DHCP基本知识点
    1 DHCP协议在RFC2131种定义，使用udp协议进行数据报传递，使用的端口是67以及68。
    2 DHCP最常见的应用是，自动给终端设备分配ip地址，掩码，默认网关，但是DHCP也同样可以给终端设备自动配置其他options，比如DNS server, 域名（比如 net130.com），time zones, NTP servers 以及其他的配置内容，更有些厂家，利用自己开发的第3放软件，把自己的一些配置信息，利用dhcp协议来实现对终端设备的自动配置。
    3 DHCP服务的系统最基本的构架是 client/server模式，并且如果client 和server不再同一个2层网络内（即广播可以到达的网络范围），则必须要有能够透过广播报文的中继设备，或者能把广播报文转化成单播报文的设备（cisco的ios就引经了这种功能）
    There are three distinct element types in a DHCP network. There must be a client and a server. If these two elements are not on the same Layer 2 network, there also must be a proxy, which usually runs on the router. The proxy is needed because the client device initially doesn't know its own IP address, so it must send out a Layer 2 broadcast to find a server that has this information. The router must relay these broadcasts to the DHCP server, then forward the responses back to the correct Layer 2 address so that the right end device gets the right configuration information.
    4 CISCO的路由器（IOS12.0 T1以后），可以配置为dhcp的中继设备，DHCP的客户端设备，也可以配置为DHCP的服务器。
    5 同一个网段DHCP服务器可以有多个，这不会影响终端设备从服务器获取配置信息，终端设备以接受到的第一组配置信息为准。以后又服务器段返回的DHCP配置信息被抛弃。
    . Most DHCP networks of any size include two or more DHCP servers for redundancy. The end devices typically just need to talk to a DHCP server at startup time, but they will not work at all without it. So redundancy is important. This also means that it is not unusual for an end device to see several responses to a DHCP request. It will generally just use the first response. However, this also underscores the importance of ensuring that all of the DHCP servers distribute the same information. Their databases of end device configuration parameters must be synchronized.
    6 DHCP 服务器往往遵守先来先服务的规则（first-come, first-served），或者说他能够建立一个IP地址和终端设备MAC地址之间的映射表（或者叫做database），由此可以保证特定的终端（也就是特定的MAC）每次开机后都能够获得此相同的ip地址。
    .The server can allocate IP addresses from a pool on a first-come, first-served basis, or it can associate IP addresses with end device MAC addresses to ensure that a particular client always receives the same address.
    用 IP Helper Addresses 命令配置DHCP中继服务
    1典型配置命令
    The ip helper-address configuration command allows the router to forward local DHCP requests to one or more centralized DHCP servers:
    Router1#configure terminal
    Enter configuration commands, one per line. End with CNTL/Z.
    Router1（config）#interface Ethernet0
    Router1（config-if）#ip helper-address 172.25.1.1 /*指定dhcp服务器的地址，表示通过Ethernet0向该服务器发送DHCP请求包*/
    Router1（config-if）#ip helper-address 172.25.10.7 /*作用同上*/
    Router1（config-if）#end
    Router1#

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