DHCP server-specific domain Problems

Source: Internet
Author: User

DHCP server settings and other issues have always attracted our attention. Next we will focus on a question: how to specify the DHCP domain for each VLAN. For example, vlan 2 corresponds to dhcp test2 vlan3 corresponds to dhcp test3. suppose:

Scenario 1: there is now a router with Multiple DHCP domains.

Then, VLAN on the switch. What is the ip address of the IP help-address on the vswitch? Which port is the IP address of the router? For example, the configuration on the router:

  1. ip dhcp excluded-address ! ip dhcp pool test 1 network default-router dns-server lease 0 0 5  
  3. ip dhcp pool test 2 network default-router dns-server lease 0 0 5 

On a vswitch:

  1. interface Vlan199 ip address ip helper-address ???????????????  
  2. int vlan 200  
  3. ip address ip helper-address ???????????  

What is the helper-address specified here? In order to obtain the corresponding DHCP scope ???

Scenario 2: Use the win2000's DHCP server in the LAN. If the ip address of the dhcp server is used, divide two scopes on the DHCP server. The first scope ----- and the second scope 192.16.000010 ------

After VLAN division, how can I ensure that the DHCP server is still used in other regions. That is, the machine in VLAN10 obtains the ip address in the first scope: that is, the IP address in the second scope is obtained from ----- what is the helper-address specified here? In order to obtain the corresponding DHCP scope ???

Case 3: Divide Multiple DHCP domains on a 3550emi. Because it can be routed. It can also be VLAN-based. So what is the helper-address specified here? In order to obtain the corresponding DHCP scope ???

  1. FW:ip dhcp pool valn x  
  2. network x.x.x.x x.x.x.x  
  3. default-router x.x.x.x 

The vlan x pool is a subset of the global pool.

  1. interface vlan x  
  2. ip helper-address x.x.x.x  
  3. The helper address can be a specific DHCP server address, or it can be the network address  
  4. if other DHCP servers are on the destination network segment. Using the network address enables other servers to respond to DHCP requests.  
  5. If you have multiple servers, you can configure one helper address for each server. 

DHCP relay:


FW: Here is a summary of a configuration case:

The int fa0/0 of router1 is connected to the dhcp server. The IP address connection scheme with router2 through s0/0 is as follows:

  1. route1:  
  2. int fa0/0 int fa0/1 int s0/0 route2:  
  3. int fa0/0 int fa0/1 int s0/0 dhcp server adderss: 


  1. router1: ! ip forward-protocol udp  
  2. ! int fa0/0 ip addr  
  3. no shutdown ! int fa0/1 ip addr  
  4. ip helper-address no shut !  
  5. int s0/0 ip addr no shut  
  6. encap ppp  
  7. !  
  8. ip route     
  9. ! router2: !  
  10. ip forward-protocol udp ! int fa0/0 ip addr  
  11. ip helper-address no shutdown !  
  12. int fa0/1 ip addr ip helper-address no shut  
  13. ! int s0/0 ip addr  
  14. no shut encap ppp ! ip route ! dhcp server: scope 11: router:  
  15. scope 12: router: scope 21: router:  
  16. scope 22:  
  17. router: 

The VLAN configuration is the same. Replace int fa with int vlan. You only need to use the VLAN as the ROUTE option of the nth SCOPE. Divide VLANs on your vswitch as usual:

Suppose there are: VLAN20 and VLAN30, you should first divide them on the vswitch (this step should be done), and then set the VLAN gateway on the vro (assuming FA0/0.1: and FA0/0.2:, set ip help-address DHCP_SERVER_ADD and ip forward udp under the VLAN20 interface and VLAN30 interface respectively. In fact, it doesn't matter where the DHCP_SERVER is connected, you only need to be able to ROUTE. The key is that when SCOPE is set on DHCP_SERVER, you can divide it into two, one of which is the network segment, the corresponding ROUTE option is, And the other SCOPE is the network segment, its corresponding ROUTE option is After completing the preceding settings, you must be able to connect to the DHCP_SERVER in both network segments.

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