Cisco ip helper-address 详解

Some time ago I wroteA simple example Cisco routers DHCP and DHCP relay configuration，Article uses a command to achieve theDHCP forwarding，But this function is exactly how to achieve it? Now let's learn more about this command。

When the terminal equipment andDHCP serverWhen not on the same broadcast domain，Intermediate devices need to specify an address to be broadcast packet forwarded out，That needs to be sethelper_addressTo achieve relay。

In fact，In the enterprise generally only need one or two DHCP servers to provide sufficient redundancy，There is no needSetting up a DHCP server in each separate broadcast domain，this wayNot only increasing the cost，More is not conducive to management。

To know how to implement this function，Let's take a look atSimple operating principle of the DHCP server：

• DHCP server you want to assign IP information to the terminal device，Two need to get important information，Terminal network segment (ie, mask)、MAC address of the corresponding device。
• DHCP relies on the former judge should assign the device to the corresponding address pool，And then rely on the latter to maintain the mapping between IP and MAC good way to ensure that no IP address conflict。

Learn the simple operating principle of DHCP，We configure it againip helper-addressRouter working process after the command：

1. DHCP client sends a request，With no ip address，So their source IP address of 0.0.0.0，And does not know the address of the destination DHCP server，So broadcast 255.255.255.255，That source IP 0.0.0.0；Destination IP address is 255.255.255.255；Source MAC address for your MAC address；Destination MAC address FF:FF:FF:FF:FF:FF
2. When the router receives the information，Then be replaced，With its own interface address(Received data packet interface，That is where the terminal network gateway)Replacing the source address 0.0.0.0，And treated with ip help-address command to replace the address specified in the destination address 255.255.255.255，That is the source IP(Gateway address)；Destination IP address（ip helper-address指定地址）；Source MAC address of the terminal MAC address；Destination MAC address（ip helper-address specifies the address of the corresponding MAC address）
3. in this way，When the DHCP server receives the information forwarded by the router，The same can get the appropriate information to determine the mapping segments and MAC address and IP address，Therefore allows the terminal to get the appropriate information。

Other than that，ip helper-address functions not only can be used for DHCP forwarding，In fact, by the global command ip forward-protocol it can forward any UDP broadcast!When you enable this command，Enabled by default 8 default UDP port forwarding。

They are yes yes：TFTP（69）、DNS（53）、Time (37)、NETBIOS services (137)、N ETBIOS datagram service (138)、BOOTP server (67)、BOOTP client (68) and Terminal Access Controller Access Control System TACACS (49)

At last，If you only used to forward DHCP，That enabled by default is likely to cause unnecessary ports router overload，所以，Please use the global command to turn off all the UDP forwarding，after thatTo open a separate DHCPForwarding

All right，So much of it! Reproduced please specify from off gridPermalink： http://www.logcg.com/archives/241.html

Other than that，If you want to learn more related content，Please click on the following link reference：

http://baike.baidu.com/view/2287054.htm

http://www.net527.cn/a/luyoujiaohuan/Ciscojishu/13468.html

http://en.wikipedia.org/wiki/UDP_Helper_Address

http://www.vlan9.com/cisco-tech/p003003161.html

http://zh.wikipedia.org/zh/DHCP

Original article written by LogStudio：R0uter's Blog » Cisco ip helper-address 详解

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