DHCP relay and server

Today I am going to talk about DHCP relay and server in detail. May be many of you will get the point about the DHCP relay and server in details. Let's talk about the start when a computer or different networked device connects to a network, the DHCP patron software program sends a broadcast question soliciting for important statistics. Any DHCP server at the network may additionally service the request.

The DHCP server manages a pool of IP addresses and records approximately patron configuration parameters together with default gateway, area call, the call servers, and time servers. On receiving a request, the server might also respond with unique facts for each patron, as previously configured by means of an administrator, or with a specific address and any other facts legitimate for the entire network and for the time period for which the allocation (lease) is valid.

A consumer generally queries for this facts immediately after booting, and periodically thereafter before the expiration of the facts. when a DHCP client refreshes an challenge, it first of all requests the same parameter values, but the DHCP server can also assign a new cope with primarily based on the venture rules set by means of administrators.

On large networks that consist of a couple of links, a unmarried DHCP server may also carrier the whole community whilst aided by DHCP relay dealers placed on the interconnecting routers. Such sellers relay messages among DHCP clients and DHCP servers positioned on one-of-a-kind subnets.


The DHCP protocol become created in order that clients may want to reap their IP cope with automatically and with out human intervention (yes that used to be an real a part of IT-ing, back within the day). The manner this works is that once a consumer connects to the network, it sends out a “broadcast” packet asking to find the DHCP server.

That was “good enough” till Vlans got here along. Vlans create obstacles  and segment your physical community into several really isolated ones (therefore the name V-LAN). one of the downsides to Vlans is that now the DHCP server and the clients can’t at once talk, due to the fact “broadcast” packets can not “bounce” networks. So, how do you avoid having a DHCP server per-Vlan, and supply the DHCP requests from the customers in a Vlan, lower back to the vital server?

DHCP relays had been invented to over come this actual hassle through basically “routing” or “proxy-ing” the consumer’s requests. The requests are broadcasted through the customers on their neighborhood network, the relay-agent catches them and forwards them to the DHCP server the use of uni cast. The back DHCP answer gets to the relay agent using uni cast as properly, and the relay agent sends the answer on the consumer’s network.

Fig 1.1- DHCP relay and server
Fig 1.1- DHCP relay and server

DHCP provides a framework for passing configuration information dynamically to hosts on a TCP/IP network. A DHCP client is an Internet host using DHCP to obtain configuration parameters such as an IP address.
 
A DHCP relay agent is any host that forwards DHCP packets between clients and servers. Relay agents are used to forward requests and replies between clients and servers when they are not on the same physical subnet. Relay agent forwarding is distinct from the normal forwarding of an IP router, where IP data grams are switched between networks somewhat transparently. By contrast, relay agents receive DHCP messages and then generate a new DHCP message to send on another interface.
Fig 1.2- DHCP relay and server
Fig 1.2- DHCP relay and server
You can configure the DHCP relay agent at both the context and VLAN interface levels of the ACE. 

  • If you configure the DHCP relay agent at the context level, the configuration applies to all interfaces associated with the context. 
  • If you configure the DHCP relay agent at the VLAN interface level, the configuration applies to that particular interface only; the remaining interfaces revert to the context level configuration.