VPC peering configurations with specific routes
You can configure route tables for a VPC peering connection to restrict access to a subnet CIDR block, a specific CIDR block (if the VPC has multiple CIDR blocks), or a specific resource in the peer VPC. In these examples, a central VPC is peered to at least two VPCs that have overlapping CIDR blocks.
For examples of scenarios in which you might need a specific VPC peering connection configuration, see VPC peering scenarios. For more information about working with VPC peering connections, see Work with VPC peering connections. For more information about updating your route tables, see Update your route tables for a VPC peering connection.
Configurations
Two VPCs that access specific subnets in one VPC
In this configuration, there is a central VPC with two subnets (VPC A), a peering
connection between VPC A and VPC B (pcx-aaaabbbb), and a peering connection
between VPC A and VPC C (pcx-aaaacccc). Each VPC requires access to the
resources in only one of the subnets in VPC A.
The route table for subnet 1 uses VPC peering connection pcx-aaaabbbb
to access the entire CIDR block of VPC B.
The route table for VPC B uses pcx-aaaabbbb to access the CIDR block of subnet 1 in VPC A.
The route table for subnet 2 uses VPC peering connection pcx-aaaacccc
to access the entire CIDR block of VPC C.
The route table for VPC C table uses pcx-aaaacccc to access the CIDR block of subnet 2 in VPC A.
| Route table | Destination | Target |
|---|---|---|
| Subnet 1 (VPC A) | VPC A CIDR |
Local |
VPC B CIDR |
pcx-aaaabbbb | |
| Subnet 2 (VPC A) | VPC A CIDR |
Local |
VPC C CIDR |
pcx-aaaacccc | |
| VPC B | VPC B CIDR |
Local |
Subnet 1 CIDR |
pcx-aaaabbbb | |
| VPC C | VPC C CIDR |
Local |
Subnet 2 CIDR |
pcx-aaaacccc |
You can extend this configuration to multiple CIDR blocks. Suppose that VPC A and VPC B have both IPv4 and IPv6 CIDR blocks, and that subnet 1 has an associated IPv6 CIDR block. You can enable VPC B to communicate with subnet 1 in VPC A over IPv6 using the VPC peering connection. To do this, add a route to the route table for VPC A with a destination of the IPv6 CIDR block for VPC B, and a route to the route table for VPC B with a destination of the IPv6 CIDR of subnet 1 in VPC A.
| Route table | Destination | Target | Notes |
|---|---|---|---|
| Subnet 1 in VPC A | VPC A IPv4 CIDR |
Local | |
VPC A IPv6 CIDR |
Local | Local route that's automatically added for IPv6 communication within the VPC. | |
VPC B IPv4 CIDR |
pcx-aaaabbbb | ||
VPC B IPv6 CIDR |
pcx-aaaabbbb | Route to the IPv6 CIDR block of VPC B. | |
| Subnet 2 in VPC A | VPC A IPv4 CIDR |
Local | |
VPC A IPv6 CIDR |
Local | Local route that's automatically added for IPv6 communication within the VPC. | |
VPC C IPv4 CIDR |
pcx-aaaacccc | ||
| VPC B | VPC B IPv4 CIDR |
Local | |
VPC B IPv6 CIDR |
Local | Local route that's automatically added for IPv6 communication within the VPC. | |
Subnet 1 IPv4 CIDR |
pcx-aaaabbbb | ||
Subnet 2 IPv4 CIDR |
pcx-aaaabbbb | Route to the IPv6 CIDR block of VPC A. | |
| VPC C | VPC C IPv4 CIDR |
Local | |
Subnet 2 IPv4 CIDR |
pcx-aaaacccc |
Two VPCs that access specific CIDR blocks in one VPC
In this configuration, there is a central VPC (VPC A), a peering connection between VPC A and
VPC B (pcx-aaaabbbb), and a peering connection between VPC A and VPC C
(pcx-aaaacccc). VPC A has one CIDR block for each peering connection.
| Route table | Destination | Target |
|---|---|---|
| VPC A | VPC A CIDR 1 |
Local |
VPC A CIDR 2 |
Local | |
VPC B CIDR |
pcx-aaaabbbb | |
VPC C CIDR |
pcx-aaaacccc | |
| VPC B | VPC B CIDR |
Local |
VPC A CIDR 1 |
pcx-aaaabbbb | |
| VPC C | VPC C CIDR |
Local |
VPC A CIDR 2 |
pcx-aaaacccc |
One VPC that accesses specific subnets in two VPCs
In this configuration, there is a central VPC (VPC A) with one subnet, a peering connection
between VPC A and VPC B (pcx-aaaabbbb), and a peering connection between
VPC A and VPC C (pcx-aaaacccc). VPC B and VPC C each have two subnets. The
peering connection between VPC A and VPC B uses only one of the subnets in VPC B. The
peering connection between VPC A and VPC C uses only one of the subnets in VPC C.
