[NB DB] NB OVN BGP Agent: Design of the BGP Driver with OVN routing¶

This is an extension of the NB OVN BGP Driver which adds a new exposing_method named ovn to make use of OVN routing, instead of relying on Kernel routing.

Purpose¶

This document presents the design decision behind the extensions on the NB OVN BGP Driver to support OVN routing instead of kernel routing, and therefore enabling datapath acceleartion.

Overview¶

The main goal is to make the BGP capabilities of OVN BGP Agent compliant with OVS-DPDK and HWOL. To do that we need to move to OVN/OVS what the OVN BGP Agent is currently doing with Kernel networking – redirect traffic to/from the OpenStack OVN Overlay.

To accomplish this goal, the following is required:

Ensure that incoming traffic gets redirected from the physical NICs to the OVS integration bridge (br-int) though one or more OVS provider bridges (br-ex) without using kernel routes and rules.
Ensure the outgoing traffic gets redirected to the physical NICs without using the default kernel routes.
Expose the IPs in the same way as we did before.

The third point is simple as it is already being done, but for the first two points OVN virtual routing capabilities are needed, ensuring the traffic gets routed from the NICS to the OpenStack Overlay and vice versa.

Proposed Solution¶

To avoid placing kernel networking in the middle of the datapath and blocking acceleration, the proposed solution mandates locating a separate OVN cluster on each node that manages the needed virtual infrastructure between the OpenStack networking overlay and the physical network. Because routing occurs at OVN/OVS level, this proposal makes it is possible to support hardware offloading (HWOL) and OVS-DPDK.

The next figure shows the proposed cluster required to manage the OVN virtual networking infrastructure on each node.

In a standard deployment br-int is directly connected to the OVS external bridge (br-ex) where the physical NICs are attached. By contrast, in the default BGP driver solution (see [NB DB] NB OVN BGP Agent: Design of the BGP Driver with kernel routing), the physical NICs are not directly attached to br-ex, but rely on kernel networking (ip routes and ip rules) to redirect the traffic to br-ex. The OVN routing architecture proposes the following mapping:

br-int connects to an external (from the OpenStack perspective) OVS bridge (br-osp).
br-osp does not have any physical resources attached, just patch ports connecting them to br-int and br-bgp.
br-bgp is the integration bridge managed by the extra OVN cluster deployed per node. This is where the virtual OVN resources are be created (routers and switches). It creates mappings to br-osp and br-ex (patch ports).
br-ex keeps being the external bridge, where the physical NICs are attached (as in default environments without BGP). But instead of being directly connected to br-int, is connected to br-bgp. Note for ECMP purposes, each nic is attached to a different br-ex device (br-ex and br-ex-2).

The virtual OVN resources requires the following:

Logical Router (bgp-router): manages the routing that was previously done in the kernel networking layer between both networks (physical and OpenStack OVN overlay). It has two connections (i.e., Logical Router Ports) towards the bgp-ex-X Logical Switches to add support for ECMP (only one switch is required but you must have several in case of ECMP), and one connection to the bgp-osp Logical Switch to ensure traffic to/from the OpenStack networking overlay.
Logical Switch (bgp-ex): is connected to the bgp-router, and has a localnet to connect it to br-ex and therefore the physical NICs. There is one Logical Switch per NIC (bgp-ex and bgp-ex-2).
Logical Switch (bgp-osp): is connected to the bgp-router, and has a localnet to connect it to br-osp to enable it to send traffic to and from the OpenStack OVN overlay.

The following OVS flows are required on both OVS bridges:

br-ex-X bridges: require a flow to ensure only the traffic targetted for OpenStack provider networks is redirected to the OVN cluster.

cookie=0x3e7, duration=942003.114s, table=0, n_packets=1825, n_bytes=178850, priority=1000,ip,in_port=eth1,nw_dst=172.16.0.0/16 actions=mod_dl_dst:52:54:00:30:93:ea,output:"patch-bgp-ex-lo"

br-osp bridge: require a flow for each OpenStack provider network to change the MAC by the one on the router port in the OVN cluster and to properly manage traffic that is routed to the OVN cluster.
cookie=0x3e7, duration=942011.971s, table=0, n_packets=8644, n_bytes=767152, priority=1000,ip,in_port="patch-provnet-0" actions=mod_dl_dst:40:44:00:00:00:06,NORMAL

OVN NB DB Events¶

The OVN northbound database events that the driver monitors are the same as the ones for the NB DB driver with the underlay exposing mode. See [NB DB] NB OVN BGP Agent: Design of the BGP Driver with kernel routing. The main difference between the two drivers is that the wiring actions are simplified for the OVN routing driver.

Driver Logic¶

As with the other BGP drivers or exposing modes ([SB DB] OVN BGP Agent: Design of the BGP Driver with kernel routing, [NB DB] NB OVN BGP Agent: Design of the BGP Driver with kernel routing) the NB DB Driver with the ovn exposing mode enabled (i.e., enabling OVN routing instead of rely on Kernel networking) is in charge of exposing the IPs with BGP and of the networking configuration to ensure that VMs abd LBs on provider networks or with FIPs can be reached through BGP (N/S traffic). Similarly, if expose_tenant_networks flag is enabled, VMs in tenant networks should be reachable too – although instead of directly in the node they are created, through one of the network gateway chassis nodes. The same happens with expose_ipv6_gua_tenant_networks but only for IPv6 GUA ranges. In addition, if the config option address_scopes is set only the tenant networks with matching corresponding address_scope will be exposed.

