OSPF Flashcards
1
Q
What must match for two routers to become OSPF neighbors?
A
Hello and Dead intervals, area ID, authentication, subnet, MTU
2
Q
What happens when the OSPF process is reset?
A
All adjacencies are lost and rebuilt
3
Q
What command sets an MD5 authentication key?
A
ip ospf message-digest-key [key-id] md5 [password]
4
Q
What is the difference between E1 and E2 external routes?
A
E1 includes internal cost, E2 uses only external cost
5
Q
How do you influence DR election?
A
Set OSPF priority (higher wins)
6
Q
What command shows OSPF DR/BDR status on an interface?
A
show ip ospf interface
7
Q
How do you change OSPF cost reference bandwidth?
A
auto-cost reference-bandwidth [value] (inside OSPF process)
8
Q
What is the significance of the OSPF Dead interval?
A
If no Hello packets are received in this time, the neighbor is declared down
9
Q
Can OSPF use loopback interfaces?
A
Yes, and they are preferred for router ID selection
10
Q
What command displays OSPF LSAs in the database?
A
show ip ospf database
11
Q
What command shows OSPF neighbors?
A
show ip ospf neighbors
12
Q
What OSPF state indicates routers are exchanging DBDs?
A
Exchange
13
Q
What command clears the OSPF process?
A
clear ip ospf process
14
Q
Which router types generate Type 5 LSAs?
A
ASBRs
15
Q
What OSPF network type does not elect a DR or BDR?
A
Point-to-Point (P2P)
16
Q
What command can trace OSPF packets?
A
debug ip ospf packet
17
Q
What command shows the dynamic routing protocols running?
A
show ip protocols
18
Q
What is the default external route type in OSPF?
A
E2
19
Q
What command resets the OSPF process?
A
clear ip ospf process
20
Q
What metric does OSPF use?
A
Cost
21
Q
Which router becomes the DR if priorities are equal?
A
Router with the highest Router ID
22
Q
How do you originate a default route in OSPF (ASBR)?
A
default-information originate
23
Q
Which command shows the contents of the OSPF LSDB?
A
show ip ospf database
24
Q
Which command configures OSPF authentication?
A
ip ospf authentication [message-digest/simple]
25
What command helps debug OSPF hello packets?
debug ip ospf hello
26
What is the default reference bandwidth in OSPF?
100 Mbps
27
How do you prevent OSPF neighbors from forming on an interface?
Make it passive or use a different area/hello setting
28
What command gives a brief summary of OSPF interfaces?
show ip ospf interface brief
29
What command shows detailed OSPF neighbor information?
show ip ospf neighbor detail
30
What does 'passive interface default' do in OSPF?
Shuts off hello packets on all interfaces by default
31
What priority value prevents a router from becoming DR/BDR?
0
32
What is the OSPF cost of a FastEthernet link (default settings)?
1
33
What is the purpose of the wildcard mask in OSPF network statements?
To specify which IP addresses are matched
34
Which router becomes the BDR if DR is elected?
Router with second-highest Router ID
35
Which area types block external LSAs (Type 5)?
Stub and Totally Stubby
36
What is the OSPF administrative distance?
110
37
What command disables OSPF Hellos on all interfaces?
passive-interface default
38
What is an internal OSPF router?
A router with all interfaces in the same area
39
What command assigns an interface to an OSPF area?
ip ospf [process ID] area [area number]
40
What is the purpose of 'auto-cost reference-bandwidth'?
Adjusts OSPF cost calculation for faster interfaces
41
What command shows OSPF interface timers and cost?
show ip ospf interface [interface]
42
Which LSA type advertises inter-area routes from ABRs?
LSA Type 3 (Summary LSA)
43
What command is used to prevent OSPF from forming adjacencies on an interface?
passive-interface [interface]
44
Which LSA type is generated by DRs on broadcast and NBMA networks?
LSA Type 2 (Network LSA)
45
What is the command to set the Dead interval on an interface?
ip ospf dead-interval [seconds]
46
What is the effect of mismatched OSPF MTU between neighbors?
Adjacency may not form (stuck in ExStart state)
47
What command shows detailed OSPF routing information?
show ip ospf
48
What OSPF state indicates routers are requesting LSAs?
Loading
49
Which area types block both external and summary LSAs?
Totally Stubby Area
50
Which LSA types are not flooded into stub areas?
Type 4 and Type 5
51
What multicast address does OSPF use for all routers?
224.0.0.5
52
What is the wildcard mask for a /24 subnet?
0.0.0.255
53
Which router types generate Type 3 LSAs?
ABRs
54
What happens if no router ID is set manually?
OSPF chooses the highest IP of a loopback, or active physical interface
55
What does 'ip ospf network point-to-point' do?
Sets the interface to use P2P OSPF network type
56
What command shows a specific OSPF neighbor?
show ip ospf neighbor
57
Which OSPF state is the first after interface comes up?
Down
58
What command displays the OSPF database for a router ID?
show ip ospf database router [router ospf ID]
59
How do you advertise a network in OSPF?
network [network address] [wildcard mask] area [area number]
60
Which LSA type is generated by every router and describes its links?
LSA Type 1 (Router LSA)
61
What is the OSPF cost of a GigabitEthernet link (default settings)?
1 (unless reference bandwidth is increased)
62
What is a Not-So-Stubby Area (NSSA)?
A stub area that can import external routes using Type 7 LSAs
63
How do you set OSPF priority on an interface?
ip ospf priority [value]
64
What command is used to configure a router as an ASBR?
Redistribute a route from another protocol into OSPF
65
What is the default Hello interval on broadcast networks?
