Which two fields in an OSPF hello packet must match before OSPF neighbors can form an adjacency? (Choose two.)

Which two fields in an OSPF hello packet must match before OSPF neighbors can form an adjacency? (Choose two.)

A.
dead interval

B.
neighbor

C.
options

D.
router priority

Explanation:
OSPF Adjacencies

OSPF creates adjacencies between neighboring routers for the purpose of exchanging routing information. Not every neighbor becomes adjacent in a broadcast environment. The Hello protocol is responsible for establishing and maintaining an adjacency.

Hello packets are sent periodically out all router functional interfaces. Two-way communi-cation is established when the router is listed in the neighbor’s Hello packet. On broadcast and NBMA networks, Hello packets are used to elect the DR/BDR.

After the two-way communication is established, the decision is made whether to form an adjacency with this neighbor. This decision is based on the neighbor state and network type. If the network type is broadcast or nonbroadcast, the adjacency is formed only with the DR and BDR routers. In all other network types, the adjacency is formed between two neighbor routers.

The first step in forming the adjacency is synchronization of the database. Each router des-cribes its link-state database in the DBD packet. Only the LSA headers are exchanged bet-ween neighbors. Master and slave election takes place during this database exchange. Each router makes a note of the LSA headers that it receives during this DBD exchange. At the end of the DBD exchange, it sends the LS request packet to request LSAs whose headers have been seen during the DBD exchange. The neighbor router then replies with the LS update packet listing the entire content of those LSAs. This LS update packet is then acknowledged by sending the link-state acknowledgment packet. At this point, all the databases are fully exchanged, and the neighbor goes into Full state.

A router can be in several neighbor states:

* Down
* Attempt
* Init
* 2-way
* Exstart
* Exchange
* Loading
* Full

Troubleshooting OSPF adjacencies can be a nightmare: if youve misconfigured the OSPF interface parameters (the timers or the subnet mask), the adjacency will not form, but the router will not tell you why. The only mechanism you can use to detect the mismatch is the debug ip ospf hello command just dont try to use it on a console session of a router running OSPF across hundreds of interfaces.