AWS VPC (Routing)

VPC Router

• Implicit (automatically created)
• Allows subnets talk to each other
• Special IP address .1 is reserved in each subnet (e.g. 10.0.0.1)

VPC Route Table (rtb)

• Local subnets do not need entries in route table to talk to each other
  • They do not "touch" router
• "Main" route table is special
  • when subnet not explicitly associated with rtb it is implicitely associated with Main route table
  • any route table can become "Main"
• Local route cannot be modified or deleted
• May contain Propagated Routes (via BGP)
  • Source
    • VPN Connection
    • Direct Connect (Gateway)
• Format
  • Destination: IP prefix
  • Target
    • Local
      • Traffic stays in the VPC
    • Instance (ENI)
    • IGW
    • VGW
    • VPC Endpoint (prefix list)
    • VPC Peering connection (pcx)

Route selection priority (same prefix)

• Local route to the VPC (no override)
  • Traffic to VPC does not leave VPC
• Longest prefix match
• Static route table entries
• Dynamic routes
  • Direct Connect
    • Shortest AS_PATH first
  • VPN
    • static routes
    • dynamic routes