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This chapter describes how to configure the Dynamic Host Configuration Protocol (DHCP) on a Cisco NX-OS device.
This chapter includes the following sections:
Your software release might not support all the features documented in this module. For the latest caveats and feature information, see the Bug Search Tool at https://tools.cisco.com/bugsearch/ and the release notes for your 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 "New and Changed Information"chapter or the Feature History table in this chapter.
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 can be enabled globally and on a per-VLAN basis. By default, the feature is disabled globally and on all VLANs. You can enable the feature on a single VLAN or a range of VLANs.
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 device that is under your administrative control. These devices include the switches, routers, and servers in the network. Any device 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 device 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 NX-OS device, 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 devices (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 device through trusted interfaces. |
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 device receives specific DHCP messages. For example, the feature adds an entry to the database when the device receives a DHCPACK message from the server. The feature removes the entry in the database when the IP address lease expires or the device 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.
Dynamic ARP inspection (DAI) and IP Source Guard also use information stored in the DHCP snooping binding database.
You can remove entries from the binding database by using the clear ip dhcp snooping binding command.
A virtual port channel (vPC) allows two Cisco NX-OS switches to appear as a single logical port channel to a third device. The third device can be a switch, server, or any other networking device that supports port channels.
In a typical vPC environment, DHCP requests can reach one vPC peer switch and the responses can reach the other vPC peer switch, resulting in a partial DHCP (IP-MAC) binding entry in one switch and no binding entry in the other switch. As a result, DHCP snooping and associated features such as dynamic ARP inspection (DAI) and IP Source Guard are disrupted. Beginning with Cisco NX-OS Release 5.1, this issue is addressed by using Cisco Fabric Service over Ethernet (CFSoE) distribution to ensure that all DHCP packets (requests and responses) appear on both switches, which helps in creating and maintaining the same binding entry on both switches for all clients behind the vPC link.
CFSoE distribution also allows only one switch to forward the DHCP requests and responses on the vPC link. In non-vPC environments, both switches forward the DHCP packets.
When the remote vPC is online, all the binding entries for that vPC link should be synchronized with the peer.
When DHCP snooping is enabled on the peer switch, the dynamic binding entries for all vPC links should be synchronized with the peer.
The device validates DHCP packets received on the untrusted interfaces of VLANs that have DHCP snooping enabled. The device forwards the DHCP packet unless any of the following conditions occur (in which case, the packet is dropped):
The device receives a DHCP response packet (such as a DHCPACK, DHCPNAK, or DHCPOFFER packet) on an untrusted interface.
The device receives a packet on an untrusted interface, and the source MAC address and the DHCP client hardware address do not match. This check is performed only if the DHCP snooping MAC address verification option is turned on.
The device receives a DHCPRELEASE or DHCPDECLINE message from an untrusted host with an entry in the DHCP snooping binding table, and the interface information in the binding table does not match the interface on which the message was received.
In addition, you can enable strict validation of DHCP packets, which checks the options field of DHCP packets, including the “magic cookie” value in the first four bytes of the options field. By default, strict validation is disabled. When you enable it, by using the ip dhcp packet strict-validation command, if DHCP snooping processes a packet that has an invalid options field, it drops the packet.
DHCP can centrally manage the IP address assignments for a large number of subscribers. When you enable Option 82, the device identifies a subscriber device that connects to the network (in addition to its MAC address). Multiple hosts on the subscriber LAN can connect to the same port on the access device and are uniquely identified.
When you enable Option 82 on the Cisco NX-OS device, the following sequence of events occurs:
The host (DHCP client) generates a DHCP request and broadcasts it on the network.
When the Cisco NX-OS device receives the DHCP request, it adds the Option 82 information in the packet. The Option 82 information contains the device MAC address (the remote ID suboption) and the port identifier, vlan-mod-port, from which the packet is received (the circuit ID suboption). For hosts behind the port channel, the circuit ID is filled with the if_index of the port channel.
Note |
For vPC peer switches, the remote ID suboption contains the vPC switch MAC address, which is unique in both switches. This MAC address is computed with the vPC domain ID. The Option 82 information is inserted at the switch where the DHCP request is first received before it is forwarded to the other vPC peer switch. |
The device forwards the DHCP request that includes the Option 82 field to the DHCP server.
