JN0-280 Free Practice Test

When troubleshooting an OSPF neighborship, you notice that the router stopped at the ExStart state. What is the cause of this result?

1. A. The priority is set to 255.
2. B. There is an interval timing mismatc
3. C. There is an area ID mismatch.
4. D. There is an MTU mismatch.

Correct Answer: D
When an OSPF (Open Shortest Path First) neighborship is stuck in the ExStart state, it usually points
to a mismatch in Maximum Transmission Unit (MTU) settings between two routers trying to establish
the adjacency. The ExStart state is where OSPF routers negotiate the master-slave relationship and
exchange DBD (Database Description) packets.
Step-by-Step Breakdown:
OSPF Neighbor States: OSPF goes through several states to establish an adjacency with a neighbor:
Down: No hello packets have been received.
Init: Hello packets are received, but bidirectional communication isn't confirmed.
2-Way: Bidirectional communication is established.
ExStart: The routers are negotiating who will be the master and who will be the slave, and begin to
exchange DBD packets.
Exchange: The routers start exchanging the database information.
Loading: The routers process the Link-State Advertisements (LSAs).
Full: The adjacency is fully established.
MTU Mismatch Issue:
During the ExStart state, both OSPF routers must agree on their MTU values. If there is an MTU
mismatch between the two routers, OSPF neighbors will fail to move from the ExStart to the
Exchange state. The router with the larger MTU setting will not accept DBD packets from the router
with a smaller MTU because the packets may exceed the smaller MTU size.
In Juniper devices, this behavior can be identified by examining the MTU settings using the show
interfaces command and ensuring both routers have matching MTU configurations. To resolve this
issue, either match the MTU settings on both routers or configure OSPF to ignore MTU mismatches
using the command set protocols ospf ignore-mtu.

Exhibit:

Referring to the exhibit, which next hop will be preferred in the routing table?

1. A. Next hop IP address 172.25.20.254 will be preferred.
2. B. Neither next hop will be preferred.
3. C. Next hop IP address 172.25.20.200 will be preferred.
4. D. Both next hops will be preferred.

Correct Answer: C
In the exhibit, we see a static route configuration with two possible next hops for the default route (0.0.0.0/0):
next-hop 172.25.20.254with the default preference of 7.
qualified-next-hop 172.25.20.200with a preference of 6.
Step-by-Step Breakdown:
Preference Value:In Junos OS, thepreferencevalue is used to determine which route should be
preferred in the routing table. The lower the preference value, the higher the priority for the route.
Comparison:In this case:
The next hop172.25.20.254has a preference of7.
Thequalified-next-hop 172.25.20.200has a preference of6.
Preferred Next Hop:Since172.25.20.200has a lower preference (6) compared to172.25.20.254(7), it
will be the preferred next hop in the routing table, assuming both next hops are reachable.
Juniper Reference:
Qualified Next Hop: In Junos, static routes with multiple next-hop options are selected based on the
preference value, with the lower value being preferred.