            Example: 
            switch(config)# show running-config dhcp
             

            Shows the DHCP snooping configuration.

             
            Step 4 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.

            SUMMARY STEPS

              1.    config t

              2.    Do one of the following options.

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

              3.    [no] ip dhcp snooping trust

              4.    show running-config dhcp

              5.    (Optional) copy running-config startup-config


            DETAILED STEPS
                Command or Action Purpose
              Step 1 config t


              Example: 
              switch# config t
switch(config)#
               

              Enters global configuration mode.

               
              Step 2 Do one of the following options.
              • interface ethernet slot / port
              • interface port-channel channel-number


              Example: 
              switch(config)# interface ethernet 2/1
switch(config-if)#
               
              • Enters interface configuration mode, where slot / port is the Layer 2 Ethernet interface that you want to configure as trusted or untrusted for DHCP snooping.
              • Enters interface configuration mode, where slot / port 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 show running-config dhcp


              Example: 
              switch(config-if)# show running-config dhcp
               

              Shows the DHCP snooping configuration.

               
              Step 5 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.

              SUMMARY STEPS

                1.    config t

                2.    [no] ip dhcp relay

                3.    (Optional) show ip dhcp relay

                4.    (Optional) show running-config dhcp

                5.    (Optional) copy running-config startup-config


              DETAILED STEPS
                  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 DHCP relay agent.

                 
                Step 3 show ip dhcp relay


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

                Displays the DHCP relay configuration.

                 
                Step 4 show running-config dhcp


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

                Displays the DHCP configuration.

                 
                Step 5 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 VRF Support for the DHCP Relay Agent

                You can configure the device to support the relaying of DHCP requests that arrive on an interface in one VRF to a DHCP server in a different VRF.

                Before You Begin

                You must enable Option 82 for the DHCP relay agent.

                SUMMARY STEPS

                  1.    config t

                  2.    [no] ip dhcp relay information option vpn

                  3.    [no] ip dhcp relay sub-option type cisco

                  4.    (Optional) show ip dhcp relay

                  5.    (Optional) show running-config dhcp

                  6.    (Optional) copy running-config startup-config


                DETAILED STEPS
                    Command or Action Purpose
                  Step 1 config t


                  Example: 
                  switch# config t
switch(config)#
                   

                  Enters global configuration mode.

                   
                  Step 2 [no] ip dhcp relay information option vpn


                  Example: 
                  switch(config)# ip dhcp relay information option vpn
                   

                  Enables VRF support for the DHCP relay agent. The no option disables this behavior.

                   
                  Step 3 [no] ip dhcp relay sub-option type cisco


                  Example: 
                  switch(config)# ip dhcp relay sub-option type cisco
                   

                  Enables DHCP to use Cisco proprietary numbers 150, 152, and 151 when filling the link selection, server ID override, and VRF name/VPN ID relay agent Option 82 suboptions. The no option causes DHCP to use RFC numbers 5, 11, and 151 for the link selection, server ID override, and VRF name/VPN ID suboptions, respectively.

                   
                  Step 4 show ip dhcp relay


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

                  Displays the DHCP relay configuration.

                   
                  Step 5 show running-config dhcp


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

                  Displays the DHCP configuration.

                   
                  Step 6 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.

                  SUMMARY STEPS

                    1.    config t

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

                    3.    (Optional) show ip dhcp snooping binding

                    4.    (Optional) show ip dhcp snooping binding dynamic

                    5.    (Optional) copy running-config startup-config


                  DETAILED STEPS
                      Command or Action Purpose
                    Step 1 config t


                    Example: 
                    switch# config t
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 show ip dhcp snooping binding


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

                    Shows the DHCP snooping static and dynamic bindings.

                     
                    Step 4 show ip dhcp snooping binding dynamic


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

                    Shows the DHCP snooping dynamic bindings.

                     
                    Step 5 copy running-config startup-config


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

                    Copies the running configuration to the startup configuration.

                     

                    This 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 Cisco Nexus 3000 Series NX-OS System Management Configuration Guide.

                    Command

                    Purpose

                    show running-config dhcp

                    Displays the DHCP snooping 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 Cisco Nexus 3000 Series NX-OS System Management Configuration Guide.

                    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.

                    SUMMARY STEPS

                      1.    (Optional) clear ip dhcp snooping binding

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

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

                      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] }

                      5.    show ip dhcp snooping binding


                    DETAILED STEPS
                        Command or Action Purpose
                      Step 1 clear ip dhcp snooping binding


                      Example: 
                      switch# clear ip dhcp snooping binding
                                              		(Optional)

                      Clears all entries from the DHCP snooping binding database.

                       
                      Step 2 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 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 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 show ip dhcp snooping binding


                      Example: 
                      switch# show ip dhcp snooping binding
                       

                      Displays the DHCP snooping binding database.

                       

                      Configuration Examples for DHCP Snooping

                      This 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