DHCP Server Port-Based Address Allocation

The DHCP Server Port-Based Address Allocation feature provides port-based address allocation support on the Cisco IOS Dynamic Host Configuration Protocol (DHCP) server for the Ethernet platform. The DHCP server provides address assignment support based on the point of attachment of the client network.

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Restrictions for DHCP Server Port-Based Address Allocation

The DHCP Server Port-Based Address Allocation feature does not support Virtual routing and forwarding (VRF) and virtual private network (VPNs).

Information About DHCP Server Port-Based Address Allocation

DHCP Server Port-Based Address Allocation Feature Design

When Cisco industrial Ethernet switches are deployed on the factory floor, they offer connectivity to the directly connected manufacturing devices. A failure manufacturing device must be repaired immediately in the existing network or replaced by a new device. The DHCP protocol recognizes DHCP clients by the client identifier (ID) option in the DHCP packet. Clients who do not include the client ID option are identified by the client hardware address. The DHCP Server Port-Based Address Allocation feature introduces the capability to ensure that the same IP address is always offered to the replacement device as the device being replaced. This IP address is always offered to the same connected port even as the client ID or client hardware address (chaddr) changes in the DHCP messages received on that port.

If this feature is configured, the port name of the interface overrides the information the client sends and the actual point of connection. Then a port on the switch becomes the client ID.

In all cases, if you connect the Ethernet cable to the same port, the same IP address is allocated through the DHCP to the attached device. The figure below shows an industrial Ethernet switch using DHCP to assign one IP address per port to directly connected manufacturing devices.

Figure 1. DHCP Server Port-Based Address Assignment to Directly Connected Manufacturing Devices

How to Configure DHCP Server Port-Based Address Allocation

Automatically Generating a Subscriber Identifier for a DHCP Message Received on a Port

Perform this task to automatically generate a unique ID, called a subscriber ID for a DHCP message received on a port.

If the DHCP Server Port-Based Address Allocation feature is configured, the subscriber ID value is used in place of the client ID to provide stable IP address assignment. The subscriber ID value is based on the short name of the port to which the directly connected device is attached. If this device is removed and replaced with a new device, the new device maintains the same subscriber ID.

The subscriber ID is used at the same point where the client ID or the client MAC address is currently captured during the DHCP IP address assignment process.

SUMMARY STEPS

    1.    enable

    2.    configure terminal

    3.    ip dhcp use subscriber-id client-id

    4.    interface type number

    5.    ip dhcp server use subscriber-id client-id


DETAILED STEPS
     Command or ActionPurpose
    Step 1 enable


    Example:
    Router> enable
     

    Enables privileged EXEC mode.

    • Enter your password if prompted.

     
    Step 2 configure terminal


    Example:
    Router# configure terminal
     

    Enters global configuration mode.

     
    Step 3 ip dhcp use subscriber-id client-id


    Example:
    Router(config)# ip dhcp use subscriber-id client-id
     

    Configures the DHCP server to globally use the subscriber ID as the client ID on all incoming DHCP messages.

    • DHCP uses the subscriber ID configured on the interface to generate the client ID. If no subscriber ID is configured then the client ID is automatically generated based on the short name of the interface. The client ID already present in the message is ignored.

    • For port based address allocation, do not configure any subscriber ID on the interface. It must be generated automatically from interface name.

     
    Step 4 interface type number


    Example:
    Router(config)# interface Ethernet 0/0
     

    (Optional) Configures an interface and enters interface configuration mode.

     
    Step 5 ip dhcp server use subscriber-id client-id


    Example:
    Router(config-if)# ip dhcp server use subscriber-id client-id
     

    (Optional) Configures the DHCP server to use the subscriber ID as the client ID on all incoming DHCP messages on the interface.

     

    Troubleshooting Tips

    Use the following command to debug any errors that you may encounter when you configure DHCP to automatically generate a unique ID:

    • debug ip dhcp server packets

    Preassigning IP Addresses and Associating Them to a Client

    Perform this task to preassign an IP address and associate it to a client identified by a client ID or MAC address.

    For port-based address assignment, you must perform the task in the Automatically Generating a Subscriber Identifier for a DHCP Message Received on a Port task to associate the client ID with the subscriber ID. The subscriber ID value is based on the short name of the port to which the directly connected device is attached.