To accomplish this, it needs to configure the extra per node ovn cluster to ensure that:

VM and LBs IPs can be advertized in a node where the traffic could be injected into the OVN overlay through the extra ovn cluster (instead of the Kernel routing) – either in the node hosting the VM or the node where the router gateway port is scheduled.
Once the traffic reaches the specific node, the traffic is redirected to the OVN overlay by using the extra ovn cluster per node with the proper OVN configuration. To do this it needs to create Logical Switches, Logical Routers and the routing configuration between them (routes and policies).

BGP Advertisement¶

The OVN BGP Agent (both SB and NB drivers) is in charge of triggering FRR (IP routing protocol suite for Linux which includes protocol daemons for BGP, OSPF, RIP, among others) to advertise/withdraw directly connected routes via BGP. To do that, when the agent starts, it ensures that:

FRR local instance is reconfigured to leak routes for a new VRF. To do that it uses vtysh shell. It connects to the existsing FRR socket ( --vty_socket option) and executes the next commands, passing them through a file (-c FILE_NAME option):

router bgp {{ bgp_as }}
  address-family ipv4 unicast
    import vrf {{ vrf_name }}
  exit-address-family

  address-family ipv6 unicast
    import vrf {{ vrf_name }}
  exit-address-family

router bgp {{ bgp_as }} vrf {{ vrf_name }}
  bgp router-id {{ bgp_router_id }}
  address-family ipv4 unicast
    redistribute connected
  exit-address-family

  address-family ipv6 unicast
    redistribute connected
  exit-address-family

There is a VRF created (the one leaked in the previous step), by default with name bgp-vrf.
There is a dummy interface type (by default named bgp-nic), associated to the previously created VRF device.
Ensure ARP/NDP is enabled at OVS provider bridges by adding an IP to it.

Then, to expose the VMs/LB IPs as they are created (or upon initialization or re-sync), since the FRR configuration has the redistribute connected option enabled, the only action needed to expose it (or withdraw it) is to add it (or remove it) from the bgp-nic dummy interface. Then it relies on Zebra to do the BGP advertisement, as Zebra detects the addition/deletion of the IP on the local interface and advertises/withdraws the route:

$ ip addr add IPv4/32 dev bgp-nic
$ ip addr add IPv6/128 dev bgp-nic
Note

As we also want to be able to expose VM connected to tenant networks (when expose_tenant_networks or expose_ipv6_gua_tenant_networks configuration options are enabled), there is a need to expose the Neutron router gateway port (cr-lrp on OVN) so that the traffic to VMs in tenant networks is injected into OVN overlay through the node that is hosting that port.

Traffic Redirection to/from OVN¶

As explained before, the main idea of this exposing mode is to leverage OVN routing instead of kernel routing. For the traffic going out the steps are the next:

If (OpenStack) OVN cluster knows about the destination MAC then that works as in deployment without BGP or OVN cluster support (no arp needed, MAC directly used). If the MAC is unknown but on the same provider network(s) range, the ARP gets replied by the Logical Switch Port on the bgp-osp LS thanks to enabling arp_proxy on it. And if it is a different range, it will reply due to the router having default routes to the outside. The flow at br-osp is in charge of changing the destination MAC by the one on the Logical Router Port on bgp-router LR.
The previous step takes the traffic to the extra OVN cluster per node, where the default (ECMP) routes are used to send the traffic to the external Logical Switch and from there to the physical nics attached to the external OVS bridge(s) (br-ex, br-ex-2). In case of known MAC by OpenStack, instead of the default routes, a Logical Route Policy gets applied so that traffic is forced to be redirected out (through the LRPs connected to the external LS) when comming through the internal LRP (the one connected to OpenStack).

And for the traffic comming in:

The flow hits the ovs flow added at the br-ex-X bridge(s) to redirect the traffic to the per node OVN cluster, changing the destination MAC by the one at the related br-ex device, which are the same used for the OVN cluster Logical Router Ports. This takes the traffic to the OVN router.
After that, thanks to having the arp_proxy enabled on the LSP on bgp-osp the traffic will be redirected to there. And due to a limitation in the functionality of arp_proxy, there is a need of adding an extra static mac binding entry in the cluster so that the VM MAC is used for destination instead of the own LSP MAC, which would lead to droping the traffic on the LS pipeline.
```
_uuid               : 6e1626b3-832c-4ee6-9311-69ebc15cb14d
ip                  : "172.16.201.219"
logical_port        : bgp-router-openstack
mac                 : "fa:16:3e:82:ee:19"
override_dynamic_mac: true
```

Driver API¶

This is the very same as in the NB DB driver with the underlay exposing mode. See [NB DB] NB OVN BGP Agent: Design of the BGP Driver with kernel routing.

Agent deployment¶

The deployment is similar to the NB DB driver with the underlay exposing method but with some extra configuration. See [NB DB] NB OVN BGP Agent: Design of the BGP Driver with kernel routing for the base.

It is needed to state the exposing method in the DEFAULT section and the extra configuration for the local ovn cluster that performs the routing, including the range for the provider networks to expose/handle:

[DEFAULT]
exposing_method=ovn

[local_ovn_cluster]
ovn_nb_connection=unix:/run/ovn/ovnnb_db.sock
ovn_sb_connection=unix:/run/ovn/ovnsb_db.sock
external_nics=eth1,eth2
peer_ips=100.64.1.5,100.65.1.5
provider_networks_pool_prefixes=172.16.0.0/16

Limitations¶

The following limitations apply:

OVN 23.06 or later is required
Tenant networks, subnet and OVN load balancers are not yet supported, and will require OVN vesion 23.09 or newer.
IPv6 not yet supported
ECMP not properly working as there is no support for BFD at the ovn-cluster, which means if one of the routes goes away the OVN cluster won’t react to it and there will be traffic disruption.
There is no support for overlapping CIDRs, so this must be avoided, e.g., by using address scopes and subnet pools.

[NB DB] NB OVN BGP Agent: Design of the BGP Driver with OVN routing