Chapter 1 - Single Area OSPFv2 Concepts Flashcards
OSPFv2 is for…
OSPFv3 is for…
…IPv4
…IPv6
What protocol is a link state protocol developed as an alternative for the distance vector RIP? It offers faster convergence and scales well.
OSPF
what is used to determine best path to destination in OSPF
Cost
OSPF link-state info includes:
network prefix, prefix length and cost
What 5 packet types does OSPF use to convey routing info and used to discover neighboring routing and exchange routing info to maintain accurate info about the network?
- Hello Packet
- Database Description (DBD) Packet
- Link-State Request (LSR) Packet
- Link-State Update (LSU) Packet
- Link-State Acknowledgement (LSA) Packet
OSPF messages are used to create and maintain 3 OSPF databases, what are they and what is created
- Adjacency Database - creates the neighbor table
- Link-State Database - creates the topology table
- Forwarding Database - creates the routing table
which OSPF database lists all neighbor routers that established bidirectional communication?
What CLI command is used to see this database?
Adjacency Database
show ip ospf neighbor
which OSPF database represents the network topology?
What CLI command is used to see this database?
Link-State Database (LSDB)
show ip ospf database
what OSPF database contains the list of routes that is generated when an algorithm is run on the link-state database?
What CLI command is used to see this database?
Forwarding Database
show ip route
What algorithm is the topology table built through?
Dijkstra Shortest-path First (SPF) - cumulative cost to reach a destination
Link-State routing steps completed by a router: Step 1. Establish Neighbor Adjacencies
OSPF-enabled router sends HELLO packets out on all OSPF-enabled interfaces to see if neighbors are present on those links. If yes, router attempts to establish neighbor adjacencies with that neighbor
Link-State routing steps completed by a router: Step 2. Exchange Link-State Advertisements (LSA)
After neighbor adjacencies are established, routers exchange LSA - contains state and cost of each directly connected link. Neighbors receiving LSA immediately flood the LSA to other directly connected neighbors.
Link-State routing steps completed by a router: Step 3. Build the Link State Database (LSDB)
LSDB is built based on LSA received
Link-State routing steps completed by a router: Step 4. Execute SPF Algorithm
SPF algorithm creates the SPF tree that is used to calculate the shortest path to each node
Link-State routing steps completed by a router: Step 5. Choose the Best Route
Best paths to each network are offered to IP routing table. Routing decisions are made based on entries in routing table
what is a group of routers that share the same link-state info in their LSDBs?
OSPF area
multiarea OSPF vs single-area OSPF
- single-area: all routers in one area (use area 0)
- multiarea: OSPF is implemented using multiple areas, in a hierarchical fashion; but all areas are connected by a backbone area.
Hierarchical-topology design option advantages to multiarea OSPF:
- smaller routing tables
- reduced link-state update overhead
- reduced frequency of SPF calculations
what are area border routers
routers that interconnect areas in a multiarea OSPF implementation
what is OSPFv3 Address Families feature
it includes support for both IPv4 and IPv6
Tools used by OSPF to help determine the fastest available route for a pakcet
link-state packets
purpose of Hello Packet
- establish and maintain adjacency with other OSPF routers
- advertise parameters on which two routers must agree to become neighbors
- elect the Designated Router (DR) and the Backup Designated Router (BDR)
purpose of Database Description Packet
contains abbreviated list of LSDB of the sending router and is used by receiving routers to check against local LSDB.
LSDB must be identical on all link-state routers within an area to construct accurate SPF tree
purpose of Link-State Request (LSR) packet
receiving routers use to request more info about any entry in the DBD
purpose of Link-State Update (LSU) packet
used to reply to Link-State Requests and to account new info - like forwarding OSPF routing updates
purpose of Link-State Acknowledgement (LSAck) packet
used to acknowledge that the LSU has been received by the router
has empty data field
what field in a Hello Packet is used in DR and BDR election?
what numbers are used?
Router Priority
0-255
1 default
higher the number = more likely to come the DR on the link
list the states OSPF progresses through while attempting to reach convergence
- Down State
- Init State
- Two-Way State
- ExStart State
- Exchange State
- Loading State
- Full State
while OSPF is attempting to reach convergence, in what state is DR/BDR elected?
Two-Way State
while OSPF is attempting to reach convergence, in what state does the router send Hello packets
Down State
which three OSPF states are involved when two routers are forming an adjacency
Down State, Init State and Two-Way State
while OSPF is attempting to reach convergence, in what state do the routers decide which router will initiate the DBD packet exchange and decide upon the initial DBD packet sequence number
ExStart State
while OSPF is attempting to reach convergence, in what state are LSRs and LSUs used to gain additional route info? And routes are processed using the SPF algorithm
Loading State
while OSPF is attempting to reach convergence, in what state is communication btwn two routers bidirectional?
Two-Way State
what IP is Hello Packet sent on
224.0.0.5
All OSPF Routers IPv4 Multicase Address
when are LSUs sent to neighbors
when network topology change is detected or every 30 mins
what is the point of electing a DR
OSPF elects a DR to be the collection and distribution point for LSAs sent and received.
