Sunday, September 11, 2016

AWS VPC VGW Multipath Routing - difference between Direct Connect and VPN

VPC VGW multi-path scenario
To connect a VPC to enterprise networks or other VPCs, we use Direct Connect or VPN. It is common to have multiple connection paths from a VPC. Routing outbound from a VPC is controlled by VGW. The question is, how does VGW which is an AWS internal logical router handle multi-path routing?

Multi-path is a requirement for high availability. Load sharing on multi-path is often desirable. How VGW handles multi-path routing is actually different based on connection type. Specifically, Direct Connect supports ECMP. VPN does not (after Oct 2015).

Direct Connect
Direct Connect supports the configuration option of redundant paths with Active/Active (BGP multipath), VGW routes traffic over multiple equal cost paths. As a result, we can leverage all bandwidth resources provisioned for DX.

With VPN, VGW currently does not support BGP multipath. VPN chooses one BGP path only.

What if we use static route instead of BGP, can static be used to load share traffic across multiple paths?
In the scenario shown in the diagram, there are dual VPN connections going to two remote CGWs, each with redundant tunnels. If static routes are defined equally, does VGW route ECMP out multiple paths?
  • VGW created prior to Oct 28 2015 supports static multipath.
  • VGW created after Oct 28 2015 selects one active path out of multiple paths defined

The scenario is tested with a new VGW in one VPC, and a pair of customer VPN appliances in aonther VPC. With 4 tunnels/paths, it seems all traffic goes to one tunnel only. AWS support confirmed the behavior that VGW only selects one path only.

Why AWS should support VPN multipath
With VPN, it may be desirable to spread load across multiple customer gateways, because those customer gateways may be Cisco or Palo Alto appliances that has licensed throughput capacity. It is more optimal to spread load across multiple destinations rather than sending all traffic to one while other paths sit idle.

Hopefully AWS will bring consistent multipath routing to VPN, with BGP multipath and static ECMP.

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