    Configure a normal DHCP pool by supplying any DHCP options and lease time. Preassigned addresses are automatically excluded from normal dynamic IP address assignment. Preassigned addresses cannot be used in host pools, but there can be multiple preassigned addresses per DHCP address pool.


    Note
    • Only one IP address can be assigned per port.
    • Preassigned addresses (also called reserved addresses) cannot be cleared by using the clear ip dhcp binding command.
    >
    SUMMARY STEPS

      1.    enable

      2.    configure terminal

      3.    ip dhcp pool name

      4.    network network-number [mask | / prefix-length]

      5.    address ip-address client-id string [ascii]

      6.    address ip-address hardware-address mac-address [hardware-number]

      7.    end

      8.    show ip dhcp pool [name]

      9.    show ip dhcp binding


    DETAILED STEPS
       Command or ActionPurpose
      Step 1 enable


      Example:
      Router> enable
       

      Enables privileged EXEC mode.

      • Enter your password if prompted.

       
      Step 2 configure terminal


      Example:
      Router# configure terminal
       

      Enters global configuration mode.

       
      Step 3 ip dhcp pool name


      Example:
      Router(config)# ip dhcp pool pool1
       

      Creates a name for the DHCP server address pool and enters DHCP pool configuration mode.

       
      Step 4 network network-number [mask | / prefix-length]


      Example:
      Router(dhcp-config)# network 10.10.10.0 /24
       

      Specifies the subnet network number and mask of the DHCP address pool.

       
      Step 5 address ip-address client-id string [ascii]


      Example:
      Router(dhcp-config)# address 10.10.10.2 client-id Et1/0 ascii
       

      Reserves an IP address for a DHCP client identified by the client ID.

      • The string argument can be an ASCII value or a hexadecimal value.

      • For port-based address allocation the string argument must be the name of the port and the ascii keyword must be specified.

       
      Step 6 address ip-address hardware-address mac-address [hardware-number]


      Example:
      Router(dhcp-config)# address 10.10.10.3 hardware-address b708.1388.f166
       

      (Optional) Reserves an IP address for a client identified by the hardware address.

      • This command is used for clients identified by the hardware address included in the fixed-size header of the DHCP message.

       
      Step 7 end


      Example:
      Router(dhcp-config)# end
       

      Returns to privileged EXEC mode.

       
      Step 8 show ip dhcp pool [name]


      Example:
      Router> show ip dhcp pool
       

      (Optional) Displays information about DHCP address pools.

       
      Step 9 show ip dhcp binding


      Example:
      Router> show ip dhcp binding


      Example:
      

        
      
 
            		 
          
      
(Optional) Displays infinite binding for the configured addresses.
      

         
      
      

      

      

      

      Preassigning IP Addresses and Associating Them to a Client
      

      
      

      Note
      
        
      
Perform this task to restrict address assignments from the DHCP address pool only to preconfigured reservations.
      

        
      
When the DHCP Server Port-Based Address Allocation feature is configured on multiple switches, devices connected to one switch may also receive an IP address assignment from the neighboring switches rather than the local DHCP address pool switch. If you want the switch to serve only the client directly connected to the switch, you can configure a group of switches with pools that share a common IP subnet and ignore the requests from other clients (not connected to this switch). 
      

>
      

    
      

      
SUMMARY STEPS
        1.   
      
          
            enable
          
        
        

        2.   
      
          
            configure
          
          
            terminal
          
        
        

        3.   
      
          
            ip
            dhcp
            pool
          
          
            name
          
        
        

        4.   
      
          
            reserved-only
          
        
        


      DETAILED STEPS
         Command or ActionPurpose
        Step 1
          
            enable
          
        
         

        Example:
        
          
        Router> enable
        

        
        
 
              		 
          
        
Enables privileged EXEC mode. 
        

          
        • 
Enter your password if prompted.
          

          • 

        

         
      
        

        Step 2
          
            configure
          
          
            terminal
          
        
         

        Example:
        
          
        Router# configure terminal
        

        
        
 
              		 
          
        
Enters global configuration mode.
        

         
      
        

        Step 3
          
            ip
            dhcp
            pool
          
          
            name
          
        
         

        Example:
        
          
        Router(config)# ip dhcp pool pool1
        

        
        
 
              		 
          
        
Configures a DHCP address pool on a DHCP server and enters DHCP pool configuration mode.
        

         
      
        

        Step 4
          
            reserved-only
          
        
         

        Example:
        
          
        Router(dhcp-config)# reserved-only
        

        
        
 
              		 
          
        
Restricts address assignments from the DHCP address pool only to the preconfigured reservations.
        

