探究OSPF邻居建立之子网掩码
在Jeff Doyle的TCP/IP路由技术 卷一上看到若网络掩码不匹配，则无法建立邻接关系，所以做个实验验证下，没想到发现了问题。设计拓扑图如下：
R—R2之间链路为192.168.12.0/24
以太网链路为192.168.1.0/24
正确配置IP地址和掩码，并配置OSPF协议。更改R3、R4的接口优先级为0，使得R2成为DR。网络正常运行，正常建立邻接关系，查看如下：
R2#sh ip ospf nei
Neighbor ID Pri State Dead Time Address Interface
3.3.3.3 0 FULL/DROTHER 00:00:37 192.168.1.3 FastEthernet1/0
4.4.4.4 0 FULL/DROTHER 00:00:34 192.168.1.4 FastEthernet1/0
1.1.1.1 0 FULL/ - 00:00:37 192.168.12.1 Serial0/0
现更改R1的s0/0接口ip地址为192.168.12.1/16。查看其是否能建立邻居
R1#sh ip ospf nei
Neighbor ID Pri State Dead Time Address Interface
2.2.2.2 0 FULL/ - 00:00:34 192.168.12.2 Serial0/0
发现依旧可以建立邻居，这是点到点的链路，再尝试以太网链路。更改R3的fa0/0接口ip地址为192.168.1.3/16，发现R3没有了OSPF邻居。而R2依旧和其他路由器保持着full状态，如下：
R2#sh ip ospf nei
Neighbor ID Pri State Dead Time Address Interface
4.4.4.4 0 FULL/DROTHER 00:00:34 192.168.1.4 FastEthernet1/0
1.1.1.1 0 FULL/ - 00:00:36 192.168.12.1 Serial0/0
在R3上 使用debug ip ospf event，结果如下：
R3#debug ip ospf ev
OSPF events debugging is on
R3#
*Mar 1 00:23:10.055: OSPF: Rcv hello from 2.2.2.2 area 0 from FastEthernet0/0 192.168.1.2
*Mar 1 00:23:10.055: OSPF: Mismatched hello parameters from 192.168.1.2
*Mar 1 00:23:10.059: OSPF: Dead R 40 C 40, Hello R 10 C 10 Mask R 255.255.255.0 C 255.255.0.0
显示hello参数不匹配，查看显示是掩码不匹配，当更改掩码后邻接关系恢复full状态。
难道点到点链路真可以正常的建立邻接关系吗？检查R1和R2的路由表，如下：
R1#sh ip route
Gateway of last resort is not set
O 192.168.12.0/24 [110/128] via 192.168.12.2, 00:08:23, Serial0/0
1.0.0.0/24 is subnetted, 1 subnets
C 1.1.1.0 is directly connected, Loopback0
2.0.0.0/32 is subnetted, 1 subnets
O 2.2.2.2 [110/65] via 192.168.12.2, 00:08:23, Serial0/0
4.0.0.0/32 is subnetted, 1 subnets
O 4.4.4.4 [110/66] via 192.168.12.2, 00:08:23, Serial0/0
O 192.168.1.0/24 [110/65] via 192.168.12.2, 00:08:23, Serial0/0
C 192.168.0.0/16 is directly connected, Serial0/0
R2#sh ip route
Gateway of last resort is not set
C 192.168.12.0/24 is directly connected, Serial0/0
1.0.0.0/32 is subnetted, 1 subnets
O 1.1.1.1 [110/65] via 192.168.12.1, 00:08:44, Serial0/0
2.0.0.0/24 is subnetted, 1 subnets
C 2.2.2.0 is directly connected, Loopback0
4.0.0.0/32 is subnetted, 1 subnets
O 4.4.4.4 [110/2] via 192.168.1.4, 00:08:44, FastEthernet1/0
C 192.168.1.0/24 is directly connected, FastEthernet1/0
O 192.168.0.0/16 [110/128] via 192.168.12.1, 00:08:44, Serial0/0
仔细观察路由表发现R1和R2的路由表中均出现了ospf域内路由，并且其metric值均为128。究其原因是因为对端传来的1类LSA，如下：
R2#sh ip ospf da router 1.1.1.1
OSPF Router with ID (2.2.2.2) (Process ID 10)
Router Link States (Area 0)
LS age: 1096
Options: (No TOS-capability, DC)
LS Type: Router Links
Link State ID: 1.1.1.1
Advertising Router: 1.1.1.1
LS Seq Number: 80000004
Checksum: 0xCECD
Length: 60
Number of Links: 3
Link connected to: another Router (point-to-point)
(Link ID) Neighboring Router ID: 2.2.2.2
(Link Data) Router Interface address: 192.168.12.1
Number of TOS metrics: 0
TOS 0 Metrics: 64
Link connected to: a Stub Network
(Link ID) Network/subnet number: 192.168.0.0
(Link Data) Network Mask: 255.255.0.0
Number of TOS metrics: 0
TOS 0 Metrics: 64
Link connected to: a Stub Network
(Link ID) Network/subnet number: 1.1.1.1
(Link Data) Network Mask: 255.255.255.255
Number of TOS metrics: 0
TOS 0 Metrics: 1
当R2从接口S0/0收到R1传过来的LSA后，查看到一个stub network 192.168.0.0，其metric为64，再加上自身接口s0/0的metric 64，则到达192.168.0.0的metric为128。这样的路由表会导致数据的非正常转发。
当在多路访问网络时，需要选举DR和BDR，选举的DR将生成2类LSA，2类LSA数据包格式如下图：
其中以太网链路需要有统一的掩码，才有DR宣告出去