Principle of DHCP Relay

DHCP Relay principle: DHCP customers use IP broadcasts to find DHCP servers on the same network segment. When the server and the client segment are in different network segments, that is, they are separated by the router, the router will not forward such broadcast packets. Therefore, it may be necessary to set up a DHCP server on each network segment, although DHCP consumes only a small part of the resources, but multiple DHCP servers, after all, bring inconvenience in management. The use of DHCP relay makes it possible for a DHCP server to serve multiple network segments at the same time. To enable the router to help forward broadcast request packets, use the ip help-address command. By using this command, the router can be configured to accept broadcast requests and then forward it to a specified IP address as a unicast. By default, ip help-address forwards the following eight UDP services:

1.Time 2.Tacacs 3.DNS 4.BOOTP/DHCP server 5. BOOTP/DHCP customer 6.TFTP 7.NetBios name service 8. NetBios Datagram service

In the case of a DHCP broadcast, the customer broadcasts a DHCP discovery packet on the local network segment. The gateway obtains this packet and forwards the DHCP packet to a specific address if a help address is configured.

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