A) On R1, enable Level 1 on the ge-0/0/1 interface. In IS-IS, both levels (Level 1 and Level 2) are enabled by default when you enable IS-IS on an interface1. Level 1 systems route within an area.
If the destination is outside an area, Level 1 systems route toward a Level 2 system. Therefore, enabling Level 1 on the ge-0/0/1 interface on R1 would allow packets to reach from R1 to R3.
D) On R3 enable Level 1 on the ge-0/0/4 interface Similarly, enabling Level 1 on the ge-0/0/4 interface on R3 would allow packets to reach from R1 to R3. These explanations are based on the IS-IS configuration documents and learning resources available at Juniper Networks1 and Cisco.

AS-path prepending is a traffic engineering technique used in a BGP export policy to artificially increase the length of the AS path attribute advertised to external peers. Since a shorter AS path is generally preferred during BGP best path selection, prepending makes a route appear less attractive to external autonomous systems, which can be used to influence inbound traffic engineering decisions.

The configuration shown in the exhibit is an example of a routing instance of type virtual-router. A routing instance is a collection of routing tables, interfaces, and routing protocol parameters that create a separate routing domain on a Juniper device. A virtual-router routing instance allows administrators to divide a device into multiple independent virtual routers, each with its own routing table.
The configuration also includes a rib-group statement, which is used to import routes from one routing table to another. A rib-group consists of an import-rib statement, which specifies the source routing table, and an export-rib statement, which specifies the destination routing table. In this case, the rib-group name is inet-to-ispi, and the import-rib statement specifies inet.0 as the source routing table. The export-rib statement specifies ispi.inet.0 as the destination routing table.
This means that the routes from inet.0 will be imported into ispi.inet.0.

A BGP route reflector allows a router to reflect routes learned from one iBGP client to other iBGP clients, eliminating the requirement for a full mesh of iBGP sessions between all routers in an autonomous system. This significantly reduces the number of iBGP sessions needed in large networks while maintaining proper route propagation within the AS.

The exhibit shows the output of the command , which displays information about the OSPF neighbors on a router1.
The output shows that the OSPF neighbor with the address 172.26.1.1 and the interface ge-0/0/3.0 is in the Exstart state1.
The Exstart state is the fourth state in the OSPF neighbor formation process, after Down, Init, and
2-Way states2. In this state, the OSPF neighbors establish a master-slave relationship and exchange database description (DBD) packets, which contain summaries of their link-state databases2.
The most common reason for OSPF neighbors to be stuck in the Exstart state is an MTU mismatch between the interfaces3. MTU stands for maximum transmission unit, which is the largest size of a packet that can be transmitted on a network segment4. If the MTU values of two OSPF neighbors are different, theymay not be able to exchange DBD packets successfully, as some packets may be dropped or fragmented due to their size exceeding the MTU limit3.
To solve this problem, you need to ensure that the MTU values of both OSPF neighbors are the same or compatible. You can use the command show interfaces to display the MTU value of an interface5. You can also use the command ping with the do-not-fragment option to test the MTU size between two routers. You can change the MTU value of an interface by using the command set interfaces interface-name mtu mtu-value in configuration mode5.