You are troubleshooting a BGP peering issue between your on-premises router and an OCI FastConnect...

Problem Context: BGP session is established, but no routes are exchanged, and basic config (ASNs, IPs) is correct.

Option A Analysis: Misconfigured keepalive timers would cause the session to drop intermittently. Since the session is confirmed as established, this is unlikely. Keepalives affect session stability, not route exchange.

Option B Analysis: A mismatch in BGP authentication keys (e.g., MD5 passwords) would prevent the session from establishing. Given the session is up, this is not the issue.

Option C Analysis: BGP prefix lists or route maps filter advertised routes. If either the on-premises router or OCI applies a filter (intentionally or misconfigured), it could block route advertisements despite an established session. This is a common issue in BGP setups and aligns with the symptoms.

Option D Analysis: MTU mismatches could cause packet loss or fragmentation, but BGP uses TCP (small packets), and session establishment indicates MTU isn’t the primary issue. Route exchange failures are more likely due to filtering than MTU.

Conclusion: Option C is the most likely cause, as filtering directly prevents route exchange without affecting session status.

From Oracle’s FastConnect documentation:

"Once a BGP session is established, routes are exchanged based on the prefixes advertised by each side. Route maps, prefix lists, or filters on either the CPE or OCI side can restrict which routes are advertised or accepted."

"If no routes appear in the routing table despite an active session, verify that no filters are blocking advertisements."This supports Option C as the most likely cause. Reference:FastConnect Overview - Oracle Help Center(docs.oracle.com/en-us/iaas/Content/Network/Tasks/fastconnect.htm).