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Cisco CCNA > OSPF > Flashcards

OSPF Flashcards

1
Q

OSPF Characteristics

A

Open standard link state protocol
Supports large networks
Very fast convergence time
Messages sent via multicast (224.0.0.5)
Uses Dijkstra’s Shortest Path First algorithm to determine the best path to learn networks

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2
Q

OSPF Operations

A

Discover neighbors
Form adjacencies
Flood Link State Database (LSDB)
Compute Shortest Path
Install best routes in routing table
Respond to network changes

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3
Q

OSPF Packet Types: Hello

A

A router will send out & listen for hello packets when OSPF is enabled on an interface, & form adjacencies with other OSPF routers on the link

Multicast to 224.0.0.5

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4
Q

OSPF Packet Types: DBD

A

DataBase Description:
Adjacent routers will tell each other the networks they know about with the DBD packet

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5
Q

OSPF Packet Types: LSR

A

Link State Request:
If a router is missing info about any of the networks in the received DBD, it will send the neighbor an LSR

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6
Q

OSPF Packet Types: LSA

A

Link State Advertisement:
A routing update

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7
Q

OSPF Packet Types: LSU

A

Link State Update:
Contains a list of LSA’s which should be updated, used during flooding

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8
Q

OSPF Packet Types: LSAck

A

Link State Acknowledgement:
Receiving routers acknowledge LSAs

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9
Q

Hello Packet Contents

A

Router ID: 32 bit unique ID for each OSPF router

Hello Interval: How often packets are sent (10 sec default)

Dead Interval: How long router waits to hear from a neighbor before declaring it out of service

Neighbors: List of adjacent OSPF routers that this router has received Hello packet(s) from

Area ID: The area configured for that interface

Router Priority: An 8 bit number used to select DR & BDR

DR & BDR IPv4 Address: If known

Authentication Flag: Authentication details if configured

Stub Area Flag: If the area is a stub area. Stub areas have a default route to their ABR rather than learning routes outside the area

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10
Q

OSPF Adjacency Matching Requirements

A

Must be in each other’s Neighbor list
Hello & Dead intervals
Area ID
IP Subnet
Authentication Flag
Stub Area Flag
MTU Size (Default 1500)

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11
Q

Command: Configure OSPF with Process ID of 1 for 10.0.0.0/24 and 10.1.0.0/24 networks in area 0

A

router ospf 1
network 10.0.0.0 0.0.0.255 area 0
network 10.1.0.0 0.0.0.255 area 0

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12
Q

OSPF Verification Commands

A

Show run | section ospf
Show ip protocols
Show ip ospf neighbor
Show ip route
Show ip ospf database
Show ip ospf int brief

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13
Q

OSPF Router ID

A

Used by OSPF routers to identify themselves
Takes form of an IP address
Defaults to being the highest IP of any loopback interfaces configured, or the highest other IP address if a loopback doesn’t exist
Can be manually specified

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14
Q

Command: Set OSPF Router ID to 2.2.2.2

A

router ospf 1
router-id 2.2.2.2

Must restart for changes to take place:
clear ip ospf process

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15
Q

Command: Set interface FastEthernet2/0 to Passive Interface

A

router ospf 1
passive-interface f2/0

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16
Q

Command: Set passive interface as default

Also, remove an interface from being passive

A

router ospf 1
passive-interface default

To remove this setting on an interface:
no passive-interface f0/0

17
Q

Command: Default OSPF route injection

A

router ospf 1
default-information originate

18
Q

OSPF Two Level Hierarchy

A

Transit Area (backbone/area 0): Does not generally contain end users

Regular Area (nonbackbone areas): Used to connect end users to the transit area

Small networks do not require hierarchical design & all routers can be area 0

19
Q

OSPF Router Types: Backbone Routers

A

Routers which have all their OSPF interfaces in Area 0

20
Q

OSPF Router Types: ABRs

A

Routers which have interfaces in multiple areas

Separates LSA flooding zones - Segregates the network

It becomes the primary point for area address summarization

Functions regularly as the source for default routes

Maintains the LSDB for each area which it’s connected to

Manual summarization (if not configured, all routes flooded everywhere)

