Networked Systems Design Flashcards
What is OSPF?
Open Shortest Path First is a link-state routing protocol developed as an alternative to RIP.
What are the advantages of OSPF over RIP?
Faster convergence and scalability to larger network implementations.
What is the purpose of areas in OSPF?
Areas help control routing update traffic within a routing domain.
What information does a link-state include?
Network prefix, prefix length, and cost.
What are the five types of OSPF packets?
- Hello packet
- Database description packet
- Link-state request packet
- Link-state update packet
- Link-state acknowledgment packet
What is the purpose of the Hello packet in OSPF?
Discovers neighbors and establishes neighbor adjacencies.
What does the Link-State Database (LSDB) represent?
Information about all other routers in the network.
What command is used to view the Adjacency Database in OSPF?
show ip ospf neighbor
What is the SPF algorithm used for in OSPF?
To calculate the shortest path to each node in the network.
What are the main steps in the link-state routing process?
- Establish Neighbor Adjacencies
- Exchange Link-State Advertisements
- Build the Link State Database
- Execute the SPF Algorithm
- Choose the Best Route
What is the best practice for single-area OSPF?
Use area 0.
What are Area Border Routers (ABRs)?
Routers interconnecting multiple OSPF areas.
What is OSPFv3?
The OSPF equivalent for exchanging IPv6 prefixes.
True or False: OSPFv3 supports both IPv4 and IPv6.
True.
What is the function of the Database Description (DBD) packet?
Checks for database synchronization between routers.
Fill in the blank: OSPF uses the _______ algorithm to determine the best paths.
SPF
What are the seven states of OSPF operation?
- Down state
- Init state
- Two-way state
- ExStart state
- Exchange state
- Loading state
- Full state
What is the routing table generated from in OSPF?
The link-state database after running the SPF algorithm.
What does the Forwarding Database contain?
List of routes generated from the link-state database.
What is the Down State in OSPF?
No Hello packets received = Down. Router sends Hello packets. Transition to Init state.
What happens in the Init State of OSPF?
Hello packets are received from the neighbor containing the Router ID of the sending router. Transition to Two-Way state.
What is the Two-Way State in OSPF?
Communication between the two routers is bidirectional. On multiaccess links, routers elect a DR and a BDR. Transition to ExStart state.
What occurs during the ExStart State?
On point-to-point networks, the two routers decide which router will initiate the DBD packet exchange and the initial DBD packet sequence number.
What is the purpose of the Exchange State in OSPF?
Routers exchange DBD packets. If additional router information is required, transition to Loading; otherwise, transition to Full state.
What is the Loading State in OSPF?
LSRs and LSUs are used to gain additional route information. Routes are processed using the SPF algorithm. Transition to Full state.
What defines the Full State in OSPF?
The link-state database of the router is fully synchronized.
How do routers establish neighbor adjacencies in OSPF?
The router sends a Hello packet containing its router ID out all OSPF-enabled interfaces to the address 224.0.0.5.
What is the OSPF router ID?
A 32-bit number formatted like an IPv4 address, assigned to uniquely identify a router among OSPF peers.
What happens when a neighboring OSPF-enabled router receives a Hello packet?
If the router ID is not in its neighbor list, it attempts to establish an adjacency with the initiating router.
What are the steps for establishing adjacency on a multiaccess network?
- Down to Init State 2. Init State 3. Two-Way State 4. Elect the DR & BDR
What occurs during the DR and BDR election?
The router with the highest router ID or priority is elected as the DR; the second highest is the BDR.
What challenges does OSPF face on multiaccess networks?
- Creation of multiple adjacencies 2. Extensive flooding of LSAs
How does OSPF use areas?
OSPF uses areas for scalability, allowing a large routing domain to be divided into smaller areas.
What databases does OSPF create and maintain?
- Adjacency database * Link-state database (LSDB) * Forwarding database
What algorithm does OSPF use to build the topology table?
The Dijkstra SPF (shortest-path first) algorithm.
What does cost represent in OSPF?
Cost is used to determine the best path to the destination.
What is the single-area OSPF best practice?
Best practice is to use area 0.
What is a DROTHER in OSPF?
A router that is neither the DR nor the BDR.
What are some important fields in the Hello packet?
Type, router ID, area ID, network mask, hello interval, router priority, dead interval, DR, BDR, list of neighbors.
