Based on the network provided in the exhibit, how to route the traffic arriving at PG-R1 for 10.1.3.1?
A.
through PG-R3, since the path through PG-R3 is through the backbone
B.
through PG-R2, because that is the only path available; no neighbor adjacency will be built between PG-R1 and PG-R2
C.
through PG-R1, since the path through PG-R1 has the lowest hop count
D.
through PG-R1, since the path through PG-R1 has the lowest total metric (10+10=20, versus 10+10+10=30 through PG-R3)
Explanation:
Choose route in intra-area firstDepending 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.
There is something completely wrong with this question.
First of all 10.1.3.0/24 will not be advertised since OSPF is not enabled on this interface.
R1 will drop the packet then, but there is no such answer.
Let’s ignore it.
Secondly R1 and R2 will not form an adjacency on 10.1.1.0/24 link as the interfaces are assigned to different areas. So the only possible path is over R3 – answer A.
What about C and D? Starting at R1 how can we pass through R1 again?
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