The DHCP server receives the packet. If the server is Option 82 capable, it can use the remote ID, the circuit ID, or both to assign IP addresses and implement policies, such as restricting the number of IP addresses that can be assigned to a single remote ID or circuit ID. The DHCP server echoes the Option 82 field in the DHCP reply.
The DHCP server sends the reply to the Cisco NX-OS device. The Cisco NX-OS device verifies that it originally inserted the Option 82 data by inspecting the remote ID and possibly the circuit ID fields. The Cisco NX-OS device removes the Option 82 field and forwards the packet to the interface that connects to the DHCP client that sent the DHCP request.
If the previously described sequence of events occurs, the following values do not change:
Circuit ID suboption fields
Suboption type
Length of the suboption type
Circuit ID type
Length of the circuit ID type
Remote ID suboption fields
Suboption type
Length of the suboption type
Remote ID type
Length of the circuit ID type
This figure shows the packet formats for the remote ID suboption and the circuit ID suboption. The Cisco NX-OS device uses the packet formats when you globally enable DHCP snooping and when you enable Option 82 data insertion and removal. For the circuit ID suboption, the module field is the slot number of the module.
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 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.
After you enable Option 82, the device uses the binary ifindex format by default. If needed, you can change the Option 82 setting to use an encoded string format instead.
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. |
You can enable the device to insert and remove Option 82 information on DHCP packets that are forwarded by the relay agent.
This figure shows an example of a metropolitan Ethernet network in which a centralized DHCP server assigns IP addresses to subscribers connected to the device at the access layer. Because the DHCP clients and their associated DHCP server do not reside on the same IP network or subnet, a DHCP relay agent is configured with a helper address to enable broadcast forwarding and to transfer DHCP messages between the clients and the server.
When you enable Option 82 for the DHCP relay agent on the Cisco NX-OS device, the following sequence of events occurs:
The host (DHCP client) generates a DHCP request and broadcasts it on the network.
When the Cisco NX-OS device receives the DHCP request, it adds the Option 82 information in the packet. The Option 82 information contains the device MAC address (the remote ID suboption) and the port identifier, vlan-mod-port, from which the packet is received (the circuit ID suboption). In DHCP relay, the circuit ID is filled with the if_index of the SVI or Layer 3 interface on which DHCP relay is configured.
Note |
For vPC peer devices, the remote ID suboption contains the vPC device MAC address, which is unique in both devices. This MAC address is computed with the vPC domain ID. The Option 82 information is inserted at the device where the DHCP request is first received before it is forwarded to the other vPC peer device. |
The device adds the IP address of the relay agent to the DHCP packet.
The device forwards the DHCP request that includes the Option 82 field to the DHCP server.
The DHCP server receives the packet. If the server is Option 82 capable, it can use the remote ID, the circuit ID, or both to assign IP addresses and implement policies, such as restricting the number of IP addresses that can be assigned to a single remote ID or circuit ID. The DHCP server echoes the Option 82 field in the DHCP reply.
The DHCP server unicasts the reply to the Cisco NX-OS device if the request was relayed to the server by the device. The Cisco NX-OS device verifies that it originally inserted the Option 82 data by inspecting the remote ID and possibly the circuit ID fields. The Cisco NX-OS device removes the Option 82 field and forwards the packet to the interface that connects to the DHCP client that sent the DHCP request.
You can configure the DHCP relay agent to forward DHCP broadcast messages from clients in a virtual routing and forwarding (VRF) instance 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. For general information about VRFs, see the Cisco Nexus 7000 Series NX-OS Unicast Routing Configuration Guide.
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 includes the following:
Name of the VRF that the interface that receives the DHCP request is a member of.
Subnet address of the interface that receives the DHCP request. When DHCP smart relay is enabled, the link selection is filled with the subnet of the active giaddr.
IP address of the interface that receives the DHCP request. When DHCP smart relay is enabled, the server identifier is filled with the active giaddr.
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.
When the DHCP relay agent receives broadcast DHCP request packets from a host, it sets giaddr to the primary address of the inbound interface and forwards the packets to the server. The server allocates IP addresses from the giaddr subnet pool until the pool is exhausted and ignores further requests.
Beginning with Cisco NX-OS Release 5.2, you can configure the DHCP smart relay agent to allocate IP addresses from the secondary IP address subnet pool if the first subnet pool is exhausted or the server ignores further requests. This enhancement is useful if the number of hosts is greater than the number of IP addresses in the pool or if multiple subnets are configured on an interface using secondary addresses.
