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、必須網絡互通，互相接收hello包；
2、必須屬於相同的區域，比如area 0；
3、必須有相同的子網和相同的網絡號，比如都是/24,都是192.168.0.0/24，除非是點到點的網絡；
4、路由器必須有相同的ospf驗證密碼；
5、ospf的hello和死亡時間都一樣，一個10秒一個40秒，默認情況下；
6、外部路由和NSSA標志必須相同。這條不懂。