As shown in the figure, R1 to R4 are four routers in AS123.

Comprehensive and Detailed In-Depth Explanation:

1. Understanding the Routing Issue

R1 and R4 are BGP routers and have an IBGP connection.

R2 and R3 are OSPF-only routers (they do not run BGP).

Networks 10.1.1.0/24 and 10.4.4.0/24 are advertised into BGP but not OSPF.

PC1 (10.1.1.1) and PC2 (10.4.4.1) need communication, but their networks are not known to OSPF routers R2 and R3.

To fix this, we need a method that ensures R2 and R3 learn the routes for 10.1.1.0/24 and 10.4.4.0/24.

2. Evaluating Each Answer Option

Option A: "Deploy policy-based routing on R1 and specify R2 as the next hop for traffic from PC1 to PC2."

Incorrect.

Policy-Based Routing (PBR) does not solve the issue of missing routes in OSPF.

Even if R1 forwards traffic to R2, R2 does not know how to reach 10.4.4.0/24, causing packet drops.

A proper routing protocol solution is required instead of PBR.

Option B: "Deploy MPLS and LDP on R1 to R4 and establish LSPs for routes 1.1.1.1/32 and 4.4.4.4/32."

Incorrect.

MPLS requires label-switched paths (LSPs) but does not distribute routing information itself.

The real problem is that R2 and R3 do not have routing entries for 10.1.1.0/24 and 10.4.4.0/24.

MPLS does not fix the missing route advertisement issue.

Option C: "Run BGP on R2 and R3, and establish full-mesh IBGP connections in AS 123."

Correct.

If R2 and R3 run BGP, they can form IBGP sessions with R1 and R4 and learn routes 10.1.1.0/24 and 10.4.4.0/24 via BGP.

R2 and R3 would then be able to forward traffic correctly between PC1 and PC2.

This is a valid method to fix the issue.

Option D: "On R1 and R4, import routes 10.1.1.0/24 and 10.4.4.0/24 into OSPF."

Correct.

By redistributing BGP routes into OSPF, R1 and R4 can ensure that R2 and R3 learn the missing networks (10.1.1.0/24 and 10.4.4.0/24) via OSPF.

This allows full reachability without running BGP on R2 and R3.

This is another valid method to fix the issue.

Final Answer:

✅ C and D are correct.

HCIP-Datacom-Advanced Routing & Switching Technology References:

BGP and OSPF Route Redistribution

IBGP Full-Mesh Requirement and Route Propagation

How Policy-Based Routing Works and Its Limitations