The following information applies to DHCP used in virtual device contexts (VDCs):
DHCP snooping binding databases are unique per VDC. Bindings in one VDC do not affect DHCP snooping in other VDCs.
The system does not limit the binding database size on a per-VDC basis.
The DHCP smart relay agent can be configured independently in default and nondefault VDCs.
This table shows the licensing requirements for DHCP.
|
Product |
License Requirement |
|---|---|
|
Cisco NX-OS |
DHCP requires no license. Any feature not included in a license package is bundled with the Cisco NX-OS system images and is provided at no extra charge to you. For an explanation of the Cisco NX-OS licensing scheme, see the Cisco NX-OS Licensing Guide. |
DHCP has the following prerequisite:
You should be familiar with DHCP before you configure DHCP snooping or the DHCP relay agent.
DHCP has the following configuration guidelines and limitations:
Use the uRFP loose mode, not uRFP strict.
Configure static routes for the interface address on the affected FHRP interfaces and redistribute the static routes into IGP.
Using the ip dhcp relay source-interface interface-name command, you can configure a different interface as the source interface.
For Cisco NX-OS Release 6.2 and later releases, you must enable the insertion of Option 82 information for DHCP packets to support the highest DHCP snooping scale.
After System Switchover, DHCP Global stats show incorrect values as they are not stored in PSS and get erased. Updating stats in PSS during packet path will affect scale.
If you use DHCP relay where DHCP clients and servers are in different VRF instances, use only one DHCP server within a VRF.
Before globally enabling DHCP snooping on the device, make sure that the devices acting as the DHCP server and the DHCP relay agent are configured and enabled.
DHCP snooping does not work with DHCP relay configured on the same nexus device.
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. When both DHCP snooping and DHCP relay are enabled on a VLAN and the SVI of that VLAN, DHCP relay takes precedence.
If an ingress router ACL is configured on a Layer 3 interface that you are configuring with a DHCP server address, ensure that the router ACL permits DHCP traffic between DHCP servers and DHCP hosts.
Access-control list (ACL) statistics are not supported if the DHCP snooping feature is enabled.
Make sure that the DHCP configuration is synchronized across the devices in a vPC link. Otherwise, a run-time error can occur, resulting in dropped packets.
Beginning with Cisco NX-OS Release 5.1, DHCP snooping is supported with FabricPath. Follow these guidelines when enabling DHCP snooping in a FabricPath network:
DHCP snooping should be enabled on CE-FabricPath boundary devices.
DHCP snooping is enabled on all access layer devices to secure the network at the access layer itself.
DHCP does not learn any binding entries on ports in FabricPath mode as users should have enabled DHCP snooping on all access layer devices. As a result, when DAI is enabled, ARP packets received on FabricPath ports are allowed.
IPSG cannot be enabled on ports in FabricPath mode.
Note |
For more information on FabricPath, see the Cisco Nexus 7000 Series NX-OS FabricPath Configuration Guide. |
DHCP smart relay and DHCP subnet broadcast support are limited to the first 100 IP addresses of the interface on which they are enabled.
You must configure a helper address on the interface in order to use DHCP smart relay and DHCP subnet broadcast support.
In a vPC environment with DHCP smart relay enabled, the subnet of the primary and secondary addresses of an interface should be the same on both Cisco NX-OS devices.
Before using POAP, make sure that DHCP snooping is enabled and firewall rules are set to block unintended or malicious DHCP servers.
Note |
For DHCP configuration limits, see the Cisco Nexus 7000 Series NX-OS Verified Scalability Guide. |
This table lists the default settings for DHCP parameters.
