Dynamic Host Configuration Protocol (DHCP)

This chapter contains the following topics:

     Overview of DHCP

     Enable DHCP

     Configure DHCP Relays

Overview of DHCP

Dynamic Host Configuration Protocol (DHCP) is a mechanism commonly used by clients to auto-discover their addressing and other configuration information when connected to a network. On ACOS devices, DHCP configuration supports IP address, subnet masks, default gateway, and classless static routes (option 121) from the DHCP server.

You can enable use of DHCP to dynamically configure IP addresses on the following types of interfaces:

     Management interface – A single IP address can be assigned.

     Ethernet data interfaces – Multiple IP addresses can be assigned.

     Virtual ethernet interfaces – Multiple IP addresses can be assigned.

     Trunk interfaces – Multiple IP addresses can be assigned.

Virtual servers and IP NAT pools are also able to use the DHCP-assigned address of a given data interface. If this option is enabled, ACOS updates the VIP or pool address any time the specified data interface’s IP address is changed by DHCP.

Notes

     DHCP can be enabled on an interface only if that interface does not already have any statically assigned IP addresses.

     On ACOS devices deployed in gateway (Layer 3) mode, Ethernet data interfaces can have multiple IP addresses. An interface can have a combination of dynamically assigned addresses (by DHCP) and statically configured addresses. However, if you plan to use both methods of address configuration, static addresses can be configured only after you finish using DHCP to dynamically configure addresses. To use DHCP in this case, you must first delete all the statically configured IP addresses from the interface.

     On vThunder models, if single-IP mode is used, DHCP can be enabled only at the physical interface level.

     On devices deployed in Transparent (Layer 2) mode:

     you can enable DHCP on the management interface and at the global level.

     The VIP address and pool NAT address (if used) should match the global data IP address of the device. Make sure to enable this option when configuring the VIP or pool.

Enable DHCP

Using the GUI

1.     Hover over Network in the navigation bar, and select Interface from the drop-down menu.

2.     Depending on the type of interface on which to configure this feature, select LAN, Virtual Ethernet or Trunk from the menu bar.

3.     Click Edit in the actions column for the interface on which to configure this feature.

4.     Expand the IP section to reveal additional configuration options.

5.     Select the checkbox in the DHCP field.

6.     Click Update.

Using the CLI

To enable DHCP on an interface, use the ip address dhcp command at the configuration level for the interface:

ACOS(config)# interface ethernet 1

ACOS(config-if:ethernet:1)# ip address dhcp

 

Configure DHCP Relays

This section contains the following:

     Overview of DHCP Relays

     Configure DHCP Relays

Overview of DHCP Relays

This section describes DHCP relay support and how to configure it.

You can configure the ACOS device to relay DHCP traffic between DHCP clients and DHCP servers located in different VLANs or subnets.

DHCP relay is supported only for the standard DHCP protocol ports:

     Boot protocol server (BOOTPS) – UDP port 67

     Boot protocol client (BOOTPC) – UDP port 68

DHCP relay service is supported for IPv4 and IPv6.

DHCP is a Client-Server protocol and relies on broadcast communication between the client and server for packet exchanges. Accordingly, the clients and the servers must be in the same broadcast domain (Layer 2 VLAN) for this to work, since Layer 3 routers typically do not forward broadcasts. However, in most deployments it is not practical to have a DHCP server in each Layer 2 VLAN. Instead, it is typical to use a common DHCP server for all VLANs and subnets in the network.

Notes

     In the current release, the helper-address feature provides service for DHCP packets only.

     The interface on which the helper address is configured must have an IP address.

     The helper address can not be the same as the IP address on any interface or an IP address used for SLB.

Configure DHCP Relays

To enable DHCP communication between different VLANs or subnets, you can use a DHCP relay. A DHCP relay acts as a mediator between the DHCP client and the DHCP server when they are not in the same broadcast domain.

To configure the ACOS device as a DHCP relay, configure the DHCP server IP address as a helper address on the IP interface connected to DHCP clients. The ACOS device intercepts broadcast DHCP packets sent by clients on the interface configured with the helper address.

The ACOS device then places the receiving interface’s IP address (not the helper address) in the relay gateway address field, and forwards the DHCP packet to the server. When the DHCP server replies, the ACOS device forwards the response to the client.

