Which port does the relay agent use when it send DHCP information back to the client?

Perform this task to configure the DHCP relay agent to forward packets to a DHCP server.

Automatic DHCP address allocation is typically based on an IP address, which may be either the gateway IP address (giaddr field of the DHCP packet) or the incoming interface IP address. In some networks, additional information may be required to further determine the IP addresses that need to be allocated. By using the relay agent information option (option 82), the Cisco IOS relay agent can include additional information about itself when forwarding client-originated DHCP packets to a DHCP server. Cisco software supports this functionality by using the ip dhcp relay information option command. The relay agent will automatically add the circuit identifier suboption and the remote ID suboption to the relay agent information option and forward them to the DHCP server.

The DHCP server can use this information to assign IP addresses, perform access control, and set quality of service (QoS) and security policies (or other parameter-assignment policies) for each subscriber of a service provider network.

The figure below shows how the relay agent information option is inserted into the DHCP packet as follows:

1. The DHCP client generates a DHCP request and broadcasts it on the network.
2. The DHCP relay agent intercepts the broadcast DHCP request packet and inserts the relay agent information option (option 82) into the packet. The relay agent information option contains related suboptions.
3. The DHCP relay agent unicasts the DHCP packet to the DHCP server.
4. The DHCP server receives the packet, uses the suboptions to assign IP addresses and other configuration parameters to the packet, and forwards the packet back to the client.
5. The suboption fields are stripped off of the packet by the relay agent while forwarding the packet to the client.

Figure 1. Operation of the Relay Agent Information Option

A DHCP relay agent may receive a message from another DHCP relay agent that already contains relay information. By default, the relay information from the previous relay agent is replaced. If this behavior is not suitable for your network, you can use the ip dhcp relay information policy {drop | keep | replace} global configuration command to change it.

To ensure the correct operation of the reforwarding policy, disable the relay agent information check by using the no ip dhcp relay information check global configuration command.

It is important to understand how DHCP options work. See the “DHCP Overview” module for more information.

Note

If the ip dhcp relay information command is configured in global configuration mode but not configured in interface configuration mode, the global configuration is applied to all interfaces. If the ip dhcp relay information command is configured in both global configuration mode and interface configuration mode, the interface configuration command takes precedence over the global configuration command. However, the global configuration is applied to interfaces without the interface configuration. If the ip dhcp relay information command is not configured in global configuration mode but is configured in interface configuration mode, only the interface with the configuration option applied is affected. All other interfaces are not impacted by the configuration. See the “Configuring Relay Agent Information Option Support per Interface” section for more information on per-interface support for the relay agent information option.

The interface configuration allows a Cisco router to reach subscribers with different DHCP option 82 requirements on different interfaces.

It is important to understand how DHCP options work. See the “DHCP Overview” module for more information.

Note

If the ip dhcp relay information command is configured in global configuration mode but not configured in interface configuration mode, the global configuration is applied to all interfaces. If the ip dhcp relay information command is configured in both global configuration mode and interface configuration mode, the interface configuration command takes precedence over the global configuration command. However, the global configuration is applied to interfaces without the interface configuration. If the ip dhcp relay information command is not configured in global configuration mode but is configured in interface configuration mode, only the interface on which the configuration option is applied is affected. All other interfaces are not impacted by the configuration.

Perform this task to enable an Internet service provider (ISP) to add a unique identifier to the subscriber-identifier suboption of the relay agent information option.

The unique identifier enables an ISP to identify a subscriber, to assign specific actions to that subscriber (for example, assignment of host IP address, subnet mask, and domain name system DNS), and to trigger accounting.

Before the introduction of this feature, if a subscriber moved, each ISP had to be informed of the change and all ISPs had to reconfigure the DHCP settings for the affected customers at the same time. Even if the service was not changed, every move involved administrative changes in the ISP environment. With the introduction of this feature, if a subscriber moves from one Network Access Server to another, there is no need for a change in the configuration on the part of the DHCP server or ISP.

You should configure the unique identifier for each subscriber.

The new configurable subscriber-identifier option should be configured on the interface connected to the client. When a subscriber moves from one interface to the other, the interface configuration should also be changed.

The server should be able to recognize the new suboption.

DHCP relay class support for client identification allows the Cisco relay agent to forward client-generated DHCP messages to different DHCP servers based on the content of the following four options:

• Option 60: vendor class identifier
• Option 77: user class
• Option 124: vendor-identifying vendor class
• Option 125: vendor-identifying vendor-specific information

Each option identifies the type of client that is sending the DHCP message.

Relay pools provide a method to define DHCP pools that are not used for address allocation. These relay pools can specify that DHCP messages from clients on a specific subnet should be forwarded to a specific DHCP server. These relay pools can be configured with relay classes inside the pool that help determine the forwarding behavior.

For example, after receiving the option in a DHCP DISCOVER message, the relay agent will match and identify the relay class from the relay pool and then direct the DHCP DISCOVER message to the DHCP server associated with that identified relay class.

In an example application, a Cisco router acting as a DHCP relay agent receives DHCP requests from two VoIP services (H.323 and the Session Initiation Protocol [SIP]). The requesting devices are identified by option 60.