|
Parameters |
Default |
|---|---|
|
DHCP feature |
Disabled |
|
DHCP snooping |
Disabled |
|
DHCP snooping on VLANs |
Disabled |
|
DHCP snooping MAC address verification |
Enabled |
|
DHCP snooping Option 82 support |
Disabled |
|
DHCP snooping trust |
Untrusted |
|
DHCP relay agent |
Enabled |
|
VRF support for the DHCP relay agent |
Disabled |
|
DHCP relay sub-option type cisco |
Disabled |
|
DHCP Option 82 for relay agent |
Disabled |
|
Subnet broadcast support for the DHCP relay agent |
Disabled |
|
DHCP smart relay agent |
Disabled |
|
DHCP server IP address |
None |
| Step 1 |
Enable the DHCP feature. When the DHCP feature is disabled, you cannot configure DHCP snooping. |
| Step 2 |
Enable DHCP snooping globally. |
| Step 3 |
Enable DHCP snooping on at least one VLAN. By default, DHCP snooping is disabled on all VLANs. |
| Step 4 |
Ensure that the DHCP server is connected to the device using a trusted interface. |
| Step 5 |
(Optional) If DHCP servers and clients are in different VRF instances, do the following:
|
| Step 6 |
(Optional) Configure an interface with the IP address of the DHCP server. |
You can enable or disable the DHCP feature on the device. By default, DHCP is disabled.
When the DHCP feature is disabled, you cannot configure DHCP snooping, the DHCP relay agent, or any of the features that depend on DHCP, such as dynamic ARP inspection and IP Source Guard. In addition, all DHCP, dynamic ARP inspection, and IP Source Guard configuration is removed from the device.
| Command or Action | Purpose | |
|---|---|---|
| Step 1 |
config t Example: |
Enters global configuration mode. |
| Step 2 |
[no ] feature dhcp Example: |
Enables the DHCP feature. The no option disables the DHCP feature and erases all DHCP configuration. |
| Step 3 |
(Optional) show running-config dhcp Example: |
(Optional)
Displays the DHCP configuration. |
| Step 4 |
(Optional) copy running-config startup-config Example: |
(Optional)
Copies the running configuration to the startup configuration. |
You can enable or disable DHCP snooping globally on the device.
Ensure that you have enabled the DHCP feature.
| Command or Action | Purpose | |
|---|---|---|
| Step 1 |
config t Example: |
Enters global configuration mode. |
| Step 2 |
[no ] ip dhcp snooping Example: |
Enables DHCP snooping globally. The no option disables DHCP snooping. |
| Step 3 |
(Optional) show running-config dhcp Example: |
(Optional)
Displays the DHCP configuration. |
| Step 4 |
(Optional) copy running-config startup-config Example: |
(Optional)
Copies the running configuration to the startup configuration. |
You can enable or disable DHCP snooping on one or more VLANs. By default, DHCP snooping is disabled on all VLANs.
Ensure that the DHCP feature 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. |
| Command or Action | Purpose | |
|---|---|---|
| Step 1 |
config t Example: |
Enters global configuration mode. |
| Step 2 |
[no ] ip dhcp snooping vlan vlan-list Example: |
Enables DHCP snooping on the VLANs specified by vlan-list . The no option disables DHCP snooping on the VLANs specified. |
| Step 3 |
(Optional) show running-config dhcp Example: |
(Optional)
Displays the DHCP configuration. |
| Step 4 |
(Optional) copy running-config startup-config Example: |
(Optional)
Copies the running configuration to the startup configuration. |
You can enable or disable DHCP snooping MAC address verification. If the device receives a packet on an untrusted interface and the source MAC address and the DHCP client hardware address do not match, address verification causes the device to drop the packet. MAC address verification is enabled by default.
Ensure that the DHCP feature is enabled.
| Command or Action | Purpose | |
|---|---|---|
| Step 1 |
config t Example: |
Enters global configuration mode. |
| Step 2 |
[no ] ip dhcp snooping verify mac-address Example: |
Enables DHCP snooping MAC address verification. The no option disables MAC address verification. |
| Step 3 |
(Optional) show running-config dhcp Example: |
(Optional)
Displays the DHCP configuration. |
| Step 4 |
(Optional) copy running-config startup-config Example: |
(Optional)
Copies the running configuration to the startup configuration. |
You can enable or disable the insertion and removal of Option 82 information for DHCP packets forwarded without the use of the DHCP relay agent. By default, the device does not include Option 82 information in DHCP packets.