         
      
        

        

        

        

        

        Configuration Examples for DHCP Server Port-Based Address Allocation
        

         
	 
        

        

        DHCP Server Port-Based Address Allocation  Example
        

        
      
        
In the following example, a subscriber ID will be automatically generated based on the short name of the interface (port) specified by the address client-id command. The DHCP server will ignore any client ID fields in the DHCP messages and use this subscriber ID as the client ID. The DHCP client is preassigned IP address 10.1.1.7.
        

      
        !
ip dhcp use subscriber-id client-id
ip dhcp subscriber-id interface-name
ip dhcp excluded-address 10.1.1.1 10.1.1.3
!         
ip dhcp pool dhcppool
 network 10.1.1.0 255.255.255.0
 address 10.1.1.7 client-id Et1/0 ascii

        

      
        
The following example shows that the preassigned address was correctly reserved in the DHCP pool:
        

      
        Router# show ip dhcp pool dhcppool
Pool test :
 Current index        IP address range                    Leased/Total
 10.1.1.1              10.1.1.1 - 10.1.1.254         0     / 254
 3 reserved addresses are currently in the pool :
Address          Client
 10.1.1.07         Et1/0
 10.1.1.20         xyz
 10.1.1.30         aabb.cc00.1501
        

    
        

        

        

        

        Additional References
        

         
	 
        

        Related Documents
         
		 
		

         
					 
         Related Topic
					 
        
 
				          		
 
					 
         Document Title
					 
        
 
				          		

        
        

         
					 
         DHCP commands: complete command syntax, command mode, command history, defaults, usage guidelines, and examples 
					 
        
 
				          		
 
					 
         
						Cisco IOS IP Addressing Services Command Reference 
					 
        
 
				          		

        
        

        

        
 
	 
        

        Standards
         
		 
		

         
					 
         Standard
					 
        
 
				          		
 
					 
         Title
					 
        
 
				          		

        
        

         
					 
         No new or modified standards are supported by this feature and support for existing standards has not been modified by this feature.
					 
        
 
				          		
 
					 
         --
					 
        
 
				          		

        
        

        

        
 
	 
        

        MIBs
         
		 
		

         
					 
         MIB
					 
        
 
				          		
 
					 
         MIBs Link
					 
        
 
				          		

        
        

         
					 
         No new or modified standards are supported by this feature and support for existing standards has not been modified by this feature.
					 
        
 
				          		
 
					 
         To locate and download MIBs for selected platforms, Cisco software releases, and feature sets, use Cisco MIB Locator found at the following URL:
					 
        
 
					 
         
						http:/​/​www.cisco.com/​go/​mibs 
					 
        
 
				          		

        
        

        

        
 
	 
        

        RFCs
         
		 
		

         
					 
         RFC
					 
        
 
				          		
 
					 
         Title
					 
        
 
				          		

        
        

         
					 
         No new or modified RFCs are supported by this feature.
					 
        
 
				          		
 
					 
         --
					 
        
 
				          		

        
        

        

        
 
	 
        

        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 Server Port-Based Address Allocation
        

        
			
        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. 
        

         
		

        Table 1 Feature Information for
		  DHCP Port-Based Address Allocation
         
				  
         Feature
					 Name 
				  
        
 
				        		
 
				  
         Releases 
				  
        
 
				        		
 
				  
         Feature
					 Information 
				  
        
 
				        		

        
        

         
				  
         DHCP
					 Server Port-Based Address Allocation 
				  
        
 
				        		
 
				   
				   
				   
				  
         Cisco IOS XE Release 3.2SE 
				  
        
 
				        		
 
				  
         The DHCP
					 Server Port-Based Address Allocation feature provides port-based address
					 allocation support on the Cisco IOS DHCP server for the industrial Ethernet
					 platform. The DHCP server provides address assignment support based on the
					 point of attachment of the client to the network. 
				  
        
 
				  
         The
					 following commands were introduced or modified: 
					 address 
						client-id, 
					 address 
						hardware-address, 
					 ip 
						dhcp 
						server 
						use 
						subscriber-id 
						client-id, 
					 ip 
						dhcp 
						subscriber-id 
						interface-name, 
					 ip 
						dhcp 
						use 
						subscriber-id 
						client-id, 
					 reserved-only, and 
					 show 
						ip 
						dhcp 
						pool.