Asymmetric Routing Due to Mismatched Local Preference
A financial institution is setting up Dedicated Interconnect with Google Cloud. They have two on-premises routers (R1 and R2) each connected to a separate Google Cloud router via VLAN attachments in two different zones (us-central1-a and us-central1-b). The on-premises routers are configured with BGP, and they advertise the corporate prefix 10.0.0.0/8. Google Cloud routers are configured with custom route advertisements. After provisioning, you notice that traffic from some on-premises subnets to GCP experiences asymmetrical routing, causing packet drops. You verify that both BGP sessions are established and that both Cloud Routers have received the 10.0.0.0/8 route. What is the most likely cause of the asymmetrical routing?
Quick Answer
The answer is mismatched BGP local preference values on the on-premises routers for routes received from Google Cloud. This is correct because local preference is the first BGP attribute evaluated when selecting the best outbound path from an on-premises network; if R1 has a higher local preference for GCP routes than R2, all outbound traffic will prefer R1, while return traffic from GCP can arrive via either router based on GCP’s routing, creating an asymmetric routing Dedicated Interconnect local preference mismatch that causes packet drops. On the Google Professional Cloud Network Engineer exam, this scenario tests your understanding of BGP path selection in dual-homed hybrid connectivity, often appearing as a trap where candidates focus on GCP-side routing instead of the on-premises side. A common memory tip is “local preference is the first tiebreaker outbound, not inbound”—think of it as the on-premises router’s “vote” for which link to use for sending traffic, and mismatched votes break symmetry.
⚠ Common exam trap
Google Cloud often tests the distinction between BGP attributes that influence inbound traffic (MED, AS-path prepend) versus outbound traffic (local preference), and the trap here is that candidates may incorrectly attribute asymmetrical routing to MED or MTU issues rather than recognizing that local preference mismatch on the on-premises side is the root cause of asymmetric outbound path selection.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
On-premises routers have mismatched BGP local preference values for routes received from Google Cloud
Asymmetrical routing in a dual-homed Dedicated Interconnect setup occurs when on-premises routers have different BGP local preference values for routes received from Google Cloud. Local preference is the first BGP attribute considered when selecting the best path outbound from the on-premises network. If R1 has a higher local preference for the GCP routes than R2, all outbound traffic from on-premises will prefer R1, while return traffic from GCP may arrive via either router (depending on GCP's routing), causing a mismatch in traffic paths and packet drops.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
On-premises routers have mismatched BGP local preference values for routes received from Google Cloud
Why this is correct
Different local preferences cause one path to be preferred for return traffic, while forward traffic may take the other path, leading to asymmetry.
- ✗
The on-premises routers are using the same AS number causing BGP loop prevention
Why it's wrong here
Using the same AS number is invalid and would prevent BGP sessions from forming; question says sessions are established.
- ✗
VLAN attachments are configured with different MTU sizes
Why it's wrong here
MTU mismatch can cause fragmentation issues but typically not asymmetrical routing; packets would be dropped rather than via different paths.
- ✗
Cloud Router is setting different BGP metric attributes for each VLAN attachment
Why it's wrong here
Google Cloud Router does not assign different metrics to different attachments unless configured with custom route propagation; by default, all routes have the same metric.
Visual reference
Quick reference
Asymmetric Encryption Algorithm Comparison
| Algorithm | Key Exchange | Signatures | Equivalent Security Key | Notes |
|---|---|---|---|---|
| RSA-3072 | Yes | Yes | 128-bit | Widely deployed; slow for bulk data |
| ECDSA P-256 | No | Yes | 128-bit | Fast signatures; standard TLS certs |
| ECDH / ECDHE | Yes | No | 128-bit | Perfect forward secrecy in TLS 1.3 |
| DH / DHE | Yes | No | 128-bit (3072-bit key) | Replaced by ECDHE in modern TLS |
| Ed25519 | No | Yes | ~128-bit | SSH keys, modern PKI |
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Same concept, more angles
1 more way this is tested on PCNE
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Variation 1. You are designing a hybrid network using Cloud VPN with dynamic routing (BGP) to connect multiple on-premises sites to Google Cloud. What is a best practice to avoid asymmetric routing when you have multiple VPN tunnels from different on-premises routers?
easy- A.Use static routes instead of BGP to have precise control over path selection
- B.Use a different BGP ASN for each on-premises router to ensure uniqueness
- ✓ C.Configure all on-premises routers with the same BGP ASN and enable ECMP on the Cloud Router
- D.Disable ECMP on the Cloud Router to avoid multipath issues
Why C: Using the same BGP ASN on all on-premises routers and enabling ECMP on the Cloud Router allows the Cloud Router to treat multiple BGP sessions as equal-cost paths. This prevents asymmetric routing by ensuring that return traffic can be load-balanced across any available tunnel, while the same ASN avoids BGP loop-prevention mechanisms that would otherwise reject routes from routers with different ASNs.
JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This PCNE practice question is part of Courseiva's free Google Cloud certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the PCNE exam.