Configuring Interoperation with Ethernet DSLAMs

The DHCP external server application uses the giaddr it receives in DHCP server-destined packets to determine the next hop for a subscriber’s access routes. However, when interoperating with Ethernet digital subscriber line access multiplexers (DSLAMs), using the giaddr sent by the DSLAM can result in traffic being dropped. To ensure that traffic is forwarded properly, you can configure the DHCP external server application to disregard the DSLAM’s giaddr and learn the subscriber’s correct next-hop address.

The dropped traffic situation can occur because of the way some DSLAMs create the giaddr that is sent to the DHCP external server application. Some Ethernet DSLAMs use a DHCP relay implementation that inserts giaddr values and relay agent options in DHCP packets that are received from end users. The intent is that this information is provided to a DHCP server, which uses the values to determine the configuration parameters for the subscriber.

However, when the DHCP external server application receives the giaddr from an Ethernet DSLAM, the application installs the subscriber access route with the Ethernet DSLAM’s IP address as the next hop, This operation results in the subscriber-destined traffic being incorrectly sent to the Ethernet DSLAM, which cannot process the traffic.

To avoid dropping the traffic in this situation, use the ip set dhcp-external disregard-giaddr-next-hop command to configure the DHCP external server application to ignore the giaddr when determining the next hop for the subscriber access routes. The E Series router then uses Address Resolution Protocol (ARP) to discover the subscriber’s IP address and sends the traffic to the learned IP address.

To configure the DHCP external server application to ignore the giaddr when determining the next hop for the subscriber access routes:

• Issue the ip dhcp-external disregard-giaddr-next-hop command from Global Configuration mode:
  host1(config)#ip dhcp-external disregard-giaddr-next-hop