Kernel

Pathvector configures two BIRD kernel protocols that synchronise BIRD's routing tables with the operating system's routing tables:

Use these names with birdc, for example birdc show route export kernel4. (Older versions left the protocols unnamed, so BIRD called them kernel1 and kernel2.)

Kernel options live under the global kernel key, see the configuration reference:

kernel:
  table: 100              # kernel table to sync with
  scan-time: 10           # seconds between kernel table scans
  learn: false            # learn routes from the kernel into BIRD
  export: true            # export routes to the kernel
  reject-connected: false # don't export connected (RTS_DEVICE) routes
  accept4: []             # BIRD protocols to always export to the kernel (IPv4)
  reject4: []             # BIRD protocols to never export to the kernel (IPv4)

Multiple kernel tables

kernel.tables lists additional kernel routing tables to export routes to, for example for policy based routing or a management VRF. Every additional table gets the same export policy as the main kernel table (kernel.table, SRD communities, statics, source addresses, ...).

kernel:
  table: 254      # main table
  tables: [ 100, 200 ]

Because BIRD only allows one kernel protocol per BIRD routing table, Pathvector creates, for each additional table N and address family:

(and the same with 6 for IPv6). Routes are only exported to the additional tables, they are not learned from them.

Static routes

kernel.statics maps a prefix to a next hop (optionally with an interface, fe80::1%eth0). Static routes are placed in their own BIRD protocols, statics4 and statics6, separate from the locally originated prefixes (static4 / static6), and are always exported to the kernel when kernel.export is enabled, including when srd-communities or source4/source6 are configured.

kernel:
  statics:
    "198.51.100.0/24": 203.0.113.1
    "2001:db8:5::/48": "fe80::1%eth0"

Selective route download (SRD)

kernel.srd-communities limits which BGP routes are installed in the kernel: if the list is not empty, only routes carrying one of these communities (and kernel.statics) are exported to the kernel. This keeps the kernel FIB small when BIRD holds full tables.

kernel:
  srd-communities:
    - 65530:100

Default routes

With default-route: true (the default), Pathvector originates 0.0.0.0/0 and ::/0 reject (unreachable) routes in the BIRD protocols default4 and default6. They are used by the announce-default peer option and are also exported to the kernel, so traffic to destinations without a more specific route is dropped.

To install a default route learned from an upstream instead, enable accept-default. This

• lets default routes pass the prefix length filter (filter-prefix-length) on import, and
• lowers the preference of the default4/default6 routes to 1, so a learned default route becomes the best route and is installed in the kernel. The locally originated default route remains as a fallback if no default is learned.

accept-default: true
peers:
  Upstream:
    asn: 64496
    neighbors:
      - 203.0.113.1

Built-in protocols

Besides the kernel protocols, Pathvector always configures a BIRD device protocol (interface discovery) and a direct protocol (routes for directly connected networks). They can be tuned with global options:

• device-scan-time: seconds between interface scans (scan time), BIRD's default if unset.
• direct-check-link: true: only import connected routes of interfaces whose link is up (check link yes).
• kernel.scan-time, kernel.learn, kernel.reject-connected and the other kernel options described above.

When these options aren't enough, list built-in protocols in disable-protocols (any of device, direct, kernel4 and kernel6) to leave them out of the generated configuration, and define your own in global-config or a manual*.conf file in the BIRD directory:

device-scan-time: 10
direct-check-link: true
disable-protocols: [ kernel4, kernel6 ]
global-config: |
  protocol kernel kernel4 { ipv4 { import all; export where source != RTS_DEVICE; }; }
  protocol kernel kernel6 { ipv6 { import all; export where source != RTS_DEVICE; }; }

When you replace the kernel protocols, the kernel options (SRD communities, statics export, source addresses, ...) no longer apply to them.