Section 1.4 - OSPFv2 Path Selection Flashcards
In OSPF, what is “cost”?
The metric used for determining the best path to a given network via OSPF. The lowest path cost to a network is the best path.
(True or False) In OSPF, the default metric for a given interface is based on the interface’s “bandwidth” setting.
True
What is the link cost algorithm for OSPF that determines the metric for a given interface by default?
link_cost = MAX((reference_bandwidth_in_bps / link_bandwidth_in_bps), 1)
What is the default reference bandwidth in OSPF (in bps)?
100,000,000 bps
How would you explicitly configure the cost of an interface in OSPF (on IOS-XR)?
conf t
router ospf 1
area 0
interface Gig0/0/0/0
cost 4000
How would you explicitly configure the cost of an interface in OSPF (on IOS-XE/IOS)?
conf t
interface Gig0/0/0/0
ip ospf cost 4000
In OSPF, the best path selection algorithm (for OSPF routes to the same network) take into account LSA type FIRST before total path cost. What is the pecking order of route types (in order of preference)?
- Intra Area
- Inter Area
- External Type 1
- NSSA Type 1
- External Type 2
- NSSA Type 2
What commands would you use to modify the reference bandwidth used in the auto-cost algorithm for OSPF (in IOS-XR)?
conf t
router ospf 1
auto-cost reference-bandwidth 4000000
NOTE: The value in that command is in Mbps
What commands would you use to modify the reference bandwidth used in the auto-cost algorithm for OSPF (in IOS-XE)?
conf t
router ospf 1
auto-cost reference-bandwidth 4000000
NOTE: The value in that command is in Mbps
What algorithm does OSPF use to determine the shortest path to a given destination, based on total path cost?
Dijkstra’s algorithm
(A.K.A. Shortest Path First (SPF) algorithm)
What is the maximum possible cost in OSPF for a given interface?
65535
Which criterion does SPF use to determine the best path?
A) Lowest delay in the path
B) Highest bandwidth of the path
C) Lowest total cost of the path
D) Lowest bandwidth of the path
C
