Allow multiple external gateways on a router


This spec proposes to allow a Neutron router to have multiple external gateways, so we can represent router designs where the router has multiple legs towards the outside world for reasons of higher capacity (typically load balanced between these legs) and higher availability (tolerating the failure of a router uplink or a router gateway port).

Problem Description

Below we describe one cloud design that required us to allow multiple external gateways on a single router. However the feature proposed in this spec is in no way specific to this design, this spec only strives to allow multiple external gateways for any design that needs it.

The Neutron router could be an active-active HA router. Compute hosts are connected to the Neutron router via an MLAG, the two sides of the router present a single IP to the computes. There is a single logical router one hop away from the Neutron router towards the external world (from here on: datacenter gateway) which is also an active-active HA router. The Neutron router and the datacenter gateway are interconnected by 2x2 point-to-point links (from each side to each side). The Neutron router in this setup has 4 links towards the external world.

+--+   +--+
|R3|   |R4| Datacenter gateway
+--+   +--+
 | \   / |
 |  \ /  |
 |   X   |  2x2 point-to-point links
 |  / \  |
 | /   \ |
+--+   +--+
|R1|   |R2| Neutron router
+--+   +--+
 | \   / |
 |  \ /  |
 |   X   |  MLAG
 |  / \
 | /
|C1|   ...  Compute hosts

Proposed Change

Overall Picture

The information in an external_gateway_info structure is used to create a gateway port. A gateway port is a router leg, where beyond packet forwarding, we perform SNAT (if enabled) and DNAT (for floating IPs and port forwardings) and which influences the default route of that router. Given multiple external_gateway_info structures we can straightforwardly create multiple gateway ports. We can also straightforwardly install the directly connected routes of the additional external gateways.

With only one external gateway - regarding packet forwarding - naturally only the internal-internal and internal-external directions exist. However with multiple external gateways the external-external direction also becomes possible. We don’t have a use case for external-external forwarding. However for the sake of simplicity of implementation this spec proposes to support that, that is to allow packet forwading from one external gateway to another. If required, a follow-up spec may propose finer control of external-external packet forwarding.

However the default route of a router cannot be multiplied without complex consequences (e.g. load balancing between multiple nexthops). So for the sake of simplicity for each router we retain one external gateway as special. The special one is used to set the default route of a router as before, while the other external gateways have no influence on the default route.

NATting and reservation of floating IPs can be performed for each external gateway as usual, but please see the ‘Out of Scope’ section.

The first implementation targets centralized routers on l3-agent.

For the special, first gateway port we keep adding the same routes as before.

The usual set of implicitly managed routes in a Neutron router are:

  • One directly connected route for each router interface’s each subnet.

  • One directly connected route for each gateway port’s each subnet.

  • One default route to the single gateway port’s subnet’s gateway_ip.

Keeping to this logic, when we add further gateway ports, we also add a directly connected route for each additional gateway port’s each subnet. Therefore the traffic destined to these subnets will be sent already via the respective gateway ports. Also traffic may reach the gateway ports from outside. However the majority of traffic will be sent via the default route.

Out of Scope

First of all, overall support for active-active HA routers by the l3-agent is not targeted here.

We intentionally only add a default route for the first, special gateway port(‘s subnet’s gateway_ip). We do not add default routes for the other gateway ports(’ subnet’s gateway_ip). Further management of routes (to make the additional external gateways actually used) is left to:

  • Managing additional routes via the extraroutes API.

  • Future improvements to neutron-dynamic-routing so a Neutron router can also receive (not just advertise) additional routes. [1]

Since the extraroutes API is already available, we believe that some use of the multiple external gateways can be realized as soon as this feature is merged.

The advertisement of any routes with the newly introduced additional external gateways as nexthops (for tenant networks or floating IPs) is also out of scope here. We believe that advertising such routes via routing protocols clearly makes sense, however:

  • This can be covered by a later spec in neutron-dynamic-routing and

  • basic use of the changes proposed here is possible without routing protocols.

Backwards Compatibility

We propose to store and expose the external gateways a bit redundantly to make backwards compatibility easier. Where possible (API, DB, RPC) keep the current scalar external gateway (or gateway port) attribute of a router as is. But also add a new router attribute for the plural: external gateways or gateway ports which contain a list of (not just the rest but) all such objects. So the object in the scalar attribute is present in this list too - preferably as the first element of the list.

This is just a high level approach that aims to leave all code unchanged where we only need to keep backwards compatible behavior.

DB Impact

The current DB schema contains the router - external gateway relations somewhat redundantly.

First in the routerports table there is no constraint on the number of ports with type network:router_gateway belonging to one router. Today we only store at most one network:router_gateway port for each router, but the DB schema allows more. [2]

Second the gw_port_id column of the routers table is a scalar. Today it stores the same port uuid as we have in the routerports table. [3]

We propose:

  • To keep the SQL schema as is, but start storing multiple network:router_gateway ports in the routerports table.

  • To also keep routers.gw_port_id scalar and there store the one special, backwards compatible external gateway (so this one is present both in the routerports table and in routers.gw_port_id).

  • To extend the neutron.db.models.l3.Router class with new attribute gw_ports that map to all relevant network:router_gateway ports stored in the routerports table.


Introduce a new API extension called multiple-external-gateways.

This extension adds a new router attribute: external_gateways. Which is a list of external_gateway_info structures, for example:

  {"network_id": ...,
   "external_fixed_ips": [{"ip_address": ..., "subnet_id": ...}, ...],
   "enable_snat": ...},

The first element in the list is special:

  • It is always the same as the original external_gateway_info.

  • It is the one that sets the default route of the Neutron router.

The order of the the rest of the list is irrelevant and ignored. Duplicates in the list (that is multiple external gateways with the same network_id) are not allowed.

Updating external_gateway_info also updates the first element of external_gateways and it leaves the rest of external_gateways unchanged. Setting external_gateway_info to an empty value also resets external_gateways to the empty list.

The external_gateways attribute cannot be set in POST /v2.0/routers or PUT /v2.0/routers/{router_id} requests, instead it can be managed via sub-methods:

  • PUT /v2.0/routers/{router_id}/add_external_gateways

    Accepts a list of external_gateway_info structures. Adding an external gateway to a network that already has one raises an error.

  • PUT /v2.0/routers/{router_id}/update_external_gateways

    Accepts a list of external_gateway_info structures. The external gateways to be updated are identified by the network_ids found in the PUT request. The external_fixed_ips and enable_snat fields can be updated. The network_id field cannot be updated.

  • PUT /v2.0/routers/{router_id}/remove_external_gateways

    Accepts a list of potentially partial external_gateway_info structures. Only the network_id field from external_gateway_info structure is used. The external_fixed_ips and enable_snat keys can be present but their values are ignored.

The add/update/remove PUT sub-methods respond with the whole router object just as POST/PUT/GET /v2.0/routers.

RPC Impact

The sync_routers message already has a gw_port field. Extend the message to also include a gw_ports field containing all gateway ports. Bump the rpc version of this message.

Upgrade Impact

The sync_routers RPC message will have a new version.

Client Impact

Relevant changes in osc and openstacksdk.


  • Unit tests.

  • Fullstack tests for l3-agent.

  • Tempest tests in neutron-tempest-plugin.



[1] [2] [3]