# Prerequisites for transit gateway policy tables in AWS Transit Gateway
<a name="tgw-policy-tables-prerequisites"></a>

Before configuring Policy-Based Routing (PBR) on a transit gateway attachment, confirm the following:
+ You have an existing transit gateway. PBR is enabled on all transit gateways by default. No additional configuration is required to activate the feature.
+ You have the IAM permissions required to call the PBR APIs. At minimum, the following actions must be allowed on the relevant resources:
  + `ec2:CreateTransitGatewayPolicyTable`
  + `ec2:DescribeTransitGatewayPolicyTables`
  + `ec2:DeleteTransitGatewayPolicyTable`
  + `ec2:AssociateTransitGatewayPolicyTable`
  + `ec2:DisassociateTransitGatewayPolicyTable`
  + `ec2:GetTransitGatewayPolicyTableAssociations`
  + `ec2:GetTransitGatewayPolicyTableEntries`
  + `ec2:CreateTransitGatewayPolicyTableEntry`
  + `ec2:ModifyTransitGatewayPolicyTableEntry`
  + `ec2:DeleteTransitGatewayPolicyTableEntry`
+ The transit gateway route tables you intend to reference as targets already exist.
+ If an attachment is currently associated with a route table, you must disassociate it before associating a policy table.

For information about resource-level IAM conditions for PBR, see the example policy in [Associate a transit gateway policy table](tgw-policy-tables-associate.md).