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After creating a network interface, you have to assign it to the instance where it will be used.

Warning
  • Stopping and restarting the instance after adding a second network interface will release any automatically assigned public IP address. If several interfaces and a public address are required, use Elastic IP addresses.
  • Additionally, adding a second network interface to a non-Amazon Linux EC2 instance causes traffic flow issues. This occurs
when there is only one routing table containing one default gateway and traffic that arrives at the second network interface leaves the instance using the first network interface or vice-versa
  • in cases of asymmetric routing where traffic to the instance arrives at one network interface and leaves the instance through the other network interface. This is blocked by AWS because a mismatch between MAC
address and IP address.

To make the second interface work, perform the following steps:

  • Add the new interface to your running instance.
  • Create a configuration file for the second interface.
  • If only one interface has an Elastic IP address , restart your network and test reachability via the Elastic IP address.
  • If both interfaces only have private IP addresses or both have public IP addresses,
    • configure an additional routing table, and then
    • set up rules in the custom routing table policy database so that traffic for each interface uses its own routing table.
  • Refer to the AWS documentation and AWS Networking and Charon-SSP for more information. Failure to use the proper steps, may make your instance unreachable!


StepDetails
Locate your network interface in the interface list and right-click on it.

The right-click opens the context menu. Select Attach.


This will open the window to enter the necessary instance information.

Select your instance and confirm entry.

Select your instance from the drop-down list and click on Attach.


The state of your interface will change from available to in-use.

Verify that instance has second interface.

Select your instance in the instance list. The description tab in instance details should now display two network interfaces:

(info) You can also attach/detach existing interfaces from the instance overview screen. Select your instance and then Actions > Networking > Attach or Detach network interface.

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Assigning an Elastic IP Address to the Network Interface

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An Elastic IP address is a persistent, public IPv4 address to be used for one of your network interfaces or instances. You can associate an Elastic IP address with any instance or network interface in your account.

(info) The advantage of associating the Elastic IP address with the network interface instead of directly with the instance is that you can move the network interface with its attributes easily from one instance to another.

(warning) The initial automatically assigned public IP address will be removed as soon as you restart the instance after adding a network interface with an Elastic IP address to your instance. Do not restart your instance before you are sure you can reach it via the Elastic IP address. The automatically assigned public IP address will also be disabled if you assign an Elastic IP address to the primary Ethernet interface of the instance.

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