Principles and configurations of DHCP relay (1)

If you know about the DHCP protocol, you must know the content of the DHCP relay. Here we will focus on the principles and configuration of DHCP relay. Let's take a look at the content of the mask.

DHCP relay principle and Configuration

Application scenarios:

When the number of hosts in a network is large, manual IP Address allocation is not only troublesome but also prone to errors. The emergence of the DHCP service greatly facilitates the allocation of Host IP addresses. Vlan division is an essential step in enterprise networking. How to enable a DHCP server to provide services for multiple CIDR blocks at the same time is what we will discuss.

DHCP relay principle:

DHCP users use IP address broadcast to find DHCP servers on the same network segment. When the server and customer segment are in different network segments, that is, they are separated by routers, the routers will not forward such broadcast packets. Therefore, you may need to set up a DHCP server on each network segment. Although DHCP only consumes a small amount of resources, it is inconvenient to manage Multiple DHCP servers. 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 forward broadcast request packets, use the ip help-address command. By using this command, the vro can be configured to Accept Broadcast requests and forward them to a specified IP address in Unicast mode. By default, the 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 DHCP broadcast, the customer broadcasts a DHCP discovery group in the guest network segment. The gateway obtains this group. If a help address is configured, the DHCP group is forwarded to a specific address.

DHCP relay Configuration

Step 1: Check whether the device supports the ios dhcp Server function

Generally, Cisco routers or access servers, and a small number of switches installed with the Routing Switching Module or multilayer switching function cards all have the ios dhcp Server function. If you have not confirmed whether your device has this function, you can perform quick detection on the CLI as follows:

 
 

  
  
router>enable  
  
  
Password:  
  
  
router#conft  
  
  
router(config)#ipdhcp? 
 
 

If the following information appears, it is a pity that your device does not support the ios dhcp Server function:

 
 

  
  
% Unrecognized command 
 
 

Otherwise, congratulations, your settings support DHCP.