Note |
DHCP relay agent support for Option 82 is configured separately. |
Ensure that the DHCP feature is enabled.
| Command or Action | Purpose | |
|---|---|---|
| Step 1 |
config t Example: |
Enters global configuration mode. |
| Step 2 |
[no ] ip dhcp snooping information option Example: |
Enables the insertion and removal of Option 82 information for DHCP packets. The no option disables the insertion and removal of Option 82 information. |
| Step 3 |
(Optional) show running-config dhcp Example: |
(Optional)
Displays the DHCP configuration. |
| Step 4 |
(Optional) copy running-config startup-config Example: |
(Optional)
Copies the running configuration to the startup configuration. |
You can enable or disable the strict validation of DHCP packets. By default, strict validation of DHCP packets is disabled.
| Command or Action | Purpose | |
|---|---|---|
| Step 1 |
config t Example: |
Enters global configuration mode. |
| Step 2 |
[no ] ip dhcp packet strict-validation Example: |
Enables the strict validation of DHCP packets. The no option disables strict DHCP packet validation. |
| Step 3 |
(Optional) show running-config dhcp Example: |
(Optional)
Displays the DHCP configuration. |
| Step 4 |
(Optional) copy running-config startup-config Example: |
(Optional)
Copies the running configuration to the startup configuration. |
You can configure whether an interface is a trusted or untrusted source of DHCP messages. By default, all interfaces are untrusted. You can configure DHCP trust on the following types of interfaces:
Layer 2 Ethernet interfaces
Layer 2 port-channel interfaces
Ensure that the DHCP feature is enabled.
Ensure that the interface is configured as a Layer 2 interface.
| Command or Action | Purpose | |
|---|---|---|
| Step 1 |
config t Example: |
Enters global configuration mode. |
| Step 2 |
Do one of the following options:
Example: |
|
| Step 3 |
[no ] ip dhcp snooping trust Example: |
Configures the interface as a trusted interface for DHCP snooping. The no option configures the port as an untrusted interface. |
| Step 4 |
(Optional) show running-config dhcp Example: |
(Optional)
Displays the DHCP configuration. |
| Step 5 |
(Optional) copy running-config startup-config Example: |
(Optional)
Copies the running configuration to the startup configuration. |
You can enable or disable the DHCP relay trusted port functionality. By default, if the gateway address is set to all zeros in the DHCP packet and the relay information option is already present in the packet, the DHCP relay agent will not discard the packet. If the ip dhcp relay information option trust command is configured globally, the DHCP relay agent will discard the packet if the gateway address is set to all zeros.
Ensure that the DHCP feature is enabled.
| Command or Action | Purpose | |
|---|---|---|
| Step 1 |
config t Example: |
Enters global configuration mode. |
| Step 2 |
[no ] ip dhcp relay information option trust Example: |
Enables the DHCP relay trusted port functionality. The no option disables this functionality. |
| Step 3 |
(Optional) show ip dhcp relay Example: |
(Optional)
Displays the DHCP relay configuration. |
| Step 4 |
(Optional) show ip dhcp relay information trusted-sources Example: |
(Optional)
Displays the DHCP relay trusted ports configuration. |
| Step 5 |
(Optional) show running-config dhcp Example: |
(Optional)
Displays the DHCP configuration. |
| Step 6 |
(Optional) copy running-config startup-config Example: |
(Optional)
Copies the running configuration to the startup configuration. |
You can configure whether a Layer 3 interface is a DHCP relay trusted or untrusted interface. By default, all interfaces are untrusted. You can configure DHCP relay trust on the following types of interfaces:
Layer 3 Ethernet interfaces and sub-interfaces
Layer 3 port-channel interfaces
Interface VLAN
Ensure that the DHCP feature is enabled.
| Command or Action | Purpose | |||
|---|---|---|---|---|
| Step 1 |
config t Example: |
Enters global configuration mode. |
||
| Step 2 |
Do one of the following options:
Example: |
|
||
| Step 3 |
[no ] ip dhcp relay information trusted Example: |
Configures the interface as a trusted interface for DHCP relay agent information. The no option configures the port as an untrusted interface.
|
||
| Step 4 |
show ip dhcp relay information trusted-sources Example: |
Displays the DHCP relay trusted ports configuration. |
||
| Step 5 |
(Optional) show running-config dhcp Example: |
(Optional)
Displays the DHCP configuration. |
||
| Step 6 |
(Optional) copy running-config startup-config Example: |
(Optional)
Copies the running configuration to the startup configuration. |
You can configure all Layer 3 interfaces as DHCP relay trusted or untrusted interfaces. By default, all interfaces are untrusted. You can configure DHCP relay trust on the following types of interfaces:
Layer 3 Ethernet interfaces and sub-interfaces
Layer 3 port-channel interfaces
Interface VLAN
When you enable the ip dhcp relay information trust-all command, any Layer 3 interface cannot be considered as untrusted irrespective of the interface-level configuration.
