Local Preference

Setting local preference on import

By default, Pathvector sets the BGP local preference of every route imported from a peer to the peer's local-pref (default 100). Related options:

• local-pref4 / local-pref6 override local-pref for one address family.
• set-local-pref: false disables setting the local preference on import.
• default-local-pref sets BIRD's default bgp_local_pref, which only applies to routes received without a local preference (eBGP), instead of overwriting it.

iBGP sessions

On iBGP, the local preference received from the neighbor usually carries policy decided at the network edge, so Pathvector doesn't overwrite it by default. A session is treated as iBGP when the peer's asn equals the global asn (or the peer's local-asn).

On an iBGP session, Pathvector only sets the local preference when one of local-pref, local-pref4, local-pref6 or set-local-pref is configured on the peer (or its template), or when optimize-inbound is enabled for the peer.

asn: 65530
peers:
  Core:            # iBGP, local preference received from the neighbor is kept
    asn: 65530
    neighbors:
      - 192.0.2.2
  Core override:   # iBGP, local preference explicitly set to 50
    asn: 65530
    local-pref: 50
    neighbors:
      - 192.0.2.3

Local preference by community

community-prefs maps a standard or large community to a local preference: routes carrying the community get that local preference on import.

community-prefs:
  "65530:0:200": 95
  "64496:100": 110

Local preference by AS

as-prefs maps an ASN to a local preference: routes whose AS path contains the ASN get that local preference on import. as-prefs4 and as-prefs6 do the same for only IPv4 or only IPv6 routes, and take precedence over as-prefs for their address family.

peers:
  Transit:
    asn: 64496
    neighbors:
      - 203.0.113.1
      - 2001:db8::1
    as-prefs:
      174: 90      # depreference paths via AS174 in both address families
    as-prefs6:
      6939: 120    # prefer IPv6 paths via AS6939

Local preference by prefix

prefix-prefs maps a prefix to a local preference: routes matching the prefix get that local preference on import. Keys use BIRD prefix pattern syntax, so 198.51.100.0/24 matches exactly that prefix, 198.51.100.0/24+ also matches more specifics and 2001:db8::/32{32,48} matches lengths 32 to 48. IPv4 and IPv6 prefixes can be mixed.

Together with as-prefs (AS path contains an ASN), this allows simple traffic engineering for a single peer without custom BIRD configuration:

peers:
  Transit A:
    asn: 64496
    neighbors:
      - 203.0.113.1
    prefix-prefs:
      "198.51.100.0/24+": 200        # prefer Transit A for this destination
      "2001:db8:1000::/36{36,48}": 200
    as-prefs:
      64511: 80                      # avoid paths through AS64511 via Transit A

For more complex matching (for example AS path regular expressions), use pre-import-accept with custom BIRD filter statements.

Precedence

When several options match the same route, they are evaluated in this order, and the last match wins:

1. local-pref (or the received value on iBGP, see above)
2. local-pref4 / local-pref6
3. community-prefs
4. as-prefs
5. as-prefs4 / as-prefs6
6. prefix-prefs

For example, with community-prefs: {"65530:0:200": 95} and as-prefs: {6939: 150}, a route via AS6939 tagged with 65530:0:200 gets a local preference of 150.