Chapter 8-10 OSPF
What is the LSA age field in the LSDB used for?
Troubleshoot when was the last time an LSA was advertised
TRUE OR FALSE All routers within the same OSPF area maintain an identical copy of the link-state database (LSDB).
True
True or False The area ID can be configured with simple decimal and dotted decimal?
True
True or false: Breaking a large OSPF topology into smaller OSPF areas can be considered a form of summarization.
True
What type of hierarchical tier does OSPF uses ?
Two-Tier hierarchical achitecture
Name all six IPV4 routing LSA types
Type 1, Router LSA Type 2, network LSA Type 3, summary LSA Type 4, ASBR summary LSA Type 5, AS external LSA Type 7, NSSA external LSA
Which LSA advertises the LSAs that originate within an area
Type 1, router LSA
Which LSA advertises a multi-access network segment attached to a DR
Type 2, network LSA
Which LSA advertises network prefixes that originated from a different area
Type 3, summary LSA
Which LSA advertises a summary LSA for a specific ASBR
Type 4, ASBR summary LSA
Which LSA advertises LSAs for routes that have been redistributed
Type 5, AS external LSA
Which LSA advertises redistributed routes in NSSAs
Type 7, NSSA external LSA
What happens within a OSPF area if a topology change occurs such as a link flap or an additional network being added within an area?
All routers in the same OSPF area calculate the SPF tree again.
Why is simplifying an OSPF topology with areas so important?
Because the larger the network gets, the more convergence, and resources is used to maintain the network by using areas it can simplify a OSPF topology while still connecting networks together
Why does OSPF does not put the DR functionality on Point to Point networks
Because this networks only two nodes can exist in a point to point network
How is an ospf router is able to determine where the Dr and BDR are?
By sending an LSA to the ALLDRouter 224.0.0.6 Multicast address
How is the process of summarizing routes on an OSPF router accomplished?
By using the OSPF process configuration command area area-id range network subnet-mask
What is the OSPF logic when selecting which router will be the DR or BDR?
1 - Highest OSPF Priority 2 - Highest Router ID
What is the logic behind OSPF routers selecting a router ID automatically?
1- The highest IP address of any up loop back interfaces. 2- The Highest IP address of any active physical interfaces
What is the advertised network for the loopback interface with IP address 10.123.4.1/30?
10.123.4.1/32
OSPF automatically assigns a link cost to an interface based on a reference bandwidth of ___________.
100 Mbps
What is the default reference bandwidth in OSPF?
100 Mbps
What OSPF neighbor state indicates Bidirectional communication has been established. If a DR or BDR is needed, the election occurs during this state.
2-way
What are the two Multicast addresses and mac addresses use in OSPF?
224.0.05 01:00:5E:00:00:05 224.0.0.6 01:00:5E:00:00:06
OSPF uses the protocol number ___________ for its inter-router communication.
89
What command restarts the ospf process?
Clear ip ospf process
What formula is used for OSPF link cost?
Cost = Reference Bandwidth / Interface Bandwidth
These packets are for summarizing database contents. Packets are exchanged when an OSPF adjacency is first being formed. These packets are used to describe the contents of the LSDB.
Database description (DBD) or DDP
Usually the Wait timer has the same time as the ?
Dead timer interval
What algorithm is used in OSPF?
Dijkstra Shortest Path First algorithm
What OSPF neighbor state indicates that the router has not received any OSPF hello packets.
Down
Special area called the backbone
Area 0
OSPF routers connected to Area 0 and another OSPF are
Area Border Routers
What OSPF neighbor state relevant to NBMA networks that do not support broadcast and require explicit neighbor configuration. This state indicates that no information has been received recently, but the router is still attempting communication.
Attempt
Why is it important to set the dead intervals longer then the hello timer?
Because so that the dead interval timer does not reach 0 between hello packets
When running multiple ospf processes what is important to keep in mind in regards to the redistribution of router?
Each process has a unique database and the routes learned in one Processes are not available to a different OSPF processes without redistribution of orutes between processes
What OSPF neighbor state is the first state in forming an adjacency. Routers identify which router will be the master or slave for the LSDB synchronization.
ExStart
What OSPF neighbor state indicates routers are exchanging link states by using DBD packets.
Exchange
A member router contains a complete copy of the LSDBs for every area in the routing domain
False
A router with an interface associated with Area 1 and Area 2 will be able to inject routes learned from one area into another area
False
TRUE OR FALSE Can a interface belong to more than one Area in OSPF?
False
True or False each router sees itself as the lowest of the Shortest Path First TREE?
False
True or false: A default route advertised with the command default information-originate in OSPF will always appear as an OSPF inter-area route.
False
True or false: OSPF is only enabled on a router interface by using the command network ip-address wildcard-mask area area-id under the OSPF router process.
False
True or false: OSPF uses the shortest total path metric to identify the best path for every internal OSPF route (intra-area and interarea).
False
True or false: The OSPF process ID must match for routers to establish a neighbor adjacency.
False
True or false: The router with the highest IP address is the designated router when using a serial point-to-point link
False
When an ABR receives a network LSA, the ABR forwards the network LSA to the other connected areas.
False
True of False is required to set the same bandwidth reference on all routers participating on an OSPF domain?
False, is good practice to place the same reference bandwidth
True or False As an OSPF router adds or removes a directly connected network link to or from its database, the router floods the link-state advertisement (LSA) out all active and inactive OSPF interfaces.
False, only active OSPF interfaces
True or False: All routers from different areas have the same LSDB?