Both VoIP services have a different back-office infrastructure, so they cannot be serviced by the same DHCP server. Requests for H.323 devices must be forwarded to the H.323 server, and requests from SIP devices must be forwarded to the SIP server. The solution is to configure the relay agent with relay classes that are configured to match option 60 values sent by the client devices. Based on the option value, the relay agent will match and identify the relay class, and forward the DHCP DISCOVER message to the DHCP server associated with the identified relay class.

The Cisco IOS DHCP server examines the relay classes that are applicable to a pool and then uses the exact match class regardless of the configuration order. If the exact match is not found, the DHCP server uses the first default match found.

It is important to understand how DHCP options work. See the “DHCP Overview” module for more information.

You must know the hexadecimal value of each byte location in the options to be able to configure the option hex command. The format may vary from product to product. Contact the relay agent vendor for this information.

Perform this task to configure DHCP relay agent support for MPLS VPNs.

Before configuring DHCP relay support for MPLS VPNs, you must configure standard MPLS VPNs.

When two relay agents are relaying messages between the DHCP client and the DHCP server, the relay agent closer to the server, by default, replaces the first option 82 information with its own option 82. The remote ID and circuit ID information from the first relay agent is lost. In some deployment scenarios, it is necessary to maintain the initial option 82 from the first relay agent, in addition to the option 82 from the second relay agent, for example, in a situation where an Intelligent Services Gateway (ISG) acting as a second relay agent is connected to a Layer 2 device. The Layer 2 device connects to the household and identifies the household with its own option 82.

The DHCP Relay Option 82 Encapsulation feature allows the second relay agent to encapsulate option 82 information in a received message from the first relay agent if the second relay agent is configured to add its own option 82 information. This configuration allows the DHCP server to use option 82 information from both relay agents. The DHCP server can use the VPN information from the second relay agent, along with the option 82 information from the first relay agent, to send correct address assignments and other configuration parameters for the client devices based on the VRF, option 60, and encapsulated option 82. The reply message from the DHCP server to the DHCP client traverses the same path as the request messages through the two relay agents to the DHCP client.

The figure below shows the processing that occurs on the two relay agents and the DHCP server when this feature is configured:

1. The DHCP client generates a DHCP message (including option 60) and broadcasts it on the network.
2. The first DHCP relay agent intercepts the broadcast DHCP request packet and inserts its own option 82 in the packet.
3. The relay agent automatically adds the circuit ID suboption and the remote ID suboption to option 82 and forwards them to the second relay agent.
4. The second relay agent encapsulates the first relay agent’s option 82 and inserts its own option 82.
5. The gateway IP address (giaddr) is set to the incoming interface on the second relay agent and the original giaddr from the first relay agent is encapsulated.
6. The second DHCP relay agent unicasts the DHCP packet to the DHCP server.
7. The DHCP server receives the packet and uses the VPN suboption information from the second relay agent, along with the option 82 information from the first relay agent, to assign IP addresses and other configuration parameters and forwards the packet back to the second relay agent.
8. When the second relay agent receives the reply message from the server, it restores the encapsulated option 82 and prior giaddr from the first relay agent. The reply message is then sent to the prior giaddr.
9. The first relay agent strips option 82 off from the packet before forwarding the packet to the client.

Figure 2. Processing DHCP Relay Agent Information Option Encapsulation Support

You only need to configure helper addresses on the interface where the UDP broadcasts that you want to forward to the DHCP server are being received. You only need to configure the ip dhcp smart-relay command if you have secondary addresses on that interface and you want the router to step through each IP network when forwarding DHCP requests. If smart relay agent forwarding is not configured, all requests are forwarded using the primary IP address on the interface.

If the ip dhcp smart-relay command is configured, the relay agent counts the number of times that the client retries sending a request to the DHCP server when there is no DHCPOFFER message from the DHCP server. After three retries, the relay agent sets the gateway address to the secondary address. If the DHCP server still does not respond after three more retries, then the next secondary address is used as the gateway address.

This functionality is useful when the DHCP server cannot be configured to use secondary pools.

Some features that are not standardized will use the private Cisco relay agent suboption numbers. After the features are standardized, the relay agent suboptions are assigned the Internet Assigned Numbers Authority (IANA) numbers. Cisco software supports both private and IANA numbers for these suboptions.

Perform this task to configure the DHCP client to use private or IANA standard relay agent suboption numbers.

Perform this task to troubleshoot the DHCP relay agent.

The show ip route dhcp command is useful to help you understand any problems with the DHCP relay agent adding routes to clients from unnumbered interfaces. All routes added to the routing table by the DHCP server and relay agent are displayed.

In the following example, DHCP relay agent 1 is configured globally to insert the relay agent information option into the DHCP packet. DHCP relay agent 2 is configured to add its own relay agent information option, including the VPN information, and to encapsulate the relay agent information option received from DHCP relay agent 1. The DHCP server receives the relay agent information options from both the relay agents, uses this information to assign IP addresses and other configuration parameters, and forwards them back to the client.

DHCP Relay Agent 1

ip dhcp relay information option 

DHCP Relay Agent 2

ip dhcp relay information option
ip dhcp relay information option vpn
ip dhcp relay information option encapsulation