Extended DHCP Relay Agent Overview
You can configure extended DHCP relay options on the router or on the switch and enable the router (or switch) to function as a DHCP relay agent. A DHCP relay agent forwards DHCP request and reply packets between a DHCP client and a DHCP server.
DHCP relay supports the attachment of dynamic profiles and also interacts with the local AAA Service Framework to use back-end authentication servers, such as RADIUS, to provide subscriber authentication or DHCP client authentication. You can attach dynamic profiles and configure authentication support on a global basis or for a specific group of interfaces.
The PTX Series Packet Transport Routers do not support authentication for DHCP relay agents.
On the routers, you can use DHCP relay in carrier edge applications such as video/IPTV to obtain configuration parameters, including an IP address, for your subscribers.
On the switches, you can use DHCP relay to obtain configuration parameters including an IP address for DHCP clients.
The extended DHCP relay agent options configured with the dhcp-relay statement are incompatible with the DHCP/BOOTP relay agent options configured with the bootp statement. As a result, you cannot enable both the extended DHCP relay agent and the DHCP/BOOTP relay agent on the router at the same time.
For information about the DHCP/BOOTP relay agent, see Configuring Routers, Switches, and Interfaces as DHCP and BOOTP Relay Agents.
You can also configure the extended DHCP relay agent to support IPv6 clients. SeeDHCPv6 Relay Agent Overview for information about the DHCPv6 relay agent feature.
To configure the extended DHCP relay agent on the router (or switch), include the dhcp-relay statement at the [edit forwarding-options] hierarchy level.
You can also include the dhcp-relay statement at the following hierarchy levels:
[edit logical-systems logical-system-name forwarding-options]
[edit logical-systems logical-system-name routing-instances routing-instance-name forwarding-options]
[edit routing-instances routing-instance-name forwarding-options]
Interaction Among the DHCP Relay Agent, DHCP Client, and DHCP Servers
The pattern of interaction among the DCHP Relay agent, DHCP client, and DHCP servers is the same regardless of whether the software installation is on a router or a switch. However, there are some difference in the details of usage.
On routers—In a typical carrier edge network configuration, the DHCP client is on the subscriber’s computer, and the DHCP relay agent is configured on the router between the DHCP client and one or more DHCP servers.
On switches—In a typical network configuration, the DHCP client is on an access device such as a personal computer and the DHCP relay agent is configured on the switch between the DHCP client and one or more DHCP servers.
The following steps describe, at a high level, how the DHCP client, DHCP relay agent, and DHCP server interact in a configuration that includes two DHCP servers.
The DHCP client sends a discover packet to find a DHCP server in the network from which to obtain configuration parameters for the subscriber (or DHCP client), including an IP address.
The DHCP relay agent receives the discover packet and forwards copies to each of the two DHCP servers. The DHCP relay agent then creates an entry in its internal client table to keep track of the client’s state.
In response to receiving the discover packet, each DHCP server sends an offer packet to the client. The DHCP relay agent receives the offer packets and forwards them to the DHCP client.
On receipt of the offer packets, the DHCP client selects the DHCP server from which to obtain configuration information. Typically, the client selects the server that offers the longest lease time on the IP address.
The DHCP client sends a request packet that specifies the DHCP server from which to obtain configuration information.
The DHCP relay agent receives the request packet and forwards copies to each of the two DHCP servers.
The DHCP server requested by the client sends an acknowledgement (ACK) packet that contains the client’s configuration parameters.
The DHCP relay agent receives the ACK packet and forwards it to the client.
The DHCP client receives the ACK packet and stores the configuration information.
If configured to do so, the DHCP relay agent installs a host route and Address Resolution Protocol (ARP) entry for this client.
After establishing the initial lease on the IP address, the DHCP client and the DHCP server use unicast transmission to negotiate lease renewal or release. The DHCP relay agent “snoops” on all of the packets unicast between the client and the server that pass through the router (or switch) to determine when the lease for this client has expired or been released. This process is referred to as lease shadowing or passive snooping.
DHCP Liveness Detection
Liveness detection for DHCP subscriber or DHCP client IP sessions utilizes an active liveness detection protocol to institute liveness detection checks for relevant clients. Clients are expected to respond to liveness detection requests within a specified amount of time. If the responses are not received within that time for a given number of consecutive attempts, then the liveness detection check fails and a failure action is implemented.
DHCP liveness detection either globally or per DHCP group.