Ensure that the DHCP feature is enabled.
| Command or Action | Purpose | |
|---|---|---|
| Step 1 |
config t Example: |
Enters global configuration mode. |
| Step 2 |
[no ] ip dhcp relay information trust-all Example: |
Configures the interfaces as trusted sources of DHCP messages. The no option configures the ports as untrusted interfaces. |
| Step 3 |
show ip dhcp relay information trusted-sources Example: |
Displays the DHCP relay trusted ports configuration. |
| Step 4 |
(Optional) show running-config dhcp Example: |
(Optional)
Displays the DHCP configuration. |
| Step 5 |
(Optional) copy running-config startup-config Example: |
(Optional)
Copies the running configuration to the startup configuration. |
You can enable or disable the DHCP relay agent. By default, the DHCP relay agent is enabled.
Ensure that the DHCP feature is enabled.
| Command or Action | Purpose | |
|---|---|---|
| Step 1 |
config t Example: |
Enters global configuration mode. |
| Step 2 |
[no ] ip dhcp relay Example: |
Enables the DHCP relay agent. The no option disables the relay agent. |
| Step 3 |
(Optional) show ip dhcp relay Example: |
(Optional)
Displays the DHCP relay configuration. |
| Step 4 |
(Optional) show running-config dhcp Example: |
(Optional)
Displays the DHCP configuration. |
| Step 5 |
(Optional) copy running-config startup-config Example: |
(Optional)
Copies the running configuration to the startup configuration. |
You can enable or disable the DHCP relay source interface. You can configure a different interface as the source of the DHCP relay agent.
Ensure that the DHCP feature is enabled.
| Command or Action | Purpose | |
|---|---|---|
| Step 1 |
configure terminal Example: |
Enters global configuration mode. |
| Step 2 |
[no ] ip dhcp relay source-interface interface-name Example: |
Enables the DHCP relay source interface. You can configure a different interface as the source of the DHCP relay agent, The no option disables the relay source interface. |
| Step 3 |
(Optional) show ip dhcp relay source-interface Example: |
(Optional)
Displays the DHCP relay source-interface configuration. |
| Step 4 |
(Optional) show running-config dhcp Example: |
(Optional)
Displays the DHCP configuration. |
| Step 5 |
(Optional) copy running-config startup-config Example: |
(Optional)
Copies the running configuration to the startup configuration. |
You can enable or disable the device to insert and remove Option 82 information on DHCP packets forwarded by the relay agent.
By default, the DHCP relay agent does not include Option 82 information in DHCP packets.
| Command or Action | Purpose | |
|---|---|---|
| Step 1 |
configure terminal Example: |
Enters global configuration mode. |
| Step 2 |
[no ] ip dhcp relay Example: |
Enables the DHCP relay feature. The no option disables this behavior. |
| Step 3 |
[no ] ip dhcp relay information option Example: |
Enables the DHCP relay agent to insert and remove Option 82 information on the packets that it forwards. The Option 82 information is in binary ifindex format by default. The no option disables this behavior. |
| Step 4 |
(Optional) show ip dhcp relay Example: |
(Optional)
Displays the DHCP relay configuration. |
| Step 5 |
(Optional) show running-config dhcp Example: |
(Optional)
Displays the DHCP configuration. |
| Step 6 |
(Optional) copy running-config startup-config Example: |
(Optional)
Saves the change persistently through reboots and restarts by copying the running configuration to the startup configuration. |
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 instance.
You must enable Option 82 for the DHCP relay agent.
| Command or Action | Purpose | |
|---|---|---|
| Step 1 |
config t Example: |
Enters global configuration mode. |
| Step 2 |
[no ] ip dhcp relay information option vpn Example: |
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: |
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. |
| Step 4 |
(Optional) show ip dhcp relay Example: |
(Optional)
Displays the DHCP relay configuration. |
| Step 5 |
(Optional) show running-config dhcp Example: |
(Optional)
Displays the DHCP configuration. |
| Step 6 |
(Optional) copy running-config startup-config Example: |
(Optional)
Copies the running configuration to the startup configuration. |
You can configure the device to support the relaying of DHCP packets from clients to a subnet broadcast IP address. When this feature is enabled, the VLAN ACLs (VACLs) accept IP broadcast packets and all subnet broadcast (primary subnet broadcast as well as secondary subnet broadcast) packets.
