You want to set up two Cloud Routers so that one has an active Border Gateway Protocol (BGP) session, and the other one acts as a standby. Which BGP attribute should you use on your on-premises router?
D is the correct answer. The question is position from a Cloud Router's perspective. That said, Cloud Routers do not support Local Preference. Google Cloud Router's Perspective (Outbound from GCP to On-prem):
To make Google Cloud prefer one Cloud Router over another for traffic from GCP to on-prem, Google Cloud Router uses the Advertised Route Priority, which is implemented as the Multi-Exit Discriminator (MED) BGP attribute.
You configure a "base priority" (which translates to the MED value) on the BGP session of your Cloud Router. A lower MED value indicates a higher priority. So, for the active Cloud Router, you'd set a lower MED (higher priority), and for the standby, a higher MED (lower priority).
The correct answer is D. Multi-exit Discriminator (MED). MED helps control which Cloud Router is preferred for outbound traffic, making one router active and the other standby.
MED is sent in the advertisements from an external AS (GCP here) to suggest to the neighbour what path the external AS prefers. Local preference is used to set from your own perspective (here on-prem) which path to prefer to a destination. Therefore, local preference is correct here.
The answer is C...
They are asking for on-premise routers TO the Cloud Routers. MED is used, more exactly - for inbound to on-premise, not to the the cloud...
Choose D. Explanation:
Multi-exit Discriminator (MED): The Multi-exit Discriminator is a BGP attribute that is used to influence the path selection process when there are multiple exit points from an autonomous system. In the context of high availability and failover scenarios, you can set different MED values for the two Cloud Routers, with the lower MED value indicating the preferred route.
When the primary Cloud Router has a lower MED value, it will be preferred by the on-premises router. If the primary router fails or the BGP session goes down, the standby Cloud Router, with a higher MED value, becomes the preferred route.
The right answer is D, MED, if you think about it, Local Preference directs the outgoing traffic, while MED Directs the incoming traffic. If the question WAS for the On-prem router, then yes, Local Preference.
D: How BGP Routers Use the Multi-Exit Discriminator for Best Path Selection
This document demonstrates the use of the bgp deterministic-med command and explains how it can affect multi-exit discriminator (MED)-based path selection.
• MED is propagated to all routers within the neighbor AS but not passed along any other autonomous systems.
The question is confusion - should explicitly ask whether for the ingress or egress decision. For Ingress to on-prem traffic should use MED; for Egress traffic out of on-prem should use LP
that may be true.. but look at it from a SP perspective. Local preference is used more so for internal BGP to exit out of an AS. MED is used to 'tell' SP which you prefer.
Answer is D.
You can configure 2 different MED values for each BGP neighbor in your single on-prem router , to influence ISP(GCP)'s 2 separate routers to select which path they send traffic towards you. The lower MED value is preferred.
Ref: https://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/13759-37.html
I was struggling with choosing between A and D. Because BGP selects shortest AS path first when sending traffic. In our On-prem router, we can actually prepend AS path for the standby BGP session. However, after learning from GCP's documentations(as referenced below) that GCP uses MED to set base priority. I decided to choose D.
Additional ref: https://cloud.google.com/network-connectivity/docs/router/concepts/overview#route-metrics
https://cloud.google.com/network-connectivity/docs/router/concepts/overview#suggested_base_priority_values
You right, for something that i don't know i could validate you that as path is not take to account. But med yes. A is wrong in GCP BGP implementation.
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