OSPF Flashcards
Once adjacencies have been formed, OSPF will then start sending these out
DBD Packets (Database Description Packets)
A router will send out and listen for these when OSPF is enabled on an interface
OSPF Hello Packets
If a router is missing information about any of the networks it received in the DBD, it will send the neighbor one of these
LSR (Link State Request)
This is the step when adjacent routers will tell each other the networks they know about with this type of packet
DBD (Database Description Packets)
These packets contain routing updates
LSA (Link State Advertisement)
These packets contain a list of Link State Advertisements (LSAs) which should be updated and that will be used during flooding
LSU (Link State Update)
This is a routers response to a Link State Request (LSR) asking for more information
Link State Advertisement (LSA)
Scenario: If a new link was added to the network or if a link went down and new information needs to get flooded everywhere so all routers can know about it this type of packet would be processed
LSU (Link State Update)
Receiving routers acknowledge Link State Advertisements (LSA) through these types of packets
LSAck (Link State Acknowledgement)
OSPF sends Hello Packets using Broadcast
False. Hello Packets use Multicast
What are the 6 Steps / Characteristics of OSPF?
- Discover Neighbors
- Form Adjacencies
- Flood the Link State Database
- Compute the Shortest Path
- Install the Best Routes Into The Routing Table
- Respond to Network Changes
What will happen if a router running OSPF has 2 interfaces with different Process IDs?
It will not share information about it’s routes with either side of the link due to separate Link State Databases
Does the network command in OSPF default to the classful boundary?
No. That’s one of the differences between OSPF and EIGRP
Do you have to enter a wildcard mask in OSPF?
Yes
What is a simple OSPF configuration?
router ospf 1
network 10.0.0.0 0.0.255.255 area 0
If a network contained routers with interfaces such as:
10.1.0.1/24
10.0.1.1/24
10.0.2.1/24
And you configured a network command for OSPF as:
network 10.0.0.0 0.0.255.255 area 0
What networks would get advertised in the routing table?
10.0.1.0/24
10.0.2.0/24
10.1.0.0 would not be advertised because it doesn’t match the 10.0.X.X space that the network command is looking for.
Use this command to see if neighbor routers have peered and formed an adjacency
show ip ospf neighbor
What are the commands to enter a Router ID for OSPF?
router ospf 1
router-id {router id}
If you need to restart OSPF what command can you use?
clear ip ospf process
What are the commands for a passive loopback interface in OSPF?
router ospf 1
passive-interface loopback0
How would you set all interfaces to passive on a router using OSPF?
router ospf 1
passive-interface default
What are the commands to enter a default route injection into OSPF?
ip route 0.0.0.0 0.0.0.0 {next-hop-address}
router ospf 1
default-information originate
This is the top level or Area 0 when it comes to OSPF routers
The Transit Area or Backbone
For a router to form a full adjacency with a neighbor, it must be configured in the same what?
Area
Routers maintain only summary information of other routers and links in their own area. True or False?
False. They maintain a full LSDB of other routers and links in their Area
Routes received from other routers in the same area appear as what kind of routes?
Intra-Area Routes
Inter-Area routes show up as what in the routing table?
O-IA
O*E2 means what in the OSPF routing table?
It’s an external route that has been redistributed in OSPF
If you have loopbacks as Router ID’s then you need to do what to integrate them into OSPF when using a multi-area network?
Make sure you target them to be in the same area in your OSPF network statements
Will the cost of an OSPF link be based on the area it’s in rather than the topology as a whole?
Yes
If an OSPF Router has interfaces in multiple areas it’s considered a what?
Area Border Router (ABR)
Routers with interfaces in multiple areas will have full LSDB information for networks in both areas. True or False
True
Scenario: Area 0 is the backbone and then there are 2 ABR routers connected in Area 1 and Area 2. Do Area 1 and 2 have full LSDB information?
No, only summary information
What is the chief characteristic of an ABR?
It separates the LSA Flooding Zones
If you don’t do summarization on ABR’s what will occur?
The same amount of targeted routes will show up in the routing table thus making using an OSPF Multi-Area network useless. You need summary routes to make sure that it’s targeting a broader range so the routers don’t respond to specific changes on one link or path
If you’re configuring an OSPF multi-area setup, what is/are the commands that would come after the network commands?
The targeted area commands that specify the summary range
area 1 range 10.0.0.0 255.255.0.0
Routers which have all of their OSPF interfaces in an area and are not in a backbone are considered what?
Normal Area Routers
How do Normal Area Routers learn their routes
From the ABRs
What kind of routes do Normal Area Routers learn from ABRs?
Inter-Area Routes
O-IA
This type of router redistributes routes from different routing protocols into OSPF Normal Area Routers
Autonomous System Boundary Router (ASBR)
The rate at which Ethernet interfaces physically transmit data is determined by what command?