Ensure that the DHCP feature is enabled.
Ensure that the DHCP relay agent is enabled.
| Command or Action | Purpose | |
|---|---|---|
| Step 1 |
config t Example: |
Enters global configuration mode. |
| Step 2 |
interface interface slot/port Example: |
Enters interface configuration mode, where slot/port is the interface for which you want to enable or disable subnet broadcast support for the DHCP relay agent. |
| Step 3 |
[no ] ip dhcp relay subnet-broadcast Example: |
Enables subnet broadcast support for the DHCP relay agent. The no option disables this behavior. |
| Step 4 |
exit Example: |
Exits interface configuration mode. |
| Step 5 |
exit Example: |
Exits global configuration mode. |
| Step 6 |
(Optional) show ip dhcp relay Example: |
(Optional)
Displays the DHCP relay configuration. |
| Step 7 |
(Optional) show running-config dhcp Example: |
(Optional)
Displays the DHCP configuration. |
| Step 8 |
(Optional) copy running-config startup-config Example: |
(Optional)
Copies the running configuration to the startup configuration. |
You can configure DHCP server IP addresses on an interface. When an inbound DHCP BOOTREQUEST packet arrives on the interface, the relay agent forwards the packet to all DHCP server IP addresses specified. The relay agent forwards replies from all DHCP servers to the host that sent the request.
Ensure that the DHCP feature is enabled.
Ensure that the DHCP server is correctly configured.
Determine the IP address for each DHCP server that you want to configure on the interface.
If the DHCP server is in a different VRF instance than the interface, ensure that you have enabled VRF support.
Note |
If an ingress router ACL is configured on an interface that you are configuring with a DHCP server address, ensure that the router ACL permits DHCP traffic between DHCP servers and DHCP hosts. |
| Command or Action | Purpose | |
|---|---|---|
| Step 1 |
config t Example: |
Enters global configuration mode. |
| Step 2 |
Do one of the following options:
Example: |
|
| Step 3 |
ip dhcp relay address IP-address [use-vrf vrf-name] Example: |
Configures an IP address for a DHCP server to which the relay agent forwards BOOTREQUEST packets received on this interface. To configure more than one IP address, use the ip dhcp relay address command once per address. |
| Step 4 |
(Optional) show ip dhcp relay address Example: |
(Optional)
Displays all the configured DHCP server addresses. |
| Step 5 |
(Optional) show running-config dhcp Example: |
(Optional)
Displays the DHCP configuration. |
| Step 6 |
(Optional) copy running-config startup-config Example: |
(Optional)
Copies the running configuration to the startup configuration. |
You can enable or disable DHCP smart relay globally on the device.
Ensure that the DHCP feature is enabled.
Ensure that the DHCP relay agent is enabled.
| Command or Action | Purpose | |
|---|---|---|
| Step 1 |
config t Example: |
Enters global configuration mode. |
| Step 2 |
[no ] ip dhcp smart-relay global Example: |
Enables DHCP smart relay globally. The no option disables DHCP smart relay. |
| Step 3 |
(Optional) show ip dhcp relay Example: |
(Optional)
Displays the DHCP smart relay configuration. |
| Step 4 |
(Optional) show running-config dhcp Example: |
(Optional)
Displays the DHCP configuration. |
| Step 5 |
(Optional) copy running-config startup-config Example: |
(Optional)
Copies the running configuration to the startup configuration. |
You can enable or disable DHCP smart relay on Layer 3 interfaces.
Ensure that the DHCP feature is enabled.
Ensure that the DHCP relay agent is enabled.
| Command or Action | Purpose | |
|---|---|---|
| Step 1 |
config t Example: |
Enters global configuration mode. |
| Step 2 |
interface interface slot/port Example: |
Enters interface configuration mode, where slot/port is the interface for which you want to enable or disable DHCP smart relay. |
| Step 3 |
[no ] ip dhcp smart-relay Example: |
Enables DHCP smart relay on the interface. The no option disables DHCP smart relay on the interface. |
| Step 4 |
exit Example: |
Exits interface configuration mode. |
| Step 5 |
exit Example: |
Exits global configuration mode. |
| Step 6 |
(Optional) show ip dhcp relay Example: |
(Optional)
Displays the DHCP smart relay configuration. |
| Step 7 |
(Optional) show running-config dhcp Example: |
(Optional)
Displays the DHCP configuration. |
| Step 8 |
(Optional) copy running-config startup-config Example: |
(Optional)
Copies the running configuration to the startup configuration. |
To display DHCP configuration information, perform one of the following tasks. For detailed information about the fields in the output from these commands, see the Cisco Nexus 7000 Series NX-OS Security Command Reference.
