Hello, and welcome to this lecture. We're on to cover some example scenarios of routine configurations in different VPC peering solutions.

For connectivity to exist between VPCs over a peered connection, abbreviated as a pcx, then specific routing must be set up on each side of the peer in the relevant subnets of the VPC to allow each VPC to know where to route traffic to.

Setting up routing on peered VPCs can only exist if the IP CIDR blocks do not overlap in any way. In each peered subnet that you want to communicate with, you must ensure that the route table being used in that subnet has a static route to either the peered VPC CIDR block or the CIDR block of the subnet in the peered VPC.

From this diagram, you can see that we have two subnets in each VPC. In VPC 1, we have a subnet A and B, and in VPC 2, subnet C and D. Subnet A in this VPC 1 has a static route in the table with a destination of the CIDR block of VPC 2 and a target of pcx-abcd1234, which is the peered gateway between the two VPCs.

Subnet B in the same VPC does not have a route pointing to the pcx. Similarly, in VPC 2, subnet C also has a route to VPC 1's CIDR block by the pcx, and subnet D does not. In this scenario, subnet A and VPC 1 has a route to the entire IP address space of VPC 2, and subnet C and VPC 2 has a route to the entire IP address space of VPC 1.

Therefore, these two subnets have a route to each other to enable connectivity. Although VPC 1 can route to the entire IP address space of VPC 2, which would include subnet D, subnet D does not have a route in its routing table to enable traffic to route to VPC 1. This is also true for VPC 2 communicating with subnet B.

For every subnet that requires connectivity to a peered VPC, a route must exist in the route table for that subnet pointing to the pcx. Let's now look at another example configuration of peering connections between VPCs. This time between three VPCs.

The peering connection between these VPCs offers connectivity to all VPCs from every other VPC. VPC 1 can communicate with 2 and 3, VPC 2 can communicate with 1 and 3, and VPC 3 can communicate with 1 and 2. In this scenario, three different peering connections are in place. As such, different pcx's are used to connect the same VPC from every other VPC. For example, the route table in VPC 3 uses pcx-wxyz6789 to communicate with VPC 1, whereas VPC 2 uses pcx-abcd1234 to communicate with VPC 1.

Care must be taken when configuring your route tables to ensure you use the correct pcx to route your traffic to peered VPCs in a multiple VPC peering scenario such as this. One final example of VPC peering. Again with three VPCs, but this time, not all the VPCs can communicate with all others plus two of the VPCs have identical CIDR blocks.

So, let's take a look at the routing in this scenario. Both VPC 1 and 3 have a peering with VPC 2. You will notice that VPC 1 and VPC 3 have identical VPC CIDR blocks as well as the same subnet CIDR range. The route table for the subnet in VPC 1 is set up correctly to point to VPC 2 via the correct target of the pcx.

However, the route table of the subnet in VPC 2 poses a problem. It states that for any traffic destined for the 10.0.0.0/16 network, then use the pcx which points to VPC 3 only. So, if a request comes from VPC 1 and a response is required, the response will be sent to VPC 3. Currently, VPC peering does not support unicast reverse path forwarding, which checks the source IP of packets and routes response packets back to the correct source.

To resolve this, you could rely on the longest periphics match roll when looking at route priorities. The route table of the 192.168.1.0/24 subnet could be changed to the following to include a new route. Now if any traffic from VPC 1 in the subnet 10.0.1.0/24 was destined for VPC 2, VPC 2 would respond correctly using the correct pcx of pcx-abcd1234, as this route has the longest periphics match.

For any other traffic on the 10.0.0.0/16 network, it will route to VPC 3. You could be more specific with your route table on VPC 2 to route to different subnets within VPC 1 and VPC 3 as shown on the route tables.

Coming up in the next lecture, I'm going to look at routing configuration when using a virtual product gateway for VPN connectivity.