What multicast address is used to send Hello packets?
224.0.0.5.
Why is DR and BDR election necessary in multiaccess networks?
To prevent chaotic network traffic from flooding and acknowledging all LSAs by every router.
What is the role of the designated router (DR) in OSPF?
To be the collection and distribution point for LSAs sent and received.
What are the three criteria for deriving a router ID in Cisco routers?
- Explicitly configured using the OSPF router-id command
- Highest IPv4 address of configured loopback interfaces
- Highest active IPv4 address of physical interfaces.
What is the recommended method to assign a router ID?
Explicitly configure it using the OSPF router-id command.
What command is used to verify the router ID?
show ip protocols.
How can a router ID be changed after it has been selected?
By reloading the router or resetting the OSPF process.
What command is preferred to reset the router ID?
clear ip ospf process.
What is the significance of the router ID during database synchronization?
The router with the highest router ID sends their database descriptor packets first.
Fill in the blank: OSPF is a _______ routing protocol.
link-state.
What command is used to reset all OSPF processes?
clear ip ospf process
What is the basic syntax for the OSPF network command?
network network-address wildcard-mask area area-id
What does the wildcard mask refer to in OSPF configuration?
The inverse of the subnet mask configured on the interface
What is the effect of sending unnecessary OSPF messages on a LAN?
Inefficient use of bandwidth, resources, and increased security risk
What command is used to prevent the transmission of routing messages through a router interface while still advertising the network?
passive-interface
What command is used to configure OSPF directly on the interface?
ip ospf process-id area area-id
What is the advantage of specifying the exact interface IPv4 address in OSPF configuration?
No wildcard mask calculation is necessary
What is the default subnet mask for loopback interfaces in OSPF?
/32
What does R1 advertise when configured with a loopback interface as a point-to-point network?
The full network
What command must be removed when switching from using network commands to using the ip ospf command?
no network command
What interface command is used to configure OSPF on GigabitEthernet 0/0/0?
ip ospf 10 area 0
What happens to the state of an OSPF neighbor when the interface goes down?
Changes from FULL to DOWN
What type of network does OSPF use for advertising the full network?
Point-to-point network
What command can be used to verify OSPF router roles?
show ip ospf interface
What is the function of the Designated Router (DR) in a multiaccess OSPF network?
Collecting and distributing LSAs
What multicast IPv4 address does the DR use?
224.0.0.5
What is the role of the Backup Designated Router (BDR)?
Maintains a relationship with all routers and takes over if the DR fails
What is the state of a router that is neither the DR nor BDR?
DROTHER
What multicast address do DROTHER routers use to send OSPF packets?
224.0.0.6
In a multiaccess OSPF topology, how are DR and BDR elected?
Based on the highest router ID
What is the router ID of the elected DR in the given multiaccess OSPF network?
3.3.3.3
What is the router ID of the BDR in the given multiaccess OSPF network?
2.2.2.2
What command is used to verify OSPF neighbor adjacencies?
show ip ospf neighbor
What OSPF neighbor state indicates full adjacency with a non-DR or BDR router?
FULL/DROTHER
What is the normal state for an OSPF router?
FULL
What is the default priority of OSPF routers during DR/BDR election?
1
Fill in the blank: The router with the highest interface priority is elected as the _______.
[Designated Router (DR)]
Fill in the blank: The router with the second highest interface priority becomes the _______.
[Backup Designated Router (BDR)]
What happens if the DR stops producing Hello packets?
The BDR promotes itself to DR
What two states can a router have with the DR?
FULL/DR and FULL/BDR
What indicates there are problems in forming OSPF adjacencies?
A router stuck in a state other than FULL
What is the significance of the 2-WAY state in a multiaccess broadcast network?
It is a normal state for non-DR or BDR routers
What does the command ‘show ip ospf interface’ reveal about a router’s role?
It confirms if the router is DR, BDR, or DROTHER
What is the first criterion for the OSPF DR/BDR election process?
The router with the highest interface priority is elected as the DR.
What happens if the interface priority value is set to 0?
That interface cannot be elected as DR nor BDR.
What is the default priority of multiaccess broadcast interfaces in OSPF?
1.
If the interface priorities are equal, how is the DR elected?
The router with the highest router ID is elected as the DR.
What triggers the OSPF election process?
The first router with an OSPF-enabled interface becoming active on the network.
