As we continue our CCENT & CCNA exam question series, today we are going to try to illustrate a drag and drop type scenario. This is not the easiest thing to do on a static blog page. But we will give it a try. Today’s CCENT/CCNA exam topic is OSPF. We will review the different states that a router can be in.
The following shows the steps undertaken by two routers when forming neighbor relationships in OSPF.
| STATE |
| INIT STATE |
| FULL STATE |
| EXSTART STATE |
| DOWN STATE |
| 2WAY STATE |
| LOADING STATE |
Now let’s pretend you were actually taking your CCNA exam and the next part we are going to give you are the descriptions for each OSPF neighbor relationship State as you might see on your certification exam.
| DESCRIPTION |
| no neighboring router has received hello packets |
| routers receive hello packet and lists neighbor by its router-ID |
| router checks for neighbor relationships and if they exist dead timer is reset |
| stage at which link state database is received and DBD (Data Base Description) packets exchanged |
| stage at which request of link state requests- (LSR’s) and sending link state updates – (LSU’s) happens |
| full synchronization and full neighbor relationship, the running of the Dijkstra’s shortest path first (SPF) algorithm |
Now what you want to do is match up the State with the Description. But before we do that, we are going to give you the theory behind the CCNA exam answers and put the actual answer all the way at the bottom of the page so you can’t see it easily.
OSPF Configuration
OPSF is a link state protocol and it undergoes several steps it takes, before the routing table is fully populated with routes. Once a router has received all of LSAs and built its local link-state database, OSPF uses Dijkstra’s shortest path first (SPF) algorithm to create an SPF tree. The SPF tree is then used to populate the IP routing table with the best paths to each network.
The steps taken in forming neighbor relationships are as follows.
Down state: this is a state before any routers running OPSF have received hello packets at this point there are no neighbors.
INIT STATE: This is the initial state and neighboring routers usually exchange hello packets, it is at this point when they discover each other’s router ID and check whether the following requirements are met and match.
- Area ID
- Network segment
- Hello and Dead timer
- OSPF authentication
2WAY STATE: This is the stage where the neighbors check to see whether they are already neighbors. If they are neighbors then the process stops there and if not it proceeds to the next stage. It should be noted that in NBMA and broadcast type of networks, only routers that have been elected the DR and the BDR move past this state.
EXSTART STATE: database description packets are exchanged and they include summary information of the link state databases and acknowledgments and reviews are sent. The information is used by the router so as to know whether there are any new routes from the neighbor.
LOADING STATE: After the EXSTART STATE, the router now gathers more detailed information from its neighbor so as to populate the Link State Database. The router with the lower router ID or priority and which is considered as the slave sends LSR (Link State Requests) packets to the master, which is usually the router with the higher priority or router ID in that segment. The master responds with LSU (Link State Update) which is detailed information on the specific networks that the slave has requested. This process continues until the neighbors have exchanged their Link State Database.
FULL STATE: this is the final state and it is when the neighbors running OSPF have the same Link State Database, from this stage the SPF (shortest Path First) algorithm can be run.
Understanding this process is crucial in troubleshooting neighbor formation in OSPF.
Ok, now that you know the theory behind this CCENT/CCNA exam question, it should be pretty easy to match them up.
| STATE | DESCRIPTION |
| DOWN STATE | router has not received hello packets |
| INIT STATE | router receive hello packet and lists neighbor by its router-ID |
| 2WAY STATE | router checks for neighbor relationships and if they exist dead timer is reset |
| EXSTART STATE | stage at which link state database is received and DBD (Data Base Description) packets exchanged |
| LOADING STATE | stage at which request of link state requests- (LSR’s) and sending link state updates – (LSU’s) happens |
| FULL STATE | complete synchronization and full neighbor relationship, the running of the Dijkstra’s shortest path first (SPF) algorithm |
This is actually very similar to the type of drag and drop question you will see on the Cisco CCENT or CCNA exam. We hope that these types of exam sample questions helps prepare you for your CCNA exam. You can get more of these exam questions in our newly designed CCNA exam simulator which gives a detailed answer why each question is right or wrong as it is very important that you understand why you answered a question incorrectly.
CCNA Exam Pass Guarantee
Now besides practicing the questions you will see on your CCENT or CCNA exam, you really should want to see these OSPF neighbor relationship states in action. What better way to do that than with your own actual CCNA lab? There is nothing like actually having the real world hands-on experience and seeing it work right before you eyes. Then the fun part is getting to break it and make it work again as you play with your CCENT and CCNA lab to see how slight different changes to your lab topology changes things.
So don’t forget to checkout our awesome CCNA Labs here!
