Прописать dhcp helper на интерфейсе

http://mellowd.co.uk/ccie/?tag=juniper-dhcp-relay

Both IOS and JunOS allows you to configure the router as a DHCP relay agent. This is how it’s done.

On IOS it’s extremely simple. All you need to do is put the following command on the interface receiving the broadcast. In this topology it’ll be the interface connected to the switch and workstation the user is on

>conf t
# int fa0
# ip helper-address 10.1.1.1
On JunOS it’s just as simple. The configuration is not put on a particular interface, rather you specify which interface will be receiving the broadcast.

> configure
# set forwarding-options helpers bootp interface em1
# set forwarding-options helpers bootp server 10.1.1.1

UPD: На практике в junos заработало так:

forwarding-options {
    dhcp-relay {
        server-group {
            OG {
                10.19.255.255;
            }
        }
        active-server-group OG;
        group ALL {
            active-server-group OG;
            interface xe-1/2/0.215 {
                overrides {
                    allow-snooped-clients;
                }
            }
            interface xe-1/3/0.3861;
        }
    }
}

Конструкция overrides allow-snooped-clients нужна для того, чтобы через данный relay могли пролетать уже завернутые в пакет dhcp запросы. Если у вас уже на каком-то влане настроен dhcp-helper, то без этой записи запрос с него не пролетит через данный dhcp-relay