You can configure extended DHCP relay options on the
router and enable the router 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. You can use DHCP relay in carrier edge
applications such as video/IPTV to obtain configuration parameters,
including an IP address, for your subscribers.
For more information about how to use the DHCP relay agent in
a video/IPTV application, see the JUNOS Feature Guide.
Note:
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 the JUNOS Policy Framework Configuration Guide.
To configure the extended DHCP relay agent on the router, include
the dhcp-relay statement at the [edit forwarding-options] hierarchy level. See the [edit forwarding-options dhcp-relay] Hierarchy Level for the complete DHCP relay
agent syntax.
You can also include the dhcp-relay statement
at the following hierarchy levels:
Interaction Among the DHCP Relay Agent, DHCP Client, and DHCP
Servers
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.
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, 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 to determine when the lease for this client
has expired or been released. This process is referred to as lease
shadowing or passive snooping.