|
Command |
Purpose |
|---|---|
|
show running-config dhcp [all ] |
Displays the DHCP configuration in the running configuration. |
|
show ip dhcp relay |
Displays the DHCP relay configuration. |
|
show ip dhcp relay address |
Displays all the DHCP server addresses configured on the device. |
|
show ip dhcp snooping |
Displays general information about DHCP snooping. |
|
show startup-config dhcp [all ] |
Displays the DHCP configuration in the startup configuration. |
Use the show ip dhcp snooping binding command to display the DHCP binding table. For detailed information about the fields in the output from this command, see the Cisco Nexus 7000 Series NX-OS Security Command Reference.
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.
Ensure that the DHCP feature is enabled.
| Command or Action | Purpose | |
|---|---|---|
| Step 1 |
(Optional) clear ip dhcp snooping binding Example: |
(Optional)
Clears all entries from the DHCP snooping binding database. |
| Step 2 |
(Optional) clear ip dhcp snooping binding interface ethernet slot/port[.subinterface-number] Example: |
(Optional)
Clears entries associated with a specific Ethernet interface from the DHCP snooping binding database. |
| Step 3 |
(Optional) clear ip dhcp snooping binding interface port-channel channel-number[.subchannel-number] Example: |
(Optional)
Clears entries associated with a specific port-channel interface from the DHCP snooping binding database. |
| Step 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] } Example: |
(Optional)
Clears a single, specific entry from the DHCP snooping binding database. |
| Step 5 |
(Optional) show ip dhcp snooping binding Example: |
(Optional)
Displays the DHCP snooping binding database. |
Use the clear ip dhcp relay statistics command to clear the global DHCP relay statistics.
Use the clear ip dhcp relay statistics interface interface command to clear the DHCP relay statistics for a particular interface.
Use the show ip dhcp snooping statistics command to monitor DHCP snooping.
Use the show ip dhcp relay statistics [interface interface] command to monitor DHCP relay statistics at the global or interface level.
Use the (Optional) show ip dhcp snooping statistics vlan [vlan-id] interface [ethernet |port-channel][id] command to know the exact statistics about snooping statistics per interface under a vlan.
|
Related Topic |
Document Title |
|---|---|
|
DHCP commands: complete command syntax, command modes, command history, defaults, usage guidelines, and examples |
Cisco Nexus 7000 Series NX-OS Security Command Reference |
|
VRFs and Layer 3 virtualization |
Cisco Nexus 7000 Series NX-OS Unicast Routing Configuration Guide |
|
vPCs |
Cisco Nexus 7000 Series NX-OS Interfaces Configuration Guide |
|
Standards |
Title |
|---|---|
|
RFC-2131 |
Dynamic Host Configuration Protocol (http://tools.ietf.org/html/rfc2131) |
|
RFC-3046 |
DHCP Relay Agent Information Option (http://tools.ietf.org/html/rfc3046) |
This table lists the release history for this feature.
|
Feature Name |
Releases |
Feature Information |
|---|---|---|
|
IP DHCP Relay Source Interface |
8.2(3) |
Added support for the DHCP relay source interface. |
|
DHCP |
5.2(1) |
Added support for DHCP smart relay. |
|
DHCP |
5.2(1) |
Added subnet broadcast support for the DHCP relay agent. |
|
DHCP |
5.1(1) |
Optimized DHCP snooping to work in a vPC environment. |
|
DHCP |
5.0(2) |
Modified the DHCP relay agent to support VRFs, added the ip dhcp relay information option vpn command, and modified the ip dhcp relay address command to add the use-vrf vrf-name option. |
|
DHCP |
5.0(2) |
Added the ip dhcp relay sub-option type cisco command to enable DHCP to use Cisco proprietary numbers 150, 152, and 151 for the link selection, server ID override, and VRF name/VPN ID relay agent Option 82 suboptions. |
|
DHCP |
4.2(1) |
Deprecated the service dhcp command and replaced it with the ip dhcp relay command. |
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