You are troubleshooting a connectivity issue between two compute instances within the same VCN.

Analyze Connectivity Successes:Instance A can ping its subnet gateway (10.0.1.1), indicating that local subnet routing and security rules are functioning for IPv4. It can also ping Instance B’s IPv6 address (fc00:1:2::20), confirming that IPv6 routing and security rules between subnets are operational.

Identify the Failure:Instance A cannot ping Instance B’s IPv4 address (10.0.2.20). Since security lists and NSGs allow all traffic, the issue is unlikely to be a security configuration problem.

Examine Routing for Instance A:The route table for Instance A’s subnet (10.0.1.0/24) has a rule directing traffic to 10.0.2.0/24 via the VCN Local Peering Gateway (LPG). In OCI, LPGs are used for intra-region VCN peering, but here, both instances are in the same VCN, so this rule is likely a misconfiguration or irrelevant unless peering is involved. However, the successful IPv6 ping suggests basic connectivity exists.

Check Return Path from Instance B:For a ping to succeed, Instance B must send ICMP replies back to Instance A (10.0.1.10). Instance B’s subnet (10.0.2.0/24) needs a route table entry to send traffic to 10.0.1.0/24. Without this, replies are dropped, causing the IPv4 ping to fail. The IPv6 success indicates that IPv6 routing is correctly configured both ways, possibly via SLAAC or default routes.

Evaluate Options:

A:Incorrect. IPv6 is enabled, as Instance A pings Instance B’s IPv6 address.

B:Correct. Missing route for 10.0.1.0/24 in Instance B’s subnet prevents IPv4 replies.

C:Incorrect. Security lists and NSGs can filter IPv6 traffic in OCI.

D:Incorrect. Ping supports IPv6, as evidenced by the successful IPv6 ping.

The most probable cause is a missing route in Instance B’s subnet route table. In OCI, each subnet has its own route table, and for instances in different subnets within the same VCN to communicate, both subnets must have appropriate routes. The successful IPv6 ping suggests that IPv6 routing is intact (likely due to default behavior or SLAAC), but IPv4 requires explicit routing. Per the Oracle Networking Professional study guide, "Route tables must be configured to direct traffic to the appropriate next hop for inter-subnet communication within a VCN" (OCI Networking Documentation, Section: Virtual Cloud Networks).