Enabling Dhcp Proxy - Cisco ASR 9000 Series Configuration Manual

Establishing Subscriber Sessions

Enabling DHCP Proxy

Initially, the DHCP client (which is a CPE) does not possess an IP address. As a result, it sends a L2 broadcast
request to get an IP address. Acting as the relay agent, BNG processes the request and forwards it to the DHCP
server. BNG also forwards responses from the DHCP server back to the DHCP client, ensuring that the end
device gets correct IP configuration information. A typical DHCP layout is depicted in this figure.
Figure 8: DHCP Network
The DHCP server allocates IP addresses for only a configurable period of time known as the lease period. If
a client device needs to retain the IP address for a period longer than the lease period, then the client must
renew the lease before it expires. To renew the lease, the client sends a unicast request to the DHCP server.
On receiving the request message, the server responds with an acknowledgment, and the client's lease is
extended by the lease time specified in the acknowledgment message.
When a control policy is applied to an access interface, it becomes a subscriber access interface. Otherwise,
it is a DHCP standalone interface. For the standalone interface, DHCP adds routes to RIB and populates ARP
entries, based on the configuration.
For the subscriber access interface, DHCP uses the policy-plane to determine whether the IP subscriber session
should be created for a client binding. This is determined based on whether a valid control policy is applied
to the access-interface on which the client binding is created. If a subscriber session is created, then a route
is added for the subscriber interface, but no ARP requests are sent out from that subscriber interface.
BNG can be configured to either act as DHCP proxy in the DHCP network.
Note DHCP relay is not supported for BNG.
Enabling DHCP Proxy
As the DHCP proxy, BNG performs all the functions of a relay and also provides some additional functions.
In the proxy mode, BNG conceals DHCP server details from DHCP clients. BNG modifies the DHCP replies
such that the client considers the proxy to be the server. In this state the client interacts with BNG as if it is
the DHCP server.
Cisco ASR 9000 Series Aggregation Services Router Broadband Network Gateway Configuration Guide, Release
4.2.x
OL-26148-02 87