Multiple-hop forwarding of DHCPv6 service requests

If a routing switch receives a unicast DHCPv6 service request forwarded by a relay agent, the request is routed to the specified server. For example, in the following figure, router "X" is a relay agent configured to forward DHCPv6 requests received from VLAN 10 to unicast Helper address 2001:db8:0:12::11. Router "Y" receives the request from router "X" on VLAN 10 and routes it to the DHCPv6 server at 2001:db8:0:12::11 on VLAN 12. In this case, router "Y" acts as an IPv6 router and not as a DHCPv6 relay.

Figure 19: Routing a unicast DHCPv6 request across a multiple-hop topology

In the following figure, router "X" is a relay agent configured to forward DHCPv6 service requests received from VLAN 10 to the "all-DHCPv6-servers" multicast Helper address (FF05::1:3) through VLAN 14. Router "Z" receives the request from router "X" on VLAN 14. Because router "Z" is configured with unicast Helper address 2001:db8:0:15::33 on VLAN 14, the service request is relayed to the DHCPv6 server at 2001:db8:0:15::33 on VLAN 15. (In this example, both router "X" and router "Z" act as DHCPv6 relay agents.)

Figure 20: Routing a multicast DHCPv6 request across a multiple-hop topology
A multi-hop relay scenario requires the following:

  • All relays in the path except the relay closest to the server have a multicast Helper address.

  • The last relay in the path to the server has a unicast Helper address.

Thus, if router "Z" was not configured with a Helper address as shown above, the relayed service request would be restricted to VLAN 14 and would not reach the server at 2001:db8:0:15::33 on VLAN 15.