True or False: The addition of a new router initiates a new election process.
False.
What occurs if the DR fails?
The BDR is automatically promoted to DR.
What command is used to set the priority of an OSPF interface?
ip ospf priority value.
What does a priority value of 0 indicate?
The interface does not become a DR or BDR.
What is the formula used to calculate the OSPF cost?
Cost = reference bandwidth / interface bandwidth.
What is the default reference bandwidth for OSPF?
100,000,000 bps.
How can you manually set the OSPF cost value on an interface?
Use the command ip ospf cost value.
What happens when the reference bandwidth is adjusted?
It affects the calculation used to determine the metric, not the actual bandwidth capacity.
What command must be configured on every router to adjust the reference bandwidth?
auto-cost reference-bandwidth Mbps.
What is the accumulated cost of an OSPF route?
The accumulated value from one router to the destination network.
How do you verify the current OSPF cost assigned to an interface?
Use the show ip ospf interface command.
What is the total cost for R1 to reach the 10.10.2.0/24 network if the link to R2 costs 10 and the loopback default cost is 1?
11.
What command is used to verify the accumulated cost for a specific network?
show ip route.
Fill in the blank: The _______ command allows you to influence path selection within OSPF.
ip ospf cost.
True or False: Changing the reference bandwidth affects the actual bandwidth on the link.
False.
How can the cost value reported by the local OSPF router be changed?
Use the interface configuration command ip ospf cost value
Example: R1(config-if)# ip ospf cost 30
What command is used to simulate a link failure between R1 and R2?
Shut down the Gigabit Ethernet 0/0/0 interface
This verifies that the routing table updates to use R3 as the next-hop router.
What is the default interval for OSPFv2 Hello packets?
10 seconds
This is the default timer value on multiaccess and point-to-point networks.
What does the Dead interval signify in OSPF?
The period the router waits to receive a Hello packet before declaring the neighbor down
Default is 40 seconds on multiaccess and point-to-point networks.
What command is used to verify OSPFv2 interface intervals?
show ip ospf interface
This command shows configured Hello and Dead intervals.
What happens when the Dead Timer expires?
R1 and R2 lose adjacency
This requires both routers to be configured with the same Hello interval.
What command is used to propagate a default static route in OSPFv2?
default-information originate
This instructs the router to propagate the default route information.
How can the Hello and Dead intervals be modified?
Using the commands ip ospf hello-interval seconds and ip ospf dead-interval seconds
Use ‘no’ commands to reset to defaults.
What indicates that a route was learned using OSPFv2?
O*E2
The asterisk signifies a good candidate for the default route, and E2 indicates it’s an external route.
What command verifies the received default route on R1?
show ip route
This shows the routing table including the default route learned via OSPF.
True or False: Hello packets are sent on interfaces set to passive by the passive-interface command.
False
Hello packets are not sent on passive interfaces.
Fill in the blank: OSPF Hello packets are transmitted to multicast address _______.
224.0.0.5
What is the command to view OSPF neighbor adjacencies?
show ip ospf neighbor
This command displays OSPF neighbor states and Dead Time.
What is the impact of increasing OSPF timers for quicker network failure detection?
Increases traffic
Quick convergence may be prioritized over extra traffic.
What command is used to check the routing table for the default route on R2?
show ip route | begin Gateway
This command filters the output to show the default route.
How does the Cisco IOS automatically adjust the Dead interval?
To four times the Hello interval
If the Hello interval is set to 5 seconds, the Dead interval becomes 20 seconds.
What command verifies that the desired interfaces are active with correct IP addressing?
show ip interface brief
What command verifies that the routing table contains all the expected routes?
show ip route
Which command is used to verify that the router has formed an adjacency with its neighboring routers?
show ip ospf neighbor
What indicates that two routers have not formed an OSPFv2 adjacency?
Router ID not displayed or not in FULL state
True or False: A non-DR or BDR router with another non-DR or BDR router will display a full adjacency.
False
What are the conditions that may prevent two routers from forming an OSPFv2 adjacency? (List at least two)
- Subnet masks do not match
- OSPFv2 Hello or Dead Timers do not match
- OSPFv2 Network Types do not match
- Missing or incorrect OSPFv2 network command
Which command provides a quick way to verify vital OSPF configuration information?
show ip protocols
What is the default administrative distance for OSPF?