False, only routers within the same area do have the same LSDB
OSPF uses ___________ packet types for inter-router communication.
Five
What OSPF neighbor state indicates Neighboring routers are fully adjacent.
Full
These packets are for discovering and maintaining neighbors. Packets are sent out periodically on all OSPF interfaces to discover new neighbors while ensuring that other adjacent neighbors are still online.
Hello
What are the five different packet types in OSPF?
Hello Database description (DBD) or DDP Linkstate request (LSR) Link-state update (LSU) Link-state ack
What command turns on the OSPF network on a loopback
IP OSPF NETWORK POINT-TO-POINT
What destination addresses does OSPF use, when feasible? (Choose two.) A - IP address 224.0.0.5 B- IP address 224.0.0.10 C- IP address 224.0.0.8 D- MAC address 01:00:5E:00:00:05 E- MAC address 01:00:5E:00:00:0A
IP address 224.0.0.5 MAC address 01:00:5E:00:00:05
Which multicast address is used for ALLDRouters
IPv4 224.0.0.6
Which multicast address is used for ALLSPFROUTERS?
IPv4 address 224.0.05
How many fields are inside a hello packet in OSPF?
NINE
Can you see the exact topology from one area to another outside area?
No your only able to the necessary information to be able to communicate to an outside area
in OSPF what is a logical grouping of routers or, more specifically
OSPF Area
What OSPF neighbor state indicates that a hello packet has been received from another router, but bidirectional communication has not been established.
Init
When a type 3 LSA is received in a nonbackbone area, what does the ABR do?
Installs the type 3 LSA for only the area where it was received
All areas in OSPF inject routing information into which area?
Into the backbone area 0
Network routes that are learned from other OSPF routers within the same area
Intra-Area Routes
What is the backbone AREA in charged of?
It distrutes routes of different areas so that different areas can communicate
What is the OSPF network type loopback feature?
It states that the IP address is always advertise a /32 prefix on a loopback interface even if the interface does not have an actual /32 prefix
How does OSPF sends to neighboring routers advertisements that contain link state and metric
Link State Advertisements (LSAs)
What is the database used by ospf neighbors to keep all advertised LSAs?
Link State Database LSDB
These packets are for flooding acknowledgments. These packets are sent in response to the flooding of LSAs, thus making flooding a reliable transport feature.
Link-state ack
These packets are for database downloads. When a router thinks that part of its LSDB is stale, it may request a portion of a neighbor's database by using this packet type
Link-state request (LSR)
These packets are for database updates. This is an explicit LSA for a specific network link and normally is sent in direct response to an LSR.
Link-state update (LSU)
What OSPF neighbor state indicates LSR packets are sent to the neighbor, asking for the more recent LSAs that have been discovered (but not received) in the Exchange state.
Loading
In order for an OSPF neighbor successfully become adjacent with is neighbors what requirements must it pass?
RIDs must be unique Interfaces must share a common subnet MTUs must match in the interface Area ID must match The Designated Router enablement must match OSPF hello and dead timer must match Authentication type and credentials must match Area type flags must match for segment
What are the benefits from segmenting OSPF domains into multiple areas?
Reduces the size of the LSDB for each area, SPF tree calculations become faster, and decreasing LSDB flooding between routers when a link flap occurs
Which LSA type exists in all OSPF areas?
Router
What are the 9 fields within a OSPF hello packet?
Router ID Authentication options Area ID Interface address mask interface priority Hello Interval Dead Interval Designated Router and Designated Backup routers Active Neighbor
What happens outside a OSPF area if a topology change occurs such as a link flap or an additional network being added within an area?
Routers outside that area do not calculate the full SPF tree again but perform a partial SPF calculation if the metric have changed or prefix is removed
What networks are set to Point to point on OSPF by default
Serial interfaces, GRE tunnels and Frame Relays
What type of network circuits dont need a Designated router?
Serial links
OSPF supports filtering of routes using what techniques?
Summarization, using the no-advertise option Area filtering, which prevents type 1 LSAs from being generated into a type 3 LSA
Explain the purpose for the DEADTimer
The Dead timer is used to determine the time when the neighboard has not check in and if the neighbor does not check in it will reconverge the network
To ensure that all routers have fully initialized which packet initiates a wait timer when OSPF hello timers do not contain a DR/BDR router for a segment?
The Wait timer
A special area in OSPF that every other area must connect to?
The backbone area or Area0
Explain the purpose for the Hello Timer
The hello packet intervals are used to discover and update router status
What command is configured to prevent a router from becoming the designated router for a network segment?
The interface command ip ospf priority 0
If a hybrid configuration of OSPF exist on a router meaning there is a network statement configuration and an interface specific configuration which configuration takes precedence over the other?
The interface specific
What can be build from the LSDB?
a topology of the advertised network
What command changes the bandwidth referene in ospf?
auto-cost reference-bandwidth
How is OSPF able to provide scalability?
by using Areas
What command sets up the default route in OSPF?
default-information originate
How many OSPF link-state announcement (LSA) types are used for routing traditional IPv4 packets?
five
Network routes that are learned from other OSPF routers from a different area using an ABR are known as
interarea routes
What command configures ospf on the interface configuration mode?
ip ospf 1 area
What command modifies the priority for ospf and where would this command be executed from?
ip ospf priority [0-255] within the interface configuration mode
What is the formula to find the number of adjecencies?
n(n-1)/2
What command enables OSPF on all interfaces?
network 0.0.0.0 255.255.255.255 area 0
What command verifies OSPF timers
show ip ospf interface
What command best shows the OSPF interface role in a router?
show ip ospt interface brief