A Site-to-Site VPN connection consists of two VPN tunnels between a customer gateway device and a virtual private gateway or a transit gateway. We recommend that you configure both tunnels for redundancy. From time to time, AWS also performs routine maintenance on your VPN connection, which might briefly disable one of the two tunnels of your VPN connection. For more information, see Tunnel endpoint replacement notifications.
When we perform updates on one VPN tunnel, we set a lower outbound multi-exit discriminator (MED) value on the other tunnel. If you have configured your customer gateway device to use both tunnels, your VPN connection uses the other (up) tunnel during the tunnel endpoint update process.
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To ensure that the up tunnel with the lower MED is preferred, ensure that your customer gateway device uses the same Weight and Local Preference values for both tunnels (Weight and Local Preference have higher priority than MED).