Speed
If you set an interface to a speed 10 command, how fast will the interface transmit data?
10Mbps
True or False - both side of the link must be configured to the same speed otherwise you will encounter transmission issues.
True
What is clock rate?
The rate at which serial interfaces physically transmit data
What speed do serial interfaces physically transmit data by default?
1.544Mbps
Do serial clock rate commands have to be set to match on both sides of the link?
Yes
What is the difference between Speed and Bandwidth?
Bandwidth is not tied to the physical speed at which data will be transferred. Speed is.
If you set the bandwidth to 50Mbps on a Gigabit interface, how fast will the data be transmitted and why?
1000Mpbs. This is because the Bandwidth command doesn’t actually affect the physically transmissible speed of the link
Why use the bandwidth policy if it doesn’t affect the physical speed transmission of the link?
Because it influences software policy. For example if you had a router with two interfaces going to separate paths and you wanted to influence which path it takes and it not do ECMP then you could set the bandwidth of that path to something lower than the other path. Thus, the other route would be preferred.
How does OSPF determine overall cost of a particular path?
It’s based on the cumulative cost of all outgoing interfaces
What is the equation to calculate the cost of a link?
Reference bandwidth divided by interface bandwidth
What is the default reference bandwidth?
100Mbps
What cost do all interfaces (FastEthernet, GigabitEthernet, etc.) default to?
1
What would the cost be for a GigabitEthernet interface taking into the equation 100/1000?
It would still be 1 because OSPF Cost defaults to 1. It can’t go below 1.
How would you change the reference bandwidth in OSPF?
auto-cost reference-bandwidth {speed}
Why would you set a reference bandwidth cost to 100000?
The goal here is to get the reference bandwidth high enough to where you don’t have to change it if you bandwidth links get changed and upgraded over time
When you change the reference bandwidth on one router what do you need to make sure you do?
Change it all every other router to ensure they are all using a consistent metric
What is more desirable - to manipulate the cost or the bandwidth on interfaces?
It’s more desirable to manipulate the cost because the bandwidth setting might affect other features other than OSPF such as QoS, etc.
How would you manipulate the cost of an OSPF interface?
interface {interface}
ip ospf cost {cost metric}
How would you view the cost of an OSPF interface?
show ip ospf interface {interface}
What are the general steps for Neighbor State Summary?
- Down - No active neighbor detected
- Initilialization - Hello packets sent and received by neighbors
- 2 WAY - Bi-Directional communication. Can see it’s own router ID received in Hello packets
- Exstart - Primary and secondary roles determined
- Exchange - DBD packets sent
- Loading - Exchange of LSRs and LSU (With LSAs within the LSU)
- Full - Neighbors full adjacent
What is the OSPF Multicast Address?
224.0.0.5
What is the default Hello interval?
10 Seconds
What is the default Dead interval?
40 Seconds
What settings must be matched for a pair of OSPF routers to form an adjacency?
- They must be in each other’s neighbor list
- Hello & Dead Intervals
- Area ID
- IP Subnet
- Authentication Flag
- Stub Area Flag
- MTU and MTU IP Size
What does MTU affect?
All Packets
What does IP MTU affect?
Only affects IP Packets
What is the command for viewing an IP MTU
show ip interface {interface}
What is the protocol number in a Layer 3 Packet Header for an OSPF Packet?
89
What does an OSPF Packet look like?
- Version of OSPF
- Packet Type
- Packet Length
- Router ID
- Area ID
- Checksum
- Auth Type
- Authentication
- Data
On point to point links, OSPF routers form a Two Way Only Adjacency?
False. They form a full adjacency
All Routers share their information with this type of router and then this router in turn shares information to ALL routers
DR Designated Router
If the DR goes down what takes it place?
The BDR Backup Designated Router
If a router isn’t considered a DR or BDR then what is it considered?
A DROTHER
How does a DR get elected?
It has the highest priority. In which case either the Router ID is the highest or it’s been manually to set to have a higher OSPF priority on the interface.
Highest Loopback Address
or
Highest Router IP Address
or
Manually Configured in OSPF
In what OSPF stage do routers get elected?
The Two Way Stage
What the is the default priority on OSPF routers?
1
What commands would you use to set a manual priority for a router to force it to be the DR?
interface {int}
ip ospf priority {priority-value}
What states are two DROTHERS considered be in?
A Two Way State
What is the multicast address that the DR and BDR are listening in on for updates?
224.0.0.6
After a DR receives multicasted updates on 224.0.0.6 what multicast address will it send those updates out of to the DROTHER routers?
224.0.0.5
What command would you use to verify a DR and a BDR?
show ip ospf int {interface}
OR
show ip ospf neighbor