21
Q

OSPF Router Types: Normal Area Routers

A

Routers which have all their OSPF interfaces in a normal area

Maintain full LSDB of other routers & links in own area

Learn IA routes to other areas from their ABRs

22
Q

OSPF Router Types: IA Routers

A

Inter-Area Routers:
Routers to other areas appear in routing table as IA routes

23
Q

OSPF Router Types: ASBR Routers

A

Routers which redistribute into OSPF

24
Q

OSPF Router Types: External Routes

A

Routes which are redistributed into OSPF appear as external routes

25
Q

Command: ABR Manual Summarization

A

router ospf 1
Network 10.1.0.0 0.0.255.255 area 0
Network 10.0.0.0 0.0.255.255 area 1
Area 0 range 10.1.0.0 255.255.0.0
Area 1 range 10.0.0.0 255.255.0.0

26
Q

OSPF: Speed

A

The rate that ethernet interfaces physically transmit at is set by the speed command

If you use the “speed 10” command on a FastEthernet interface, it will physically transmit at 10Mbps

27
Q

OSPF: Clock Rate

A

The rate that serial interfaces physically transmit it set by the “clock rate” command

Serial interfaces transmit at 1.544Mbps by default

If you use the “clock rate 64000” command on a serial interface, it will physically transmit at 64Kbps

28
Q

OSPF: Bandwidth

A

Bandwidth usually matches physical transmission rate of interface

The bandwidth setting on an interface does not affect the physical transmission rate (that is set by “speed” or “clock rate”

The bandwidth command affects software policy on the router, such as which path will be selected by EIGRP/OSPF, or how much bandwidth will be guaranteed to a traffic type by QoS

29
Q

OSPF Metric Calculation

A

For destinations in its own area, a router looks at all available links to get there, & chooses the path with the lowest overall cost

For destinations in another area, a router looks at all available links to get to the ABR & chooses the path with the lowest overall cost to the ABR.

It’s then up to the ABR to choose the best path onward from there

30
Q

SPF Algorithm

A

Calculates overall cost for each available path to each destination network
Then selects lowest cost path

Overall cost = cumulative cost of all outgoing interfaces
You should ensure the cost is set the same on the interfaces on both sides of a link or you can get asymmetric routing

31
Q

Reference Bandwidth

A

The cost is automatically derived from the interface bandwidth
Cost = Reference Bandwidth / Interface Bandwidth
The default reference bandwidth is 100Mbps

Best possible cost is 1

OSPF treats all interfaces of 100+Mbps as equal (can cause undesirable routing)

32
Q

Command: Change OSPF Reference Bandwidth

A

Router ospf 1
Auto-cost reference-bandwidth 100000

33
Q

Command: Manipulate OSPF Metric by changing the cost on FastEthernet0/0

A

interface f0/0
ip ospf cost 50

34
Q

Command: Verify OSPF cost on FastEthernet0/0

A

show ip ospf interface FastEthernet0/0

35
Q

OSPF Neighbor State Summary

A

Down: No active neighbor detected
INIT: Hello packet received from neighbor
2-Way: Own router ID in received hello
Exstart: Primary & secondary roles determined
Exchange:Database description packets sent
Loading: Exchange of LSRs & LSUs
Full: Neighbors fully adjacent

36
Q

Command: Change Hello Timer

A

ip ospf hello-interval 5

37
Q

DR & BDR

A

Designated Router & Backup Designated Router:

The router with the highest priority becomes DR
2nd highest priority becomes BDR
Default priority is 1; higher the better (0-255)
Highest Router ID is used in case of a tie
Selected at 2-Way Stage
No election on point-to-point links
Establish FULL neighbor state with all routers on network segment

When a link state changes on a router in a multiaccess segment
Sends LSU packet to 224.0.0.6 (all designated routers)
DR multicasts update to 224.0.0.5 (all OSPF routers)

38
Q

Command: Set OSPF priority to 100 on FastEthernet0/0

A

interface f0/0
ip ospf priority 100

39
Q

Command: Make sure a router is not a DR/BDR on FastEthernet0/0

A

interface f0/0
ip ospf priority 0

Cisco CCNA (38 decks)

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