JN0-281 Sample Questions & Answers
OSPF and BGP routing protocols for the data center carry the most weight, alongside spine-leaf and traditional architectures, Ethernet switching with VLANs, static and aggregate routing, link aggregation groups, and graceful restart for high availability.
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- Question 1Beginner
Layer 2 Switching and VLANs · Port modes
A Juniper administrator is configuring a QFX Series switch. They need to configure a specific interface to carry traffic for multiple VLANs to a connected server that supports 802.1Q tagging. Which configuration command correctly sets the interface mode?
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Correct answer: B
In Junos OS, to allow an interface to carry multiple VLANs using 802.1Q tagging, the interface mode must be set to 'trunk'. The command 'set interfaces unit 0 family ethernet-switching interface-mode trunk' accomplishes this.
- Question 2Beginner
Layer 2 Switching and VLANs · Ethernet switching/bridging concepts and operations
While troubleshooting Layer 2 connectivity on a Juniper switch, you verify the contents of the Ethernet switching table. What information is primarily contained in this table?
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Correct answer: C
The Ethernet switching table (often called the MAC address table) maps MAC addresses to the specific ports/interfaces where they were learned and associates them with their respective VLANs to facilitate Layer 2 forwarding.
- Question 3Intermediate
Layer 2 Switching and VLANs · Integrated Routing and Bridging (IRB)
You are configuring an Integrated Routing and Bridging (IRB) interface on a QFX switch to provide Layer 3 gateway services for VLAN 100. Which configuration step associates the Layer 3 logical interface with the Layer 2 VLAN?
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Correct answer: B
To link a VLAN to a Layer 3 IRB interface in Junos, you must configure the 'l3-interface' statement within the VLAN configuration hierarchy, pointing to the specific IRB logical unit (e.g., irb.100).
graph LR subgraph Configuration V[VLAN 100] -- references --> I[irb.100] I -- has --> IP[IP Address] end - Question 4Advanced
Layer 2 Switching and VLANs · VLAN Tagging
Case Study:
A data center administrator is troubleshooting a connectivity issue between two servers, Server A and Server B, located in different racks. The servers are in the same subnet (192.168.10.0/24) and VLAN (VLAN 10). The switches are connected via a trunk link allowing VLAN 10.
Upon checking the MAC address table on the switch connected to Server A, the administrator sees the MAC address of Server B learned on the trunk port. However, Server A cannot ping Server B.
Further investigation reveals that the trunk port on the remote switch (connected to Server B) has the 'native-vlan-id' set to 10, while the local switch trunk port has the default native VLAN configuration.
What is the most likely cause of the communication failure?
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Correct answer: A
The remote switch has 'native-vlan-id 10', meaning it sends VLAN 10 traffic untagged. The local switch has the default native VLAN (usually VLAN 1), so it expects VLAN 10 traffic to be TAGGED. When the local switch receives untagged traffic from the remote switch, it associates it with its own native VLAN (VLAN 1), not VLAN 10, resulting in a loss of connectivity for that specific VLAN context.
- Question 5Intermediate
Layer 2 Switching and VLANs · Ethernet switching/bridging
Which Junos OS command is used to display the status of Spanning Tree Protocol (STP) on a specific interface to determine if it is in a forwarding or blocking state?
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Correct answer: B
The command 'show spanning-tree interface ' provides detailed STP information for a specific port, including its role (Root, Designated, Altn) and state (FWD, BLK).
- Question 6Intermediate
Protocol-Independent Routing · Static, aggregate, and generated routes
In a Junos OS routing table, which type of route allows traffic to be discarded if it matches the route, often used to prevent routing loops when summarizing address blocks?
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Correct answer: C
Aggregate routes are used to summarize routing information. By default, an aggregate route in Junos has a next-hop of 'discard' or 'reject'. This ensures that if a packet matches the summary route but not a more specific contributing route, it is dropped rather than forwarded to a default route, preventing loops.
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