10 seconds
66
In which OSPF state are Hello packets sent but no received yet?
Init
67
Can routers with different OSPF process IDs form a neighbor relationship?
Yes
68
Which LSA type is used for external routes redistributed into OSPF?
LSA Type 5 (External LSA)
69
What is the command to set the Hello interval on an interface?
ip ospf hello-interval [seconds]
70
Which OSPF state indicates full adjacency and synchronized databases?
Full
71
What OSPF state indicates the start of database exchange?
ExStart
72
What command can verify OSPF adjacency formation?
debug ip ospf adj
73
Which OSPF state indicates bidirectional communication?
2-Way
74
How do you set the OSPF network type?
ip ospf network [network type]
75
What command sets the router ID manually?
router ospf [ID] -> router-id [x.x.x.x]
76
What does the OSPF 2-Way state indicate?
DR/BDR election completed (on broadcast/NBMA networks)
77
How does OSPF react to topology changes?
Recalculates SPF and updates routing table accordingly
78
What area must all other OSPF areas connect to?
Area 0 (Backbone)
79
How do you enable OSPF on interface g0/0 in area 0?
int g0/0
ip ospf 1 area 0
80
How do you start the OSPF process on a router?
router ospf [process ID]
81
What is the purpose of OSPF?
To calculate the shortest path using Dijkstra's SPF algorithm
82
What is the OSPF priority range?
0 to 255
83
What multicast address does OSPF use for DR/BDR?
224.0.0.6
84
What is the default Dead interval on broadcast networks?
40 seconds
85
What OSPF router type connects multiple areas?
ABR (Area Border Router)
86
Which LSA type advertises routes to an ASBR?
LSA Type 4 (ASBR Summary LSA)
87
Which router types generate Type 4 LSAs?
ABRs (about ASBRs)
88
What router type redistributes external routes into OSPF?
ASBR (Autonomous System Boundary Router)
89
What command shows the OSPF neighbors and their states?
show ip ospf neighbor
90
What OSPF network type requires DR/BDR elections?
Broadcast and Non-Broadcast Multi-Access (NBMA)
91
What command sets simple authentication in OSPF?
ip ospf authentication-key [password]
92
What is a Type 7 LSA?
External LSA used in NSSA
93
What is the initial OSPF neighbor state?
Down
94
What OSPF state means a Hello was received but not acknowledged?
Init
95
What OSPF state shows bidirectional communication is established?
2-Way
96
What OSPF state begins master/slave negotiation for database exchange?
ExStart
97
What OSPF state involves sending Database Description (DBD) packets?
Exchange
98
What OSPF state involves requesting and sending LSAs?
Loading
99
What is the final OSPF neighbor state?
Full
100
Which OSPF state is required for routers to add each other to the LSDB?
Full
101
What is the purpose of the ExStart state in OSPF?
Master/slave negotiation
102
What indicates the OSPF adjacency is fully synchronized?
Full
103
What is LSA Type 1 called?
Router LSA
104
What does a Router LSA describe?
All of a router's links and their states within an area
105
Who generates Type 1 LSAs?
Every OSPF router
106
What is LSA Type 2 called?
Network LSA
107
What does a Network LSA describe?
A broadcast or NBMA network and its attached routers
108
Who generates Type 2 LSAs?
The Designated Router (DR) on a multi-access network
109
What is LSA Type 3 called?
Summary LSA
110
What does a Summary LSA describe?
Routes between areas (inter-area networks)
111
Who generates Type 3 LSAs?
Area Border Routers (ABRs)
112
What is LSA Type 4 called?
Summary ASBR LSA
113
What is the purpose of a Summary ASBR LSA?
To advertise the location of an ASBR to other areas
114
Who generates Type 4 LSAs?
ABRs
115
What is LSA Type 5 called?
External LSA
116
What does an External LSA describe?
Routes external to the OSPF domain (e.g., BGP, static)
117
Who generates Type 5 LSAs?
Autonomous System Boundary Routers (ASBRs)
118
What is LSA Type 7 called?
NSSA External LSA
119
Where is a Type 7 LSA used?
In Not-So-Stubby Areas (NSSA)
120
What is the purpose of a Type 7 LSA?
To describe external routes in NSSA areas
121
Who generates Type 7 LSAs?
ASBRs located in NSSA areas
122
What are LSA Types 9, 10, and 11 called?
Opaque LSAs
123
What are Opaque LSAs used for?
Advanced features like MPLS Traffic Engineering and Graceful Restart
124
What does OSPF cost represent?
The cost or "metric" to reach a destination network
125
What is the default OSPF cost formula?
Cost = Reference Bandwidth / Interface Bandwidth
126
What is the default reference bandwidth in OSPF?
100 Mbps (100
127
What is the OSPF cost of a 100 Mbps interface by default?
1
128
What is the OSPF cost of a 10 Mbps interface by default?
10
129
What is the OSPF cost of a 1 Gbps interface by default?
1 (same as 100 Mbps
130
What command sets the reference bandwidth?
auto-cost reference-bandwidth
131
What command sets cost on an interface?
ip ospf cost
132
Why would you change the reference bandwidth?
To allow OSPF to distinguish between faster links like 1 Gbps or 10 Gbps
133
What happens if you don't change the reference bandwidth?
All interfaces ≥100 Mbps will have the same cost (1)
134
Where do you apply 'ip ospf cost'?
Directly under the interface configuration
135
What is the benefit of manually setting interface cost?
Gives you full control over path selection in OSPF
136
Can OSPF automatically calculate cost based on bandwidth?
Yes
137
What is the drawback of using default cost calculation on modern networks?
It doesn’t differentiate between fast links (e.g.
