8 - Implementing OSPF IPV4 Flashcards
What does the ‘ospf network ‘ do?
- finds the routers interfaces that match the network/wildcard mask
- enables OSPF on those interfaces
- DISCOVERS neighbors
- CREATES RELATIONSHIPS with neighbors
- ASSIGNS INTERFACES to AREAS
What does wildcard 0.0.0.0 do?
all 4 octets must exactly match
What does wildcard 255.255.255.255 do?
all addresses will match
What are the 3 steps an OSPF router does to add routes to its routing table?
- create neighbor relationships
- build and flood LSAs so that each router in that area has identical LSDB
- computes its IP routes INDEPENDENTLY using SPF and adds them to routing table
What show command verifies neighbors?
show ip ospf neighbors
What show command verifies the LSDB?
show ip ospf database
What show command verifies the routes?
show ip route
What is the difference between a Local route and a Connected route?
Local is the local interface with /32 mask, Connected is the subnet the interface connects to.
What are two ways to stop and start the OSPF process?
- ) Reboot the router
2. ) Use the command ‘clear ip ospf process’
What show command confirms a router’s OSPF ID?
show ip ospf
If an interface is configured to be passive what happens?
- Quits sending OSPF Hellos on the interface.
- Ignores received Hellos on the interface.
- Does not form neighbor relationships over the interface.
Does a subnet whose interface is passive still get advertised?
Yes
What are two ways to configure interfaces as passive?
1) in router-config mode use ‘passive-interface fa0/1’ or 2) in router-config mode use ‘passive-interface default’ and then for each interface you don’t want to be passive use ‘no passive-interface fa0/0’
What show command will reveal a passive-interface?
show ip ospf interface fa0/0
What kind of routers have interfaces in more than one area?
ABRs, Area Boundary Routers
What show commands reveal what area an interface is in?
- show ip ospf interfaces fa0/0
* show ip ospf interfaces brief
Which show commands identifies a router as a DR or BDR
- show ip ospf interface brief
- show ip ospf interface
- show ip ospf neighbor
How are the DR and BDR elected?
The highest Router ID wins and is DR, the second highest Router ID is BDR
How do Intra-area Routes appear in the ‘show ip route’ cmd output?
They show up as an O route (OSPF) and WITHOUT the IA code
What is the formula OSPF uses to calculate the cost of an interface?
Convert to kilobits/sec and use Reference Bandwidth / interface_bandwidth
How does an engineer change the cost of a particular interface?
Method 1: go on the interface and configure the cost using ‘ip ospf cost ‘.
Method 2: Change the bandwidth of the interface using ‘bandwidth or
Method 3: Go into OSPF router-config and change the reference bandwidth using the command ‘auto-cost reference-bandwith 10000’ changes reference bandwidth to 10Gbps.
What is the command to enable OSPF directly on the interface?
interface fa0/0
ip ospf 1 area 23
If OSPF is enabled using a network command in router-config what does the output of the ‘show ip ospf interface fa0/0’ show?
Attached via Network Statement
If OSPF is enabled using a ‘ip ospf 1 area 23 command in Interface config mode what does the output of the ‘show ip ospf interface fa0/0’ show?
Attached via Interface Enable
What 4 items make up the OSPF Configuration Checklist
- router ospf
- router-id
- network 10.2.2.0 0.0.0.255 area 0
- passive-interface gig0/1
What 3 ‘show’ cmds verify 1) neighbor relationships 2) builds and floods LSAs 3) add routes
- show ip ospf neighbor
- show ip ospf database
- show ip route
What are some default interface costs?
- 1.54 Mbps = 64
- 10 Mbps = 10
- 100 Mbps = 1
What is the command to manually configure the interface cost?
Interface subcommand: ip ospf cost
How do you change the Reference Bandwidth?
In router subcommand mode:
auto-cost reference-bandwidth
example: auto-cost reference-bandwidth 10000 will accomodate links up to 10Gbps.
How does the reference-bandwidth formula work?
reference_bandwidth / actual bandwidth = cost