Use the GUI to Configure a DHCP Relay

To configure a helper address for the IP interface connected to the DHCP clients:

1.     Hover over Network in the navigation bar, and select Interface from the drop-down menu.

2.     Depending on the type of interface on which to configure this feature, select LAN, Virtual Ethernet or Trunk from the menu bar.

3.     Click Edit in the actions column for the interface on which to configure this feature.

4.     Expand the IP section to reveal additional configuration options.

5.     Specify an IP address for the IP Helper Address field.

6.     Click Add.

7.     You can add up to 2 helper addresses per interface.

8.     Click Update.

Use the CLI to Configure a DHCP Relay

The following commands configure two helper addresses. The helper address for DHCP server 100.100.100.1 is configured on Ethernet interface 1 and on Virtual Ethernet (VE) interfaces 5 and 7. The helper address for DHCP server 20.20.20.102 is configured on VE 9.

NOTE:                               You can configure up to 2 IP helper addresses per Ethernet interface.

ACOS(config)# interface ethernet 1

ACOS(config-if:ethernet:1)# ip helper-address 100.100.100.1

ACOS(config-if:ethernet:1)# exit

ACOS(config)# interface ve 5

ACOS(config-if:ve:5)# ip helper-address 100.100.100.1

ACOS(config-if:ve:5)# exit

ACOS(config)# interface ve 7

ACOS(config-if:ve:7)# ip helper-address 100.100.100.1

ACOS(config-if:ve:7)# exit

ACOS(config)# interface ve 9

ACOS(config-if:ve:9)# ip helper-address 20.20.20.102

 

Use the show ip helper-address command shows summary DHCP relay information:

ACOS(config)# show ip helper-address

Interface  Helper-Address            RX            TX      No-Relay         Drops

---------  --------------  ------------  ------------  ------------  ------------

eth1       100.100.100.1              0             0             0             0

ve5        100.100.100.1           1669          1668             0             1

ve7                                1668          1668             0             0

ve8        100.100.100.1              0             0             0             0

ve9        20.20.20.102               0             0             0             0

 

Use the detail parameter to view additional detailed DHCP relay information:

ACOS# show ip helper-address detail

IP Interface: eth1

------------

 Helper-Address: 100.100.100.1

  Packets:  

           RX: 0

              BootRequest Packets : 0

              BootReply Packets   : 0

           TX: 0

              BootRequest Packets : 0

              BootReply Packets   : 0

 No-Relay: 0

 Drops:

           Invalid BOOTP Port  : 0

           Invalid IP/UDP Len  : 0

           Invalid DHCP Oper   : 0

           Exceeded DHCP Hops  : 0

           Invalid Dest IP     : 0

           Exceeded TTL        : 0

           No Route to Dest    : 0

           Dest Processing Err : 0

 

IP Interface: ve5

------------

 Helper-Address: 100.100.100.1

  Packets:  

           RX: 16

              BootRequest Packets : 16

              BootReply Packets   : 0

           TX: 14

              BootRequest Packets : 0

              BootReply Packets   : 14

 No-Relay: 0

 Drops:

           Invalid BOOTP Port  : 0

           Invalid IP/UDP Len  : 0

           Invalid DHCP Oper   : 0

           Exceeded DHCP Hops  : 0

           Invalid Dest IP     : 0

           Exceeded TTL        : 0

           No Route to Dest    : 2

           Dest Processing Err : 0

 

IP Interface: ve7

------------

 Helper-Address: None

  Packets:  

           RX: 14

              BootRequest Packets : 0

               BootReply Packets   : 14

           TX: 14

              BootRequest Packets : 14

              BootReply Packets   : 0

 No-Relay: 0

 Drops:

           Invalid BOOTP Port  : 0

           Invalid IP/UDP Len  : 0

           Invalid DHCP Oper   : 0

           Exceeded DHCP Hops  : 0

           Invalid Dest IP     : 0

           Exceeded TTL        : 0

           No Route to Dest    : 0

           Dest Processing Err : 0

Descriptions for the fields in both outputs are available in the Command Line Interface Reference.

The following command clears the DHCP relay counters:

ACOS# clear ip helper-address statistics

 

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