OSPF

Open shortest path first is an Open Standard Link State routing protocol which works by using Dijkastra algorithm to initially construct the shortest paths and follows that by populating the routing table with resulting best paths.

  

steps required to change Neighborship into adjacency?
1.Two-way communication (using Hello Protocol).
2.Database Synchronization which means exchange of Database Description (DD) packets, Link State Request (LSR) packets, Link State Update (LSU) packets.
After Database synchronization is complete, the two routers are considered adjacent.

The LSAs (Link-State Advertisements) are used by OSPF routers to exchange routing and topology information.

Each router then checks its topology database and sends Link State Request (LSR) message requesting all LSAs that was not found in its topology table. Other router responds with the Link State Update (LSU) that contains all LSAs requested by the neighbor.

OSPF Router ID?
Router Id is used to identify the Router. Highest IP address of the router's loopback interfaces is chosen as the Router ID, If no loopback is present than highest IP address of the router's physical interfaces will be chosen as Router ID.

difference between an OPPF neighbor and an adjacent neighbor?
LSAs are exchanged only among adjacent routers not among neighbor routers.

different neighbour states in OSPF ?
OSPF routers need to go through several state before establishing a neighbor relationship -
1. Down - No Hello packets have been received on the interface.
2. Attempt - In Attempt state neighbors must be configured manually. It applies only to nonbroadcast multi-access (NBMA) networks.
3. Init - Router has received a Hello message from the other OSFP router.
4. 2way - the neighbor has received the Hello message and replied with a Hello message of his own. Bidirectional Communication has been established. In Broadcast network DR-BDR election can occur after this point.
5. Exstart - DR & BDR establish adjacencies with each router in the network. Master-slave election will takes place (Master will send its DBD first).
6. Exchange - Routing information is exchanged using DBD (Database Descriptor) packets, Link-State Request (LSR) and Link-State Update packets may also be sent.
7. Loading - LSRs (Link State Requests) are send to neighbors for every network it doesn't know about. The Neighbor replies with the LSUs (Link State Updates) which contain information about requested networks. After all the requested information have been received, other neighbor goes through the same process.
8. Full - All neighbor routers have the synchronized database and adjacencies has been established.

ifferent OSPF LSA Types?
1. Router LSA (Type1)  - Each router generates a Type 1 LSA that lists its active interfaces, IP addresses, neighbors and the cost. LSA Type 1 is flooded only within an area.
2. Network LSA (Type2) - Type2 LSA is sent out by the designated router (DR) and lists all the routers on the segment it is adjacent to. Type 2 LSA are flooded only within an area.
3. Summary LSA (Type3) - Type 3 LSAs are generated by Area Border Routers (ABRs) to advertise networks from one area to the rest of the areas in Autonomous System.
4. Summary ASBR LSA (Type4) - Generated by the ABR. It contain routes to ASBRs.
5. External LSA (Type5) - External LSAs are generated by ASBRs and contain routes to networks that are external to the current Autonomous System.
6. Not-So-Stubby Area LSA (Type7) - Stub areas do not allow Type 5 LSAs.  A Not So Stubby Area (NSSA) allows advertisement of Type 5 LSA as Type 7 LSAs. Type LSA is generated by an ASBR inside a Not So Stubby Area (NSSA) to describe routes redistributed into the NSSA.

OSPF Network types ?
1.Broadcast
2.Non-Broadcast (NBMA)
3.Point-to-Point
4.Point-to-multipoint
5.Point-to-multipoint non-broadcast

OSPF Virtual Link?
OSPF requires the use of a backbone area (area 0) with each area connecting to area 0 through an ABR. However in some cases, regular area might not have a convenient point of connection to the backbone area. In this case, OSPF uses virtual link to connect that regular area to backbone area virtually. 

An OSPF virtual link allows two ABRs that connect to the same non-backbone area to form a neighbor relationship through that non-backbone area.

Explain OSPF Stub Area and different types of Stub Areas?
Stub Area
Sometimes we need to control the advertisement of external routes into an area. This area is called Stub area. Stub areas are not capable of importing routes external to ospf.Type 4 & Type 5 LSA are filtered from Stub areas

 Totally Stubby Area
Like stub areas, totally stubby areas do not receive type 4 or 5 LSAs from their ABRs. However, they also do not receive type 3 LSAs.

OSPF calculate its metric or cost?
OSPF uses Cost as its metric. The formula to calculate the OSPF cost is reference bandwidth divided by interface bandwidth. For example, in the case of Ethernet, it is 100 Mbps / 10 Mbps = 10