Because DHCP requests are broadcast packets that normally do not propagate outside of their IP subnet, a DHCP relay agent intercepts such requests and forwards them as unicast messages to a configured DHCP server.
When forwarding a DHCP message, the relay agent sets the GIADDR in the packet to the IP address of its ingress interface. This allows DHCP clients to use a DHCP server on a remote network. From both a scalability and a security point of view, it is recommended that the DHCP relay agent is positioned as close as possible to the client terminals.
DHCP relay is used in a Layer 3 environment, and therefore is only supported in IES services and VPRN services. VPRN is only supported on the 7750 SR.
When DHCP clients and servers are in different VPRN routing instances of which one is the Base routing instance, route leaking (GRT-leaking) should be used to relay DHCPv4 and DHCPv6 messages between a VPRN and the Global Routing Table (GRT).
While DHCP relay is not implemented in a VPLS, it is still possible to insert or modify Option 82 information.
In a routed CO environment in the 7750 SR, the subscriber interface’s group interface DHCP relay is stateful.