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Open Shortest Path First (OSPF) > OSPF Basics > Flashcards

OSPF Basics Flashcards

1
Q

What does OSPF packet header include? (9)

A
  • Version
  • Type
  • Packet Length
  • Router ID
  • Area ID
  • Checksum
  • Authentication Type
  • Authentication Data
  • Data
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2
Q

Does OSPF send periodic routing updates?

A

no

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3
Q

What are the 4 basic characteristics of OSPF?

A
  1. open standard
  2. classless
  3. interior
  4. link-state
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4
Q

What algorithm does OSPF use for topology calculation?

A

Dijkstra SPF algorithm

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5
Q

What is the role of the Dijkstra algorithm?

A

build the overall map of the network topology represented by state of the links between the routers

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6
Q

Do routers know about every single interface packet needs to go through to reach the destination?

A

yes

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7
Q

What is the IP protocol number of OSPF?

A

89

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8
Q

Does OSPF use TCP or UDP for transport?

A

neither

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9
Q

What is the OSPF version for IPv4 and IPv6?

A

IPv4: OSPFv2
IPv6: OSPFv3

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10
Q

How many tables are constructed by OSPF?

A

3

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11
Q

What are the three tables constructed by OSPF?

A
  1. Neighbor Database
  2. Link State Database
  3. Routing Table
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12
Q

What is the purpose of the Neighbor Table (Neighbor Database)?

A

keep track of its neighboring routers with which it has established OSPF adjacencies

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13
Q

What does the Neighbor Table (Neighbor Database) contain? (11)

A
  1. Neighbor Router ID
  2. Neighbor IP Address
  3. Neighbor Priority
  4. Neighbor State
  5. Neighbor Interface
  6. Neighbor Area
  7. Neighbor DR and BDR
  8. Neighbor Router Dead Timer
  9. Neighbor Authentication Type
  10. Next Hop Address (the neighbor’s IP on the link)
  11. Exit interface (what interface to use to send the packet to the neighbor)
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14
Q

What is Router ID?

A
  • IP address-like ID of the OSPF device used to identify a device
  • each OSPF device requires a Router Identifier (Router ID)
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15
Q

What happens if the same Router ID shows up in a multiple places of a tree?

A

the overall SPF calculations will be broken and routing loops may occur

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16
Q

How is the Router ID represented?

A

32bit dotted-decimal value that looks like an IPv4 IP address (also applicable to OSPFv3)

17
Q

Which Router ID value is reserved and cannot be used?

A

0.0.0.0

18
Q

Is it true that multiple OSPF process on same node need unique RID values?

A

yes, also includes VRFs

19
Q

What is the Router ID (RID) selection order of precedence?

A
  1. Manually configured under the global OSPF configuration section
  2. Highest numeric IPv4 on a loopback interface (the interface cannot be shutdown)
  3. Highest numeric IPv4 address on any non-loopback interface (can be in down/down state but cannot be shutdown)
20
Q

How is a routing table calculated in OSPF?

A

on each router, LSDb is provided as an input to SPF algorithm, which then generates a routing table

21
Q

What is a passive interface?

A

router prevents sending Hello messages through interfaces configured as passive

22
Q

What type of updates does OSPF use?

A

Event Driven Incremental Updates

23
Q

How do Event Driven Incremental Updates work?

A
  • no need to wait for a timer to expire so update can be triggered
  • uses multicast and unicast addresses
  • non-OSPF devices do not need to process updates
24
Q

What is the time period for which advertised OSPF routes are valid?

A

60 minutes - the router that advertised the network periodically refreshes the route each 30 minutes

25
Q

What does OSPF use as a metric?

A

Cost

26
Q

What is cost based on?

A

interface speed

27
Q

What is the formula for the cost calculation?

A

Reference Bandwidth/[interface speed in bits]

28
Q

What is the value of the Reference Bandwidth?

A

100 000 000 bits (100 Mbps)

29
Q

What is the problem with the OSPF Reference Bandwidth and the formula?

A

default OSPF cost formula doesn’t differentiate between interfaces with bandwidth faster than 100 Mbps - means that if there are multiple connections from a router that are 100 Mb and more, all of these will have the same metric of one, as OSPF cost calculation cannot go below 1

30
Q

Considering 1 Gbps and 10 Gbps links are common, what should be done with the default Reference Bandwidth?

A

the value should be changed (needs to be adjusted on other OSPF routers as well)

31
Q

OSPF was designed to replace which protocol?

A

RIP

32
Q

Why was OSPF invented? (5)

A
  1. faster convergence time
  2. larger network diameter
  3. consideration of bandwidth and delay
  4. ECMP support
  5. better scaling
33
Q

What does it mean that OSPF is a link-state protocol?

A

each OSPF router builds an identical map of the network by sharing link-state advertisements (LSAs)

34
Q

What does an LSA contain in high-level? (3)

A
  1. operational interfaces
  2. associated cost to the interfaces
  3. connected IP network
35
Q

What does Autonomous System refer to in the context of OSPF?

A

set of OSPF routers connected to the same area

Open Shortest Path First (OSPF) (9 decks)

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