How to Resolve BGP Session Failures on Cloud Interconnect?

You have provisioned a Cloud Interconnect connection with a VLAN attachment. You configured Border Gateway Protocol (BGP) between your on-premises router and your Cloud Router. After deploying and testing the connection, you discover that the BGP session is not established between your on-premises router and the Cloud Router. Which two actions should you take to resolve this issue? (Choose two.)

  1. From the Google Cloud console, run gcloud compute routers get-status to verify the Address Resolution Protocol (ARP) learned.
  2. Verify that you have configured the on-premises router's subinterface with a subnet mask of /31. Source Reference Answer
  3. Verify that you have configured the on-premises router's eBGP multihop with a minimum hop length of 4.
  4. Verify that you have configured the on-premises router's BGP security parameters to use MD5 authentication.
  5. From the Google Cloud console, run gcloud compute interconnects get-diagnostics to verify the Address Resolution Protocol (ARP) learned. Source Reference Answer

Community Votes

BE
33%
AE
33%
AB
33%

33% of anonymous learners picked answer BE. Votes are pick records left by other test-takers — they are not the verified answer.

Community Insight

Tests your ability to isolate BGP failures by distinguishing interconnect-level diagnostics from router-level status checks, commonly trapping candidates who skip physical-layer verification.

Troubleshooting BGP adjacency over Cloud Interconnect requires verifying physical-link ARP diagnostics and validating point-to-point subnet masks, with candidates frequently divided between BE and AE due to subnet sizing debates.

Candidates often select A instead of E, incorrectly assuming gcloud compute routers get-status displays physical ARP states when it only reports BGP session and prefix information.

Community Discussion (3 comments)

mohitms1996 👍 1 Selected: AE
❌ B. Verify that you have configured the on-premises router's subinterface with a subnet mask of /31. Incorrect approach: Google Cloud uses /29 subnets for VLAN attachments, not /31. A /31 subnet mask is used for point-to-point links but is not applicable for Cloud Interconnect VLAN attachments.
Popa 👍 1 Selected: BE
The correct answers are B and E.
RKS_2021 👍 1 Selected: AB
(A) gcloud compute routers get-status → This command helps troubleshoot BGP session issues by showing ARP information, learned prefixes, and BGP status. If ARP isn't learned correctly, BGP won't establish. (B) Subnet mask of /31 → Google Cloud uses a /30 or /31 subnet for VLAN attachments. If your on-premises router isn't configured correctly with one of these subnet masks, BGP won't work.

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Expert Analysis

Why the Answer Is Correct

Options B and E target the foundational requirements for establishing BGP over Cloud Interconnect. Configuring a /31 subnet mask on the on-premises subinterface ensures efficient point-to-point addressing compatible with Google’s peering expectations, while running gcloud compute interconnects get-diagnostics confirms successful Layer 2/Layer 3 handshakes through ARP learning on Google’s edge routers. These steps effectively isolate whether the failure originates from local misconfiguration or underlying transport disruptions.

Why the Other Options Are Wrong

Option A is misleading because router status commands focus on BGP neighbor states and route advertisements rather than interconnect physical diagnostics. Option C unnecessarily enables eBGP multihop, which contradicts the direct L3 connectivity model inherent to Cloud Interconnect deployments. Option D incorrectly assumes MD5 authentication is mandatory, whereas sessions establish successfully without it unless explicitly enforced on both peers.

Community Comment Notes

The tied vote distribution highlights ongoing confusion regarding subnet sizing, with comment [1] correctly noting that Google provisions a /29 for VLAN attachments but affirming that /31 remains a valid customer-side peering configuration. Comment [3] supports AE by detailing how router diagnostics assist broader BGP troubleshooting, though it overlooks the necessity of interconnect-level checks first. Candidate discussions consistently emphasize isolating transport-layer issues before adjusting BGP protocol parameters.

Official Reference

Exam Strategy

Always separate Layer 2/Layer 3 transport diagnostics from BGP protocol troubleshooting when analyzing Cloud Interconnect failures. Verify ARP resolution and subnet alignment first, as these physical prerequisites must succeed before BGP session negotiation can even begin.

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