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Exam Professional Cloud Network Engineer topic 1 question 15 discussion

Actual exam question from Google's Professional Cloud Network Engineer
Question #: 15
Topic #: 1
[All Professional Cloud Network Engineer Questions]

Your on-premises data center has 2 routers connected to your Google Cloud environment through a VPN on each router. All applications are working correctly; however, all of the traffic is passing across a single VPN instead of being load-balanced across the 2 connections as desired.
During troubleshooting you find:
"¢ Each on-premises router is configured with a unique ASN.
"¢ Each on-premises router is configured with the same routes and priorities.
"¢ Both on-premises routers are configured with a VPN connected to a single Cloud Router.
"¢ BGP sessions are established between both on-premises routers and the Cloud Router.
"¢ Only 1 of the on-premises router's routes are being added to the routing table.
What is the most likely cause of this problem?

  • A. The on-premises routers are configured with the same routes.
  • B. A firewall is blocking the traffic across the second VPN connection.
  • C. You do not have a load balancer to load-balance the network traffic.
  • D. The ASNs being used on the on-premises routers are different.
Show Suggested Answer Hide Answer
Suggested Answer: D 🗳️
by Jos at June 22, 2020, 9:53 a.m.

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glk
Highly Voted 3 years, 11 months ago
Answer is D: Cloud Router doesn't use ECMP across routes with different origin ASNs For cases where you have multiple on-premises routers connected to a single Cloud Router, the Cloud Router learns and propagates routes from the router with the lowest ASN. Cloud Router ignores advertised routes from routers with higher ASNs, which might result in unexpected behavior. For example, you might have two on-premises routers advertise routes that are using two different Cloud VPN tunnels. You expect traffic to be load balanced between the tunnels, but Google Cloud uses only one of the tunnels because Cloud Router only propagated routes from the on-premises router with the lower ASN. reference: https://cloud.google.com/network-connectivity/docs/router/support/troubleshooting#ecmp
upvoted 16 times
AzureDP900
2 years ago
Agree, D. The ASNs being used on the on-premises routers are different.
upvoted 1 times
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Windows98
Highly Voted 4 years ago
D - GCP doesn't run ECMP across different ASNs
upvoted 9 times
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saraali
Most Recent 3 months, 3 weeks ago
Selected Answer: D
Reason:The correct answer is D. The ASNs being used on the on-premises routers are different. Cloud Router does not perform Equal-Cost Multi-Path (ECMP) routing across BGP sessions with different Autonomous System Numbers (ASNs). Cloud Router selects the BGP route from the router with the lowest ASN and ignores the routes from the router with the higher ASN. This leads to the observed issue where traffic is not load-balanced, leading to traffic being routed through just one VPN tunnel instead of being load-balanced across both. To resolve this, ensure that both on-premises routers use the same ASN for BGP sessions with Cloud Router
upvoted 1 times
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xhilmi
11 months, 3 weeks ago
Choose D: ASN (Autonomous System Number): An ASN is a unique identifier assigned to an autonomous system for the purpose of routing traffic on the Internet. In BGP (Border Gateway Protocol), each router in a network is assigned a unique ASN. To resolve this issue, ensure that both on-premises routers have the same ASN or use the same ASN for both routers if possible. This will help achieve the desired load balancing across the two VPN connections.
upvoted 3 times
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didek1986
1 year, 3 months ago
Selected Answer: D
All Cloud Routers that are associated with a single hub must use the same Google ASN. To select an ASN, follow the recommendations in the Cloud Router documentation.
upvoted 1 times
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rr4444
1 year, 4 months ago
Everyone is saying D, but that link saying only same ASN is needed no longer has that content Searched Google also Still the same now? Migth have changed, but I don't see anything Cloud Router or Cloud VPN release notes for
upvoted 2 times
rr4444
1 year, 4 months ago
Actually D The docs are in a totally different place https://cloud.google.com/network-connectivity/docs/network-connectivity-center/concepts/asn-requirements#asn_assignment
upvoted 1 times
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pk349
1 year, 10 months ago
D: Autonomous System Numbers: An Autonomous System (AS) is a set of Internet routable IP prefixes belonging to a network or a collection of networks that are all managed, controlled and supervised by a single entity or organization. Google Cloud uses only one of the tunnels because Cloud Router only propagated routes from the on-premises router with the lower ASN."
upvoted 2 times
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pfilourenco
2 years ago
Selected Answer: D
The correct answer is D. Cloud Router doesn't use ECMP across routes with different origin ASNs
upvoted 1 times
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GCP72
2 years, 3 months ago
Selected Answer: D
The correct answer is D.
upvoted 1 times
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kumarp6
2 years, 11 months ago
Answer is : D
upvoted 2 times
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Vidyasagar
3 years, 8 months ago
D is right
upvoted 1 times
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groovygorilla
3 years, 11 months ago
Agree with glk, answer is D. This reference says it all: https://cloud.google.com/network-connectivity/docs/router/support/troubleshooting#ecmp Cloud Router doesn't use ECMP across routes with different origin ASNs Cloud Router doesn't use ECMP across routes with different origin ASNs Cloud Router doesn't use ECMP across routes with different origin ASNs
upvoted 2 times
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[Removed]
4 years ago
Ans - D
upvoted 1 times
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genesis3k
4 years, 1 month ago
Correct answer is D. Please refer below: "you might have two on-premises routers advertise routes that are using two different Cloud VPN tunnels. You expect traffic to be load balanced between the tunnels, but Google Cloud uses only one of the tunnels because Cloud Router only propagated routes from the on-premises router with the lower ASN." https://cloud.google.com/network-connectivity/docs/router/support/troubleshooting#ecmp
upvoted 3 times
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Aniyadu
4 years, 1 month ago
The answer seems to be D. As per standard practices we can only one ASN configured in on-premise. https://cloud.google.com/network-connectivity/docs/vpn/concepts/topologies
upvoted 1 times
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passtest100
4 years, 3 months ago
change to A. The BGP session is esblished. so B is wrong. BGP(EBGP and IBGP) by default has only one optimal route in routing table. So whether ASN is the same or different, the issue still exists. Only if the routes are different, routes of the two router will be in the routing table.
upvoted 1 times
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passtest100
4 years, 3 months ago
sorry，typo. it should be B is the possible answer.
upvoted 1 times
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