What is a DHCP Relay Agent?

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We talked about the basic operations of DHCP in our previous blog post, "What is DHCP?”. Today, we will find out what kind of agent a DHCP relay agent is.

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What is a DHCP Relay Agent?

  1. 1. NETMANIAS TECH-BLOG Please visit www.netmanias.com to view more posts What is a DHCP Relay Agent? August 1, 2013 | By Chris (tech@netmanias.com) | Netmanias Tech-Blog | www.netmanias.com We talked about the basic operations of DHCP in our previous blog post, "What is DHCP?”. Today, we will find out what kind of agent a DHCP relay agent is. (a) Network w/o DHCP relay agent PC Broadcast Domain (1 Subnet) L2 SW Internet Broadcast Domain (1 Subnet) L2 SW DHCP Server DHCP server unable to receive any DHCP messgaes Router Broadcast packets discarded by router DHCP Discover/Request (Broadcast) (b) Network w/ DHCP relay agent PC1 Broadcast Domain (1.1.1.0/24) L2 SW 1.1.1.254/24 Broadcast Domain (2.1.1.0/254) L2 SW DHCP Discover/Request (Broadcast) 100.1.1.1 DHCP Server DHCP messages delivered to DHCP server through DHCP relay agent 100.1.1.254/24 Internet Router DHCP Relay Agent DHCP relay agent converting broadcast ones and forwarding to DHCP server DHCP Discover/Request (Unicast) Generally, DHCP messages are broadcasted. So, in order for the messages to be exchanged between a DHCP client (PC) and DHCP server, both the client and server have to reside on the same subnet. That is because routers do not forward any broadcast IP packet (i.e. one with a destination MAC address of FF:FF:FF:FF:FF:FF and a destination IP address of 255.255.255.255) to other interfaces. Thus, a broadcast DHCP packet sent by a DHCP client cannot be delivered to DHCP server(s) on different subnet(s) through a router (shown in Figure (a)). This restriction requires all individual subnets have its own DHCP server for DHCP operations, which is practically not feasible in network operators' networks or corporate networks (Too many DHCP servers are required in the network!). To address this problem, the concept of a DHCP relay agent has long been adopted. As shown in Figure 1 (b), enabling the DHCP relay agent function in the router allows DHCP messages to be exchanged between DHCP 1
  2. 2. Netmanias Tech-Blog: What is a DHCP Relay Agent? clients and DHCP servers residing on different subnets. The core function of this DHCP relay agent is to convert broadcast DHCP packets into unicast messages and then forward them to DHCP servers. DHCP message flows in a network with a DHCP relay agent are as follows: 1. [DHCP client (PC) -> DHCP server] DHCP Discover Message When a client PC broadcasts a DHCP Discover message, a DHCP relay agent receives and converts the message (SIP=DHCP Relay Agent, DIP=DHCP Server), and forwards it to the DHCP server (here, SIP=Source IP address, DIP=Destination IP address in IP header). 2. [DHCP client (PC) <- DHCP server] DHCP Offer Message When the DHCP server unicasts a DHCP Offer message to the DHCP relay agent (SIP=DHCP Server, DIP=DHCP Relay Agent), the DHCP relay agent converts the received message and broadcasts it to the client PC (Note: messages are not broadcasted in all cases. Please click the link below and refer to our technical document “Understanding the Detailed Operations of DHCP”). 3. [DHCP client (PC) -> DHCP server] DHCP Request Message When the client PC broadcasts a DHCP Request message, the DHCP relay agent converts the received message (SIP=DHCP Relay Agent, DIP=DHCP Server), and forwards it to the DHCP server. 4. [DHCP client (PC) <- DHCP server] DHCP Ack Message When the DHCP server unicasts a DHCP Ack message to the DHCP relay agent (SIP=DHCP server, DIP=DHCP Relay Agent), the DHCP relay agent converts and broadcasts the message to the client PC (Note: Messages are not broadcasted in all cases. Please click the link below and refer to our technical document “Understanding the Detailed Operations of DHCP”). 2
  3. 3. Netmanias Research and Consulting Scope 99 00 01 02 03 04 05 06 07 08 09 10 11 12 13 eMBMS/Mobile IPTV CDN/Mobile CDN Transparent Caching BSS/OSS Services Cable TPS Voice/Video Quality IMS Policy Control/PCRF IPTV/TPS LTE Mobile Network Mobile WiMAX Carrier WiFi LTE Backaul Data Center Migration Carrier Ethernet FTTH Wireline Network Data Center Metro Ethernet MPLS IP Routing CDN Transparent Caching Analysis Networks eMBMS LTE IMS Infrastructure Services Analyze trends, technologies and market Report Technical documents Blog One-Shot gallery Concept Design DRM POC Training Wi-Fi We design the future protocols IP/MPLS We design the future Carrier Ethernet We design the future Consulting Visit http://www.netmanias.com to view and download more technical documents. Future About NMC Consulting Group (www.netmanias.com) NMC Consulting Group is an advanced and professional network consulting company, specializing in IP network areas (e.g., FTTH, Metro Ethernet and IP/MPLS), service areas (e.g., IPTV, IMS and CDN), and wireless network areas (e.g., Mobile WiMAX, LTE and Wi-Fi) since 2002. Copyright © 2002-2013 NMC Consulting Group. All rights reserved. 3

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