You work as a network engineer for the PassGuide Company. Study the exhibit carefully. In this network, PG-R1 is redistributing 10.1.5.0/24 into OSPF, and PG-R2 is originating 10.1.4.0/24 as an internal route. PG-R6 has received packets destined to 10.1.4.1 and 10.1.5.1. Which statement is true about the path or paths these two packets will take?
A.
The packet destined to 10.1.5.1 will follow the optimum path through the network, PG-R4 to PG-R1, while the packet destined to 10.1.4.1 will follow a suboptimal path through the network.
B.
There is not enough information provided to determine which packet will take an optimal or suboptimal path through the network.
C.
Both packets will follow optimal paths through the network to their destinations
D.
The packet destined to 10.1.4.1 will follow an optimal path through the network, while the packet destined to 10.1.5.1 will follow a suboptimal path through the network
Explanation:
Costs dictate path to ABR selectedDepending on the point where a network is sourced, there are various types of routes that could be present in a OSPF domain.
When there are multiple routes to a particular network in a OSPF domain, the type of the route influences the route that is selected and installed by the router in the routing table.
In OSPF, routes that are learned by a router from OSPF sources within the same area are known as inta-area routes.
Routes that originate from an OSPF router in a different area are considered as inter-area routes.
Certain networks could belong to a domain outside OSPF, which could then be redistributed into the OSPF by an Autonomous System Boundary Router (ASBR).
Such routes are considered external routes.
They can be further divided into external type-1 or external type-2 routes, depending on how they are advertised while being redistributing on the ASBR.
The difference between these two types is the way in which the metric for the route is calculated.
OSPF-running routers use these criteria to select the best route to be installed in the routing table:1. Intra-area routes.
2. Inter-area routes.
3. External Type-1 routes.
4. External Type-2 routes.
B) If there are multiple routes to a network with the same route type, the OSPF metric calculated as cost based on the bandwidth is used for selecting the best route.
The route with the lowest value for cost is chosen as the best route.
C) If there are multiple routes to a network with the same route type and cost, it chooses all the routes to be installed in the routing table, and the router does equal cost load balancing across multiple paths.
I think the question is somehow screwed.
According to the information provided area 2 is NSSA, so redistributed network 10.1.5.0/24 will not be injected there. NSSA does not provide default route, so packet destined to 10.1.5.1 will be dropped on R6. Can we call it suboptimal path and choose “D”?
On the other hand network 10.1.4.0/24 will be passed from area 0 to area 2 over R4 and R5.
The cost on R6 from R4 is 10+5+10=25 and from R5 is 10+15=25 which means load balancing.
I have made a test on emulated routers to make sure my analysis is correct.
Now – which answer to choose?