OSPF Packet Types & States | OSPF Interview Questions & Answer Part-1
I will not explain only the theoretical concepts like in other articles or books, but my main purpose of posts is to explain & share the practical experience that I faced during project executions & implementation. I will write all interview Questions related to OSPF that will be helpful for Fresh & experienced engineers.
Below are the high-level target topics of OSPF that I will explain in different parts. First interview Question then explain the summary answer of all Questions in each post.
- OSPF Packet Types
- OSPF States
- OSPF LSA Types
- Network Types
- Route Types
- Special Areas
- OSPF Header
- Hello Packet Header
- LSA Header
- Optional Bits
- OSPF IPV6
There are so many topics but my target is to cover those that are mostly asked in interviews & during project practical implementation like OSPF packet types & OSPF states for this post.
In this post OSPF Part-1, I will cover The OSPF Packet types & States.
OSPF Packet Types & States Interview Questions:
- What is the purpose of the Hello packet?
- Why use a Database Description (DBD) Packet?
- What is LSR & when will send LSR Packet?
- How many LSAs in the Link state update (LSU)?
- Why use the LSA acknowledge packet in OSPF?
- What does it mean if the OSPF Router is in Down state?
- What is meant by init state in OSPF?
- What is the Two-way state in OSPF?
- What is the state of DRother Routers in Broadcast Network of OSPF?
- What is exstart state in OSPF?
- Master & slave selection are in which state of OSPF?
- What are the criteria for selecting a master & initiate communication in OSPF?
- Which packet is sending in the Exstart & two-way state of OSPF?
- Which packets are sending during the Exchange state of OSPF?
- What is different b/w loading & full state of OSPF?
- Which packets sending during loading & full state of OSPF?
- SPF Algorithm is applied in which state?
- If ospf router is stuck in the init state what is the reason?
- If the Router is stuck in the Exstart & exchange state of OSPF, what to check first?
- If MTU is mismatched then Router will remain in which state?
Let me summarize answers of all the above OSPF packet types & OSPF States Questions
There are five packet types in OSPF
- Hello Packet.
- Data base description Packet (DBD)
- Link state request (LSR)
- Link state update(LSU)
- LSA Acknowledge
Hello Packet
The first one hello packet is used for the OSPF discovery & maintaining Neighbor relation b/w OSPF routers. I will explain the Hello packet Header in the next few posts.
Database Description Packet:
DBD packet is used for the synchronization of Data bases of OSPF Routers. For example, if some new routes/Networks are added in one Router then during the DBD exchange both router routes are not the same then LRS & LSU will send & synchronize the Database.
LSR & LSU :
After DBD negotiation if the Database is not synchronized, then Link state request(LSR) packet is sent for specific routes from the neighbor router then the requested records will send through Link state update. LSU contains multiple LSAs depending on LRS request.
Keep in mind that after the completion of the DBD packet successfully then LSR packet will send.
OSPF States:
DOWN State:
If OSPF is not configured on Neighbor routers then no OSPF detection in this case OSPF state will in down state in the local Router.
Init State:
When OSPF is configured on Route-A & Router-B, after configurations of Router-A. it will send a hello packet through the OSPF multicast address in this case Router-A will be in the init state means hello packet send but not received.
Two-Way State:
Then when Router-B finds the Router ID of Router-A,then it will send a hello response to Router-A in this case Router-A will be in two way state as it will receive its own router ID in the hello packet from Router-B.
Two-way state is normal in broadcast network like if there is DR/DBR & DRother then states b/w DRother to DRother is two-way not full as its state is full with DR & BDR.
Exstart State:
Next is the exstart state in this master/slave role will be decided that will initiate communication, this role depends on large IP address not on DR or BDR.
If MTU is mistmatch in DBD packet then ospf state will be stucked in Exstart state.
Exchange State:
The DBD packet is sending in the Exchange state of OSPF. If any routes are missing not same Database then LSR & LSU will send in loading state.in this state all routes best & backup will be built Database.
Loading & Full State:
In loading state there are all routes means best & non best & its not in routing table.
Now shortest path first (SFP) algorithm will be applied on the Loading state & only select the best routes from Database to make one Routing table. This is the difference b/w Loading & Full state of OSPF.
Attempt State:
OSPF attempt state is used in NBMA network where configure neighbor manually not through OSPF multicast address ,after manual neighbor command its send unicast hello packet toward neighbor ip address but not received response in this case its in attempt state.
Attempt Vs Init State in OSPF:
if you compare this attempt state with Init state , in Init hello packet received through dynamic multicast but own router’s id is not found in hello packet yet.But attempt is through manual neighbor command explained above.
Attempt state is used in the Non-Broadcast Multi-Access (NBMA) Network type. I will explain this point in network-type articles as well.
In the next posts I will cover the remaining topic of OSPF, if you have any good questions in your mind I am happy to see your comments.
If you want to learn more in deep then below links for reference.