OSPF Exam Questions With Correct
Answers
The three main components of the OSPF routing protocol include: - answerAdjacency
database - Creates the neighbor table
Link-state database (LSDB) - Creates the topology table
Forwarding database - Creates the routing table
These tables contain a list ...
The three main components of the OSPF routing protocol include: - answer✔✔Adjacency
database - Creates the neighbor table
Link-state database (LSDB) - Creates the topology table
Forwarding database - Creates the routing table
These tables contain a list of neighboring routers to exchange routing information with and are
kept and maintained in RAM.
The CPU processes the neighbor and topology tables using Dijkstra's SPF algorithm. The SPF
algorithm is based on the cumulative cost to reach a destination.
OSPF exchanges messages to convey routing information using five types of packets. -
answer✔✔Type 1: Hello packet - Used to establish and maintain adjacency with other OSPF
routers.
Type 2: Database Description (DBD) packet - Contains an abbreviated list of the sending router's
LSDB and is used by receiving routers to check against the local LSDB. The LSDB must be
identical on all link-state routers within an area to construct an accurate SPF tree.
Type 3: Link-State Request (LSR) packet - Receiving routers can then request more information
about any entry in the DBD by sending an LSR.
Type 4: Link-State Update (LSU) packet - Used to reply to LSRs and to announce new
information. LSUs contain seven different types of LSAs.
Type 5: Link-State Acknowledgment (LSAck) packet - When an LSU is received, the router
sends an LSAck to confirm receipt of the LSU. The LSAck data field is empty.
EXAM STUDY MATERIALS July 26, 2024 9:30 AM
, To maintain routing information, OSPF routers complete the following generic link-state routing
process to reach a state of convergence: - answer✔✔1. Establish Neighbor Adjacencies - OSPF-
enabled routers must recognize each other on the network before they can share information. An
OSPF-enabled router sends Hello packets out all OSPF-enabled interfaces to determine if
neighbors are present on those links. If a neighbor is present, the OSPF-enabled router attempts
to establish a neighbor adjacency with that neighbor.
2. Exchange Link-State Advertisements - After adjacencies are established, routers then
exchange link-state advertisements (LSAs). LSAs contain the state and cost of each directly
connected link. Routers flood their LSAs to adjacent neighbors. Adjacent neighbors receiving the
LSA immediately flood the LSA to other directly connected neighbors, until all routers in the
area have all LSAs.
3. Build the Topology Table - After LSAs are received, OSPF-enabled routers build the topology
table (LSDB) based on the received LSAs. This database eventually holds all the information
about the topology of the network.
4. Execute the SPF Algorithm - Routers then execute the SPF algorithm. The gears in the figure
are used to indicate the execution of the SPF algorithm. The SPF algorithm creates the SPF tree.
From the SPF tree, the best paths are inserted into the routing table. Routing decisions are made
based on the entries in the routing table.
multiarea OSPF - answer✔✔All areas must connect to the backbone area (area 0). Routers
interconnecting the areas are referred to as Area Border Routers (ABR).
It can divide one large autonomous system (AS) into smaller areas, to support hierarchical
routing. With hierarchical routing, routing still occurs between the areas (interarea routing),
while many of the processor intensive routing operations, such as recalculating the database, are
kept within an area.
For instance, any time a router receives new information about a topology change within the area
(including the addition, deletion, or modification of a link) the router must rerun the SPF
algorithm, create a new SPF tree, and update the routing table. The SPF algorithm is CPU-
intensive and the time it takes for calculation depends on the size of the area.
EXAM STUDY MATERIALS July 26, 2024 9:30 AM
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