Influence best path selection

1. Influence best path selection

influence the best path selection. When a Cloud router advertises prefixes to a BGP pier, it includes a priority for each prefix in the advertisement. The advertisement is the BGP message, and it includes a root priority. The root priority is stored in the BGP multi-exit discriminator or MED attribute, which influences root selection. You can change the advertised root priority value on each Cloud VPN tunnel or VLAN attachment. This value is then assigned to the BGP MED attribute. The MED value affects the BGP best path selection. By changing the advertised root priority, you influence the best path selection. For more information about other factors that influence the best path selection, see the BGP protocol standard on the ietf.org website or a networking tutorial. The advertised root priority value applies to all prefixes advertised by the BGP session associated with the Cloud VPN tunnel or VLAN attachment. Base priorities are whole numbers from 0-65535. The highest possible base priority is zero. In other words, a lower number indicates a higher priority. The default base priority is 100. If you don't specify a base priority, the default priority is used. Next, let's talk about the region to region costs that can be added to the base priority to set the MED attribute on the BGP session. This cost only applies when the VPC is in global dynamic routing mode. Region to region costs are from 201 through 9,999. The value depends on the distance, latency, and other factors between two regions. Google generates the region to region cost values and you can't modify them. Next, let's look at a sample topology to see how this works. In the graphic, you see an example of region to region costs that Google calculates. Regions that are closer have a lower region cost for traffic that flows between them. For example, the regional cost between US Central 1 and Europe West 1 is lower than the cost between Europe West 1 and US West 1. For more information, see advertise prefixes and priorities in the Google Cloud documentation.

2. Let's practice!

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