Two routers do not become neighbors unless the following conditions are met.
- Two way communication between routers is possible. Determined by flooding Hello packets.
- Interface should belong to the same area;
- Interface should belong to the same subnet and have the same network mask, unless it has network-type configured as point-to-point;
- Routers should have the same authentication options, and have to exchange same password (if any);
- Hello and Dead intervals should be the same in Hello packets;
- External routing and NSSA flags should be the same in Hello packets.
1、必须网络互通，互相接收hello包；
2、必须属于相同的区域，比如area 0；
3、必须有相同的子网和相同的网络号，比如都是/24,都是192.168.0.0/24，除非是点到点的网络；
4、路由器必须有相同的ospf验证密码；
5、ospf的hello和死亡时间都一样，一个10秒一个40秒，默认情况下；
6、外部路由和NSSA标志必须相同。这条不懂。