Cisco – OSPF

What command is used to modify the router ID to a loopback address that is using OSPF?
interface loopback number
What command is used to restart the OSPF process?
clear ip ospf process
What are the 3 different types of networks that are defined by OSPF?
– Point-to-point – A network that joins a single pair of routers – Broadcast – A multiaccess broadcast network, such as Ethernet – Nonbroadcast multiaccess (NBMA) – A network that interconnects more than 2 routers but that has no broadcast capabilities. 
What are the default OSPF hello and dead intervals on point-to-point links?
Ten seconds for the hello interval, and 40 seconds for the dead interval.
What type of adjancencies are established when a router is added to an already existing OSPF broadcast network?
Any router added to an already existing OSPF network establishes adjacencies wth the DR and BDR only.
How are designated routers and backup designated routers determined?
The router with the highest priority value is the DR.

The router with the second highest priority value is the BDR

What is the default priority for an OSPF interface?
The default priority for an OSPF interface is 1. In case of a tie, the router with the highest router ID becomes the DR. The second highest router ID becomes the BDR.

What does a priority of 0 mean for a router that is running OSPF?
A router with a priority of 0 cannot become the DR or BDR. Setting an interface’s OSPF priority to 0 takes effect immediately, causing a new election process to take place if that router was the DR or BDR
How does the BDR determine whether if the DR is out of service or not?
The BDR uses the wait timer to determine the status of the DR. If the BDR does not confirm that the DR is forwarding LSAs before the wait timer expires, the BDR assumes that the DR is out of service.
What interface configuration command can be used to configure the OSPF priority?
ip ospf priority; number
What are the default OSPF hello and dead interval on NBMA interfaces?
The default OSPF hello and dead intervals on NBMA interface are 30 seconds and 120 seconds, respectively.
What are the different modes for OSPF operation in an NBMA network?
– Point-to-multipoint nonbroadcast- Broadcast- Point-to-point;
What command selects the OSPF mode on a particular interface?
ip ospf network {broadcast | non-broadcast | point-to-multipoint [non-broadcast] | point-to-point}
What is the default OSPF mode on a point-to-point Frame Relay subinterface?
The default OSPF mode on a point-to-point Frame Relay subinterface is the point-to-point mode.
What is the default OSPF mode on a Frame Relay multipoint subinterface?
The default OSPF mode on a Frame Relay multipoint subinterface is the nonbroadcast mode.
What is the default OSPF mode on a main Frame Relay interface?
The default OSPF mode on a main Frame Relay interface is nonbroadcast mode.
What command is used to statically define adjacent relationships in NBMA networks using the nonbroadcast mode?
neighbor ip-address [priority; number] [poll-interval number] [cost number]
In nonbroadcast mode, where are neighbor statement required?
In nonbroadcast mode, neighbor statements are required only on the DR and BDR.

In a hub-and-spoke topology, where should neighbor statements be placed?
In a hub-and-spoke topology, the neighbor statements must be placed onthe hub, which must be configured to become the DR by being assigned a higher priority.
What command is used to define subinterfaces on a router?
interface serial number.subint-number {multipoint | point-to-point}
Which OSPF mode(s) require the manual configuration of neighbors?
Non-broadcast and point-to-multipoint non-broadcast
What command is used to track OSPF adjacencies?
debug ip ospf adj
What are the 4 different router types supported by OSPF?
– Internal router – Routers that have all of their interfaces in the same area.;- Backbone router – Routers that sit in the perimeter of the backbone area and have at least 1 interface connected to area 0;- ABR – Routers that have interfaces attached to multiple areas, maintain separate LSDBs for each area to which they connect, androute traffic destined for or arriving from other areas.;- ASBR – Routers that have a least 1 interface attached to an external internetwork.;
What is the maximum number of routers Cisco recommends being in a single area?
An area should have no more than 50 routers
What is the maximum number of neighbors that a single router should have?
Each router should have no more than 60 OSPF neighbors.

What is the maximum number of areas that a single router should be in?
A router should not be in more than 3 areas.
What are the different LSA types supported by OSPF?
– Type 1 – Router LSA- Type 2 – Network LSA- Type 3 and 4 – Summary LSAs- Type 5 – AS external LSA- Type 6 – Multicast OSPF LSA- Type 7 – Defined for not-so-stubby-areas (NSSA)- Type 8 – External attibutes LSA for BGP;
Describe the 4 different route designators used by OSPF.
– O – OSPF intra-area and network LSA;- O IA – OSPF interarea (summary LSA);- O E1 – Type 1 external routes;- O E2 – Type 2 external routes;
In what order are the best paths calculated?
1. First, all routers calculate the paths to destinations within their area (type 1 and 2)2. Second, routers calculate the best paths to the other area in the internetwork.

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3. Finally, routers calculate the best path to external AS’s 

What is the difference between type 1 and type 2 external routes?
Type O E1 external routes calculate the cost by adding the external cost to the internal cost of each link the packet crosses. Type O E2 have the external cost equal to the external cost only.