ovn-bgp-agent/doc/source/contributor/bgp_advertising.rst

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.