110
What command examines the OSPFv2 process ID and router ID?
show ip ospf
What command provides a detailed list for every OSPFv2-enabled interface?
show ip ospf interface
Fill in the blank: The basic syntax for the network command in OSPF is network ______ area area-id.
network-address wildcard-mask
What is the purpose of the passive-interface command?
To stop transmitting routing messages through a router interface but still allow that network to be advertised
What multicast IPv4 address is used by the DR to distribute LSAs?
224.0.0.5
What happens if the DR stops producing Hello packets?
The BDR promotes itself and assumes the role of DR
What address do DROTHERs use to send OSPF packets to the DR and BDR?
224.0.0.6
True or False: Only the DR and BDR listen for the multicast address 224.0.0.6.
True
What command is used to verify OSPFv2 adjacencies?
show ip ospf neighbor
What is the highest priority criteria for deriving the router ID for an OSPF router?
Explicitly configured using the OSPF router-id command
List the order of criteria Cisco routers use to derive the router ID.
- Explicitly configured router ID
- Highest IPv4 address of configured loopback interfaces
- Highest active IPv4 address of physical interfaces
What command provides a summary of OSPFv2-enabled interfaces?
show ip ospf interface brief
What does the command ‘show ip ospf interface’ display?
Process ID, local router ID, network type, OSPF cost, DR and BDR information, adjacent neighbors
What is the significance of a loopback interface advertised as a /32 host route?
It simulates a real LAN
What command is used to verify OSPFv2 adjacencies?
show ip ospf neighbor
This command displays the OSPF neighbor relationships and their states.
What are the possible states of neighbors in multiaccess networks?
- FULL/DROTHER
- FULL/DR
- FULL/BDR
- 2-WAY/DROTHER
These states indicate different levels of adjacency in OSPF.
How is the DR (Designated Router) elected in OSPF?
Based on the highest interface priority
If priorities are equal, the router with the highest router ID is elected.
What happens if the interface priority is set to 0?
The interface cannot be elected as DR nor BDR
Setting priority to 0 effectively disables participation in DR/BDR elections.
What is the default priority of multiaccess broadcast interfaces in OSPF?
1
This is the initial setting for interface priority unless configured otherwise.
Are OSPF DR and BDR elections pre-emptive?
False
If the DR fails, the BDR is automatically promoted to DR without a new election.
What command is used to set the priority of an interface in OSPF?
ip ospf priority value
The value can range from 0 to 255.
What does a lower OSPF cost indicate?
A better path
OSPF uses cost as a metric for route selection.
What is the formula to calculate OSPF cost?
Cost = reference bandwidth / interface bandwidth
This formula helps determine the cost associated with a specific route.
What command can adjust the reference bandwidth in OSPF?
auto-cost reference-bandwidth
This command allows for the adjustment of OSPF cost calculations based on interface speed.
What does the command ip ospf cost value do?
Sets the OSPF cost value for an interface
This value can be manipulated to influence routing decisions.
What happens when the Dead interval expires before a Hello packet is received?
OSPF removes the neighbor from its LSDB
This is part of OSPF’s mechanism to maintain accurate routing information.
What is the default Dead interval for OSPF on multiaccess and point-to-point networks?
40 seconds
This is set to 4 times the Hello interval by default.
Which command can be used to verify OSPFv2 interface intervals?
show ip ospf interface
This command provides detailed information about OSPF interface configurations.
What is the role of an ASBR in OSPF?
Router between an OSPF routing domain and a non-OSPF network
ASBR stands for Autonomous System Boundary Router.
What command must be used to propagate a default route from an ASBR?
ip route 0.0.0.0 0.0.0.0 [next-hop-address | exit-intf]
This command sets up a default static route on the ASBR.
What is the command to verify the default route settings on the ASBR?
show ip route
This command displays the routing table, including default routes.
What command can be used to check if OSPF is operating as expected?
show ip ospf
This command provides information about OSPF operations and statistics.
What does the command ip ospf hello-interval value do?
Sets the OSPF Hello interval
This interval determines how often Hello packets are sent.
What does the command ip ospf dead-interval value do?
Sets the OSPF Dead interval
This interval determines how long to wait before removing a neighbor.
What command is used to clear the OSPF process?
clear ip ospf process
This command resets the OSPF process, useful for troubleshooting.
