# Distributed Virtual Routing DVR

<code class="expression">space.vars.product\_name</code> Networking Service uses Distributed Virtual Routing (DVR) as the primary mode to configure networking under the hood. This document describes DVR.

DVR configuration has the following characteristics:

* Distributed Layer 3 (L3) space across all hypervisor hosts. No dedicated network nodes required.
* North-South and East-West traffic is handled at each hypervisor host.

## Advantages of DVR

* High performance (L3 at compute node)
* Single hop Floating IP (FIP) i.e. North-South traffic
* East-West traffic also is routed at source
* Failure domain reduced to a single compute node
* Great load sharing in terms of L3 routing & DHCP servers
* Easier for maintenance when it comes to re-purposing the otherwise central network node

## DVR Deployment

Components in a DVR deployment are distributed as shown below.

![](https://content.gitbook.com/content/SNWOoFOMzRblbHdwmlrR/blobs/8DymD8sBXVSQoQM3yhjC/1605050264.png)

The Network components in the above picture consist of the dhcp-agent and the l3-agent. DHCP agent is responsible for managing DHCP servers for tenant networks along with DNS service while the L3 agent is responsible for routing i.e. cross networks both east-west and north-south traffic. You can assign one or more nodes to run SNAT. VMs without Floating IPs will connect external to the cloud through this SNAT service node. VMs with floating IPs however will directly communicate external to the cloud from the host/server it is running in.

Following table lists the key characteristics of DVR:

| **Feature**      | **Feature Detail**       | **Distributed Virtual Routing - L3**                                                                                                                |
| ---------------- | ------------------------ | --------------------------------------------------------------------------------------------------------------------------------------------------- |
| DHCP server      | Setup                    | On any hypervisor. User has the control to choose which hypervisors host DHCP servers. This is scalable to 'N' where N is the number of hypervisors |
|                  | Failure Domain           | Active-Active                                                                                                                                       |
|                  | Failover                 | Moves to another hypervisor/node with DHCP service enabled assuming there are > 2                                                                   |
| L3 or IP Routing | Setup                    | On all hypervisors for East West traffic and on SNAT enabled hypervisors for North South traffic.                                                   |
|                  | SNAT                     | On a particular hypervisor. Load balanced to the hypervisors enabled with SNAT                                                                      |
|                  | Floating / Elastic IPs   | On the specific hypervisor where VM runs.                                                                                                           |
|                  | North-South traffic flow | SNAT through SNAT hypervisor but when using FIP, directly through hypervisor running the VM.                                                        |
|                  | East-West traffic flow   | Directly at hypervisor.                                                                                                                             |
|                  | Redundancy               | Active-active only makes sense for SNAT. (In our roadmap)                                                                                           |
|                  | Failure Domain           | Loss of hypervisor alone. If hypervisor running SNAT, loss of SNAT till failover.                                                                   |
|                  | Failover                 | SNAT router failover to other hypervisor after timeout.                                                                                             |
| IP usage         | External IPs             | 1 for external n/w gateway interface per L3 virtual router (SNAT), 1 each for FIP gateway/namespace (=no. of hypervisors) + 1 FIP per VM.           |
|                  | Internal IPs             | 2 per tenant network per router (1 for SNAT/ 1 for Gateway)                                                                                         |


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