Chap 8 - OSPF Flashcards
What is SPT
Shortest Path Tree
How does OSPF provide for scaling?
By using multiple areas within the routing domain.
What multicast addresses does OSPF use?
224.0.0.5 and 224.0.0.6
What multicast MAC address do all OSPF routers use?
01:00:5E:00:00:05
What multicast MAC address is used for all DR routers?
01:00:5E:00:00:06
What are the 5 types of packets used by OSPF?
- Hello
- DD
- Link State Request (LSR)
- Link State Update (LSU)
- Link State Ack
What are OSPF Hello packets used for?
- Discovering neighbors
- Periodic checks to make sure the neighbor is still alive
What are DD packets used for?
Summarizing database contents when adjacency is first formed
What are Link State Requests used for?
Sent by a router requesting LSAs from its neighbor
What are Link State Updates for?
Sent in response to an LSR containing database updates
What are Link State Acknowledgements for?
- Acknowledges the flooding of LSAs.
- Provides reliable transport
What are 9 fields in a Hello packet?
- Router ID
- Authentication options
- Area ID
- Interface mask
- Interface priority
- Hello Interval
- Dead Interval
- DR and BDR IP addresses
- Active neighbors
What are the 8 neighbor states?
- Down
- Attempt
- Init
- 2-way
- ExStart
- Exchange
- Loading
- Full
Describe the Down state.
When the router has not received any Hello packets
Describe the Attempt state.
On NBMA networks, no information has been received recently but its still trying.
Describe the Init state.
Router has received a hello packet from its neighbor, but the receiving router’s ID was not included in the hello packet.
What happens in the 2-way state.
Bidirectional communication has started. DR and BDR elected at the end of this state
What 2 things happen during the ExStart state.
- Master/Slave roles are determined. Highest router-id is Master
- Initial sequence number is decided.
What happens during the Exchange state.
DD’s are exchanged summarizing the contents of the LSDB
What 3 things happen during the Loading state.
- Routers send link-state request packets based on the DD summary it received
- Neighbor responds with Link State Update packets
- All LSUs are acknowledged with Link-state Acks
Describe the Full state.
LSDBs are in sync, routers are fully adjacent.
What are 2 methods of enabling OSPF on an interface?
- network statement
- interface config ‘ip ospf (process_id) area (area_number)
If 2 network statements are entered that both include an interface what happens?
The more specific network statement wins.
If the interface configuration is used will it match on secondary interfaces?
Yes, unless the ‘secondaries none’ parameter is added to the command.
If an OSPF configuration includes both a network statement and an interface command which takes precedence?
The interface command takes precedence.
If a router ID is not specified what is used for the router ID?
- Highest loopback IP address first
- Highest physical interface IP address second
Will the network on an interface still be advertised if the network is configured as passive?
Yes, network still advertised but interface won’t send out Hellos, won’t process received OSPF packets and won’t establish neighborships.
What are the 8 requirements for OSPF routers to become neighbors?
- Unique router IDs
- Authentication matches
- Area ID matches
- Area flags match
- Interfaces on a common subnet
- MTUs match
- Hello and Dead timers match
- DR enablement matches
When a default route is advertised what kind of route is it?
E2 External
What is the reference bandwidth in OSPF?
100 Mbps
What is the formula used to calculate the cost of an interface?
100 Mbps / interface bandwidth
What are 5 interface costs?
- T1 = 64
- 10 Mbps Ethernet = 10
- 100 Mbps Ethernet = 1
- 1 Gbps Ethernet =1
- 10 Gbps Ethernet = 1
If the reference bandwidth is changed what does it affect and where else should it be done?
- Affects all interfaces in that OSPF process
- Should be made the same on all OSPF routers
For LSA purposes what range does an interface cost need to be in?
1 - 65,535
What happens when you change the Hello timer?
The Dead timer changes to 4 times the Hello timer.
What should be taken into consideration when you choose which routers should be DR and BDR?
DR and BDR routers need to have adequate memory and CPU
How are DR and BDR routers elected?
Router with the highest interface priority is DR, second highest is BDR. If interface priorities tie then routers with highest and second highest router IDs win.
After the DR has been elected and the BDR election takes place what does the DR do?
The DR does not place its RID in the BDR field of the Hello packet.
What has to happen in order to elect an new DR or BDR?
The OSPF process on the DR or BDR has to have gone down or restarted.
What is the default priority of OSPF interfaces?
The default is 1.
How does a router know it needs to elect a DR / BDR?
If Hellos don’t contain a DR/BDR the router initiates a Wait timer (equal to the Dead timer). Once Wait timer expires router participates in the election.
What is the best way to modify a router so that it becomes the DR or BDR?
Change the interface priority to a value higher than the default (1).
Can the DR or BDR roles be preempted?
No. The OSPF process on the DR or BDR needs to be restarted.
How can you make sure a router doesn’t participate in the DR / BDR election?
Change its interface priority to 0.
What will happen if the priority of an interface on the DR is changed to 0?
Router will remove itself as the DR. The BDR will become the DR and a new BDR will be elected.
What are the 5 OSPF media types?
- Broadcast
- Non-broadcast
- Point-to-point
- Point-to-multipoint
- Loopback
Display the Hello and Dead timers.
show ip ospf interface
Determine if a router is the DR or BDR of a network.
show ip ospf interface brief (appears under State)
Identify the Network Type of an interface.
show ip ospf interfaces gig0/1
