Explanation
A keepalive OAM probe is a mechanism that can be used to monitor the state of a GRE tunnel and detect any failures in the tunnel path. A keepalive OAM probe consists of sending periodic packets from one end of the tunnel to the other and expecting a reply. If no reply is received within a specified time, the tunnel is considered down and the line protocol of the tunnel interface is changed to down1.
To configure a keepalive OAM probe for a GRE tunnel, you need to specify two parameters: the keepalive-time and the hold-time. The keepalive-time is the interval between each keepalive packet sent by the local router. The hold-time is the maximum time that the local router waits for a reply from the remote router before declaring the tunnel down2.
According to the Juniper Networks documentation, the hold-time value must be two times the keepalive-time value for a GRE tunnel2. This is because the hold-time value must account for both the round-trip time of the keepalive packet and the processing time of the remote router. If the hold-time value is too small, it may cause false positives and unnecessary tunnel flaps.
In the exhibit, the configuration shows that the keepalive-time is set to 10 seconds and the hold-time is set to
15 seconds for the gr-1/1/10.1 interface. This means that the local router will send a keepalive packet every 10 seconds and will wait for 15 seconds for a reply from the remote router. However, this hold-time value is not two times the keepalive-time value, which violates the recommended configuration. This may cause traffic drops if the remote router takes longer than 15 seconds to reply.
Therefore, option D is correct, because the hold-time value must be two times the keepalive-time value for a GRE tunnel. Option A is incorrect, because BFD is not required for GRE tunnels; BFD is another protocol that can be used to monitor tunnels, but it is not compatible with GRE keepalives3. Option B is incorrect, because the “event link-adjacency-loss” option is not related to GRE tunnels; it is an option that can be used to trigger an action when a link goes down4. Option C is incorrect, because LLDP does not need to be removed from the gr-1/1/10.1 interface; LLDP is a protocol that can be used to discover neighboring devices and their capabilities, but it does not interfere with GRE tunnels5.
References:
1: Configuring Keepalive Time and Hold time for a GRE Tunnel Interface 2: keepalive | Junos OS | Juniper Networks 3: Configuring Bidirectional Forwarding Detection 4: event link-adjacency-loss | Junos OS | Juniper Networks 5: Understanding Link Layer Discovery Protocol

Explanation
The default MAC age-out timer on an EX Series switch is 300 seconds12. The MAC age-out timer is the maximum time that an entry can remain in the MAC table before it “ages out,” or is removed31. This configuration can influence efficiency of network resource use by affecting the amount of traffic that is flooded to all interfaces1. When traffic is received for MAC addresses no longer in the Ethernet routing table, the router floods the traffic to all interfaces1.

The output shown in the exhibit is a brief display of the Ethernet switching table, which shows the learned Layer 2 MAC addresses for each VLAN and interface1.
The command show ethernet-switching table displays the Ethernet switching table with brief information, such as the destination MAC address, the VLAN name, the forwarding state, and the interface name1.
The command show route forwarding-table displays the routing table information for each protocol family, such as inet, inet6, mpls, iso, and so on2. It does not show the Ethernet switching table or the MAC addresses.
The command show ethernet-switching table extensive displays the Ethernet switching table with extensive information, such as the destination MAC address, the VLAN name, the forwarding state, the interface name, the VLAN index, and the tag type1. It shows more details than the brief output shown in the exhibit.
The command show route forwarding-table family ethernet-switching displays the routing table information for the ethernet-switching protocol family, whichshows the destination MAC address, the next-hop MAC address, and the interface name3. It does not show the VLAN name or the forwarding state.

B is correct because the loops do not exist is one of the conditions that are verified by the receiving router to ensure that the received BGP route is valid. A loop in BGP means that a route has been advertised by the same AS more than once, which can cause routing instability and inefficiency1. To prevent loops, BGP uses the AS-path attribute, which lists the AS numbers that a route has traversed from the origin to the destination2. The receiving router checks the AS-path attribute of the received route and discards it if it finds its own AS number in the list2. This way, BGP avoids accepting routes that contain loops.
C is correct because the next hop is reachable is one of the conditions that are verified by the receiving router to ensure that the received BGP route is valid. The next hop is the IP address of the next router that is used to forward packets to the destination network3. The receiving router checks the next hop attribute of the received route and verifies that it has a valid route to reach it3. If the next hop is not reachable, the received route is not usable and is rejected by the receiving router3. This way, BGP ensures that only feasible routes are accepted.

On Juniper switches, the default VLAN is set to a VLAN ID of 1 by default. This means that all interfaces on the switch are members of VLAN until they are specifically assigned to another VLAN. Therefore, option A is correct.
The default VLAN ID can be changed. This allows network administrators to configure the switch to use a different VLAN as the default, if necessary. Therefore, option D is correct.

The error in the exhibit occurs because an access interface can only be a member of one VLAN, but it is currently configured with the “default” VLAN and is trying to be added to VLAN “v11.” To resolve the issue, the correct command is:
delete ge-0/0/2 unit 0 family ethernet-switching vlan members default
This command removes the interface from the “default” VLAN, allowing it to be part of VLAN
“v11” as intended.

Explanation
In OSPF, the state of the neighbor relationship is determined by the exchange of OSPF packets between routers1. The state “2Way” as shown in the exhibit indicates that bi-directional communication has been established between the two OSPF routers1. This is the normal state for a neighbor that is not the Designated Router (DR) or Backup Designated Router (BDR) on a broadcast, non-broadcast multi-access (NBMA), or point-to-multipoint network1. These neighbors are often referred to as “DRothers”1. Therefore, option B is correct.

Explanation
The maximum allowable MTU size for a default GRE tunnel without IPv4 traffic fragmentation is 1476 bytes1. This is because GRE packets are formed by the addition of the original packets and the required GRE headers1. These headers are 24-bytes in length and since these headers are added to the original frame, depending on the original size of the packet we may run into IP MTU problems1. The most common IP MTU is 1500-bytes in length (Ethernet)1. When the tunnel is created, it deducts the 24-bytes it needs to encapsulate the passenger protocols and that is the IP MTU it will use1. For example, if we are forming a tunnel over FastEthernet (IP MTU 1500)the IOS calculates the IP MTU on the tunnel as: 1500-bytes from Ethernet –
24-bytes for the GRE encapsulation = 1476-Bytes1.