Use this configuration when you have a central VPC that has a single set of resources, such as Active Directory services, that other VPCs need to access. The central VPC does not require full access to the VPCs that it's peered with.
The route table for VPC A uses the peering connections to access only specific subnets in the peered VPCs. The route table for subnet 1 uses the peering connection with VPC A to access the subnet in VPC A. The route table for subnet 2 uses the peering connection with VPC A to access the subnet in VPC A.
| Route table | Destination | Target |
|---|---|---|
| VPC A | VPC A CIDR |
Local |
Subnet 1 CIDR |
pcx-aaaabbbb | |
Subnet 2 CIDR |
pcx-aaaacccc | |
| Subnet 1 (VPC B) | VPC B CIDR |
Local |
Subnet in VPC A CIDR |
pcx-aaaabbbb | |
| Subnet 2 (VPC C) | VPC C CIDR |
Local |
Subnet in VPC A CIDR |
pcx-aaaacccc |
Routing for response traffic
If you have a VPC peered with multiple VPCs that have overlapping or matching CIDR blocks, ensure that your route tables are configured to avoid sending response traffic from your VPC to the incorrect VPC. AWS does not support unicast reverse path forwarding in VPC peering connections that checks the source IP of packets and routes reply packets back to the source.
For example, VPC A is peered with VPC B and VPC C. VPC B and VPC C have matching CIDR
blocks, and their subnets have matching CIDR blocks. The route table for subnet 2 in
VPC B points to the VPC peering connection pcx-aaaabbbb to access the
VPC A subnet. The VPC A route table is configured to send traffic destined for the
VPC CIDR to peering connection pcx-aaaaccccc.
| Route table | Destination | Target |
|---|---|---|
| Subnet 2 (VPC B) | VPC B CIDR |
Local |
Subnet in VPC A CIDR |
pcx-aaaabbbb | |
| VPC A | VPC A CIDR |
Local |
VPC C CIDR |
pcx-aaaacccc |
Suppose that an instance in subnet 2 in VPC B sends traffic to the Active Directory
server in VPC A using VPC peering connection pcx-aaaabbbb. VPC A sends
the response traffic to Active Directory server. However, the VPC A route table is
configured to send all traffic within the VPC CIDR range to VPC peering connection
pcx-aaaacccc. If subnet 2 in VPC C has an instance with the same IP
address as the instance in subnet two of VPC B, it receives the response traffic
from VPC A. The instance in subnet 2 in VPC B does not receive a response to its
request to VPC A.
To prevent this, you can add a specific route to the VPC A route table with the CIDR of
subnet 2 in VPC B as the destination and a target of pcx-aaaabbbb. The
new route is more specific, therefore traffic destined for the subnet 2 CIDR is
routed to the VPC peering connection pcx-aaaabbbb
Alternatively, in the following example, the VPC A route table has a route for each subnet for each VPC peering connection. VPC A can communicate with subnet B in VPC B and with subnet A in VPC C. This scenario is useful if you need to add another VPC peering connection with another subnet that falls within the same address range as VPC B and VPC C —you can simply add another route for that specific subnet.
| Destination | Target |
|---|---|
VPC A CIDR |
Local |
Subnet 2 CIDR |
pcx-aaaabbbb |
Subnet 1 CIDR |
pcx-aaaacccc |
Alternatively, depending on your use case, you can create a route to a specific IP address in VPC B to ensure that traffic routed back to the correct server (the route table uses longest prefix match to prioritize the routes):
| Destination | Target |
|---|---|
VPC A CIDR |
Local |
Specific IP address in subnet 2 |
pcx-aaaabbbb |
VPC B CIDR |
pcx-aaaacccc |
Instances in one VPC that access specific instances in two VPCs
In this configuration, there is a central VPC (VPC A) with one subnet, a peering
connection between VPC A and VPC B (pcx-aaaabbbb), and a peering connection
between VPC A and VPC C (pcx-aaaacccc). VPC A has a subnet with one
instance for each peering connection. You can use this configuration to limit peering
traffic to specific instances.
Each VPC route table points to the relevant VPC peering connection to access a single IP address (and therefore a specific instance) in the peer VPC.
| Route table | Destination | Target |
|---|---|---|
| VPC A | VPC A CIDR |
Local |
Instance 3 IP address |
pcx-aaaabbbb | |
Instance 4 IP address |
pcx-aaaacccc | |
| VPC B | VPC B CIDR |
Local |
Instance 1 IP address |
pcx-aaaabbbb | |
| VPC C | VPC C CIDR |
Local |
Instance 2 IP address |
pcx-aaaacccc |
One VPC that accesses two VPCs using longest prefix matches
In this configuration, there is a central VPC (VPC A) with one subnet, a peering connection
between VPC A and VPC B (pcx-aaaabbbb), and a peering connection between
VPC A and VPC C (pcx-aaaacccc). VPC B and VPC C have matching CIDR blocks.
