Geographically Distributed Subnet

The Geographically distributed subnet service helps to implement the availability of infrastructure from the Internet between regions or locations using the same IP address space with automatic default gateway redundancy.

Glossary

Term Definition
VLAN (network) Unique networks isolated from each other at the Layer 2, created on the network devices of the location for delimitation of client network segments.
VLANs (networks) are created on network devices in each of the locations
Geographically distributed L2 domain A combined L2 domain of two VLANs in different locations, over which a common geographically distributed IP subnet works
Subnets (IP addresses) A range of public IP addresses limited by the size of a prefix (mask) provided by the client.
Subnets (IP addresses) are assigned to one geographically distributed L2 domain
VRRP address (VIP)
Virtual IP address
A virtual IP address that is used:
- on Selectel network devices as a default gateway or next-hop for client hosts
- on client servers that provide a highly available service

Learn more about networks for dedicated servers in the instruction.

What is Required to Enable the Service

Have dedicated VLANs available in each of the locations of the service.

Select the region where the master router will be located (through which routing from/to the Internet will be performed). We recommend choosing the region in which the main infrastructure is located.

Note: if, for example, most of the infrastructure that should be accessible from the Internet is located in the St. Petersburg region, then it is more optimal to choose St. Petersburg as the master in terms of routing (in particular, to minimize network delays).

Specify the subnet size (/29 and larger). The subnet will be allocated from a separate range. It is not possible to use existing subnets that are generally allocated in locations. Two IP addresses from the dedicated subnet will be taken on the Selectel network equipment for configuring VRRP for reserving the default gateway.

Schedule a convenient time for enabling the service, since the service is enabled with a few minutes of downtime for all subnets in the locations where the service is enabled.

VRRP is configured on the Selectel side by default. If you need to ensure not only the use of shared IP addressing, but also redundancy, the scheme can be converted into a fault-tolerant one by configuring VRRP on the client server side.

Existing Limitations

Limitations when enabling the service:

  • a distributed subnet can only cover 2 locations in one or two regions;
  • connection is not possible for servers located on a shared subnet;
  • standard IP subnets allocated to an account in each location cannot be converted to a geographically distributed subnet.

The connection is carried out on two IP addresses:

  • Virtual IP address (VIP) that is used on the master router;
  • IP address from a dedicated subnet on the second router that is additionally used as a service one.

How a Geographically Distributed Subnet Works

* VRRP for geographically distributed service redundancy configured by the client

The client’s servers in different locations will be accessible to each other via L2, since an L2 tunnel is organized on the equipment between the two routers via MPLS.

In the event of the infrastructure failure in one of the locations, the resource will still be available from the Internet in another location. Internet traffic will be directed to a reserved host (VRRP virtual IP address on the client side) to the remaining location.

Example

Let’s say there is an important Internet resource www.mysite.ru that must be accessible from the Internet. You also need to implement a physically distributed infrastructure between two locations/regions, with the ability to automatically redirect traffic from the Internet to an available location.

Let’s consider an example of creating such an infrastructure, which includes:

  • servers located in two different locations/regions of Selectel;
  • shared IP subnet for 2 locations;
  • VRRP providing default gateway redundancy and VRRP configured on client’s servers. Please note that this will ensure the availability of the Internet resource from any location in case of problems in another one.

The sequence of creating such an infrastructure:

  1. You need to order a dedicated VLAN in each location.
  2. When ordering the service, the dedicated VLANs in both locations are combined into a single geographically distributed L2 domain with a common IP addressing. An IP subnet is allocated from a special range when ordering the service.
  3. From the Selectel side, when enabling the service, VRRP is automatically configured on the network equipment with a master in one location and a backup in another (you need to specify the location for the master).
  4. You need to configure VRRP between hosts located in different locations on the servers. After that, the scheme becomes fault-tolerant.

    Please note that in order to implement VRRP from the client’s side, it can be used, for example, keepalived.

Ordering the Service

To order the service in the Control panel:

  1. Go to the Geographically distributed subnet tab in the Servers and colocation section.
  2. Click Order service.
  3. Select a service and click Pay.
  4. Specify the payment period in the new window.
  5. Click Pay for Service.