OSPF protocol
Overview
autonomous system (AS)
Interior Gateway Protocol (IGP) (including RIP, OSPF, ISIS)
Exterior Gateway Protocol (EGP) (including BGP) Ali Baba, commonly known as the Flying Platform
OSPF is a link state routing protocol 
OSPF (Open Shortest Path First protocol) work process
shortest path: the path cost is low, the higher the bandwidth. Information has been studying the link state
OSPF Area
OSPF area (Area 0 is the backbone area, and only one, other areas are non-backbone area)
in order to adapt to large networks, OSPF AS divided into multiple areas within
each OSPF router maintains a complete link-state in your area information
Area ID
Area ID can be expressed as a decimal number
can be expressed as a decimal digital
backbone area Area 0
for the inter-area routing information dissemination
Router ID
IP address that uniquely identifies the router within the OSPF area
Router ID selection rule
1. Select lookback router IP address of the interface on the highest value of
2. If there is no lookback interfaces, select the highest IP address in the physical port
3. Router ID may be specified (selected manually) using the router-id command
TIP: the second is not recommended for use (except for a row of routing addresses all down, otherwise the loopback address has been helpful)
DR (primary) and BDR (backup)
network topology is more reliable, and the central star structure routing problems can not occur otherwise, all routes were not working properly.
Other routers (DRothers) and DR and BDR only adjacency relationship
DR和BDR的选举方法
1.自动选举DR和BDR
（1）网段上Router ID最大的路由器将被选举为DR，第二大的将被选举为BDR
2.手动选择DR和BDR
（1）优先级范围是0~255，数值越大，优先级别越高，默认为1
（2）如果优先级相同，则需要比较Router ID
（3）如果路由器的优先级被设置为0，它将不参与DR和DBR的选举
DR和BDR的选举过程
路由器的优先级可以影响一个选举过程，但是它不能强制更换已经存在的DR或BDR路由器
OSPF的组播地址
224.0.0.5：承载地址，DR和BDR将错误地址发送给其它的路由（用来发话）；
224.0.0.6：监听其它路由的反馈信息。
(RIPv2：224.0.0.9)
OSPF的度量值为COST
COST=108/BW
最短路径是基于接口指定的代价（cost）计算的
TIP：Cost值越低越好，度量值是带宽。
OSPF数据包
承载在IP数据包内，使用协议号89
OSPF的包类型
OSPF启动的第一个阶段是使用Hello报文建立双向通信的过程
七个状态
1、down状态：没有打招呼确定邻居状态之前
2、init状态：收到hello包之后激活
3、2-Way状态：建立交互（选举DR和BDR，只选出两个最大的，但是不确定DR和BDR）
4、ExStart：准启动状态（确定DR和BDR身份）
5、Exchange状态：交换数据（DBD报文，LSAck）
6、Loading状态：加载（数据类型：LSR/LSU/LSA/LSAck）
7、Full状态：收敛
OSPF将网络划分为四种类型
1.点到点网络
2.广播多路访问网络
3.非广播多路访问网络
4.点到多点网络
OSPF路由协议
四种路由类型：DR、BDR、ABR、ASBR
五大区域：骨干区域、标准区域、末梢区域、存末梢区域、非存末梢区域
五大数据包类型：hello、DBD、LSR、LSU(LSA)、LSACK
六种链路状态通告（LSA）：type1、type2、type3、type4、type5、type7
七种状态：
从以下几方面考虑OSPF的使用
1.网络规模
2.网络拓扑
3.其他特殊要求
4.路由器自身要求
OSPF features
1 may be adapted to large-scale network
2. routing convergence speed change
3. No routing loop
4. supports variable length subnet mask VLSM
5. The zoning support
6. Support protocol packets to multicast addresses.
Comparison of OSPF and RIP's
Subnet inverted (0,0 variant 1 variant 1)
Ospf of trust 110, rip 120
Ospf multi-zone configuration
generated because multiple OSPF areas
1, to improve the scalability of the network
2, fast convergence 
area division is divided by the interface of the router; route divided by the ISIS.
Inter-domain (ABR, ASR)