You use VPC peering connection pcx-aaaabbbb to route traffic between VPC A
and a specific instance in VPC B. All other traffic destined for the CIDR address range
shared by VPC B and VPC C is routed to VPC C through pcx-aaaacccc.
VPC route tables use longest prefix match to select the most specific route across the
intended VPC peering connection. All other traffic is routed through the next matching
route, in this case, across the VPC peering connection pcx-aaaacccc.
| Route table | Destination | Target |
|---|---|---|
| VPC A | VPC A CIDR block |
Local |
Instance X IP address |
pcx-aaaabbbb | |
VPC C CIDR block |
pcx-aaaacccc | |
| VPC B | VPC B CIDR block |
Local |
VPC A CIDR block |
pcx-aaaabbbb | |
| VPC C | VPC C CIDR block |
Local |
VPC A CIDR block |
pcx-aaaacccc |
Important
If an instance other than instance X in VPC B sends traffic to VPC A, the response traffic might be routed to VPC C instead of VPC B. For more information, see Routing for response traffic.
Multiple VPC configurations
In this configuration, there is a central VPC (VPC A) is peered with multiple VPCs in a spoke configuration. You also have three VPCs (VPCs X, Y, and Z) peered in a full mesh configuration.
VPC D also has a VPC peering connection with VPC X (pcx-ddddxxxx). VPC A
and VPC X have overlapping CIDR blocks. This means that peering traffic between VPC A
and VPC D is limited to a specific subnet (subnet 1) in VPC D. This is to ensure that if
VPC D receives a request from VPC A or VPC X, it sends the response traffic to the
correct VPC. AWS does not support unicast reverse path forwarding in VPC peering
connections that checks the source IP of packets and routes reply packets back to the
source. For more information, see Routing for response traffic.
Similarly, VPC D and VPC Z have overlapping CIDR blocks. Peering traffic between VPC D and VPC X is limited to subnet 2 in VPC D, and peering traffic between VPC X and VPC Z is limited to subnet 1 in VPC Z. This is to ensure that if VPC X receives peering traffic from VPC D or VPC Z, it sends the response traffic back to the correct VPC.
The route tables for VPCs B, C, E, F, and G point to the relevant peering connections
to access the full CIDR block for VPC A, and the VPC A route table points to the
relevant peering connections for VPCs B, C, E, F, and G to access their full CIDR
blocks. For peering connection pcx-aaaadddd, the VPC A route table routes
traffic only to subnet 1 in VPC D and the subnet 1 route table in VPC D points to the
full CIDR block of VPC A.
The VPC Y route table points to the relevant peering connections to access the full CIDR blocks of VPC X and VPC Z, and the VPC Z route table points to the relevant peering connection to access the full CIDR block of VPC Y. The subnet 1 route table in VPC Z points to the relevant peering connection to access the full CIDR block of VPC Y. The VPC X route table points to the relevant peering connection to access subnet 2 in VPC D and subnet 1 in VPC Z.
| Route table | Destination | Target |
|---|---|---|
| VPC A | VPC A CIDR |
Local |
VPC B CIDR |
pcx-aaaabbbb | |
VPC C CIDR |
pcx-aaaacccc | |
Subnet 1 CIDR in VPC D |
pcx-aaaadddd | |
VPC E CIDR |
pcx-aaaaeeee | |
VPC F CIDR |
pcx-aaaaffff | |
VPC G CIDR |
pcx-aaaagggg | |
| VPC B | VPC B CIDR |
Local |
VPC A CIDR |
pcx-aaaabbbb | |
| VPC C | VPC C CIDR |
Local |
VPC A CIDR |
pcx-aaaacccc | |
| Subnet 1 in VPC D | VPC D CIDR |
Local |
VPC A CIDR |
pcx-aaaadddd | |
| Subnet 2 in VPC D | VPC D CIDR |
Local |
VPC X CIDR |
pcx-ddddxxxx | |
| VPC E | VPC E CIDR |
Local |
VPC A CIDR |
pcx-aaaaeeee | |
| VPC F | VPC F CIDR |
Local |
VPC A CIDR |
pcx-aaaaaffff | |
| VPC G | VPC G CIDR |
Local |
VPC A CIDR |
pcx-aaaagggg | |
| VPC X | VPC X CIDR |
Local |
Subnet 2 CIDR in VPC D |
pcx-ddddxxxx | |
VPC Y CIDR |
pcx-xxxxyyyy | |
Subnet 1 CIDR in VPC Z |
pcx-xxxxzzzz | |
| VPC Y | VPC Y CIDR |
Local |
VPC X CIDR |
pcx-xxxxyyyy | |
VPC Z CIDR |
pcx-yyyyzzzz | |
| VPC Z | VPC Z CIDR |
Local |
VPC Y CIDR |
pcx-yyyyzzzz | |
VPC X CIDR |
pcx-xxxxzzzz |
