BGP path attributes and best path selection: weight, local preference, AS path, MED and more

Attribute types

Each BGP route carries path attributes. RFC 4271 groups them into four types:

• Well-known mandatory: in every update. AS_PATH, NEXT_HOP and ORIGIN.
• Well-known discretionary: understood by everyone but optional to send. LOCAL_PREF and ATOMIC_AGGREGATE.
• Optional transitive: passed on even by routers that do not understand them. COMMUNITIES and AGGREGATOR.
• Optional non-transitive: not passed on. MULTI_EXIT_DISC (MED).

Cisco adds weight, a value that never leaves the router.

The selection order

Before comparing anything, BGP ignores paths whose next hop is unreachable. Then, on Cisco IOS:

1. Highest weight. Default 32768 for routes the router originates, 0 for learned routes.
2. Highest local preference. Default 100, shared with all iBGP peers.
3. Locally originated (network, aggregate or redistribute) over learned.
4. Lowest AIGP (accumulated IGP metric), if used. It suits large networks spanning several ASes under one operator.
5. Shortest AS_PATH.
6. Lowest origin: IGP before EGP before incomplete.
7. Lowest MED, compared only between paths from the same neighbouring AS unless configured otherwise.
8. eBGP over iBGP.
9. Lowest IGP metric to the next hop.
10. Tie-breakers: oldest eBGP path, lowest router ID, shortest cluster list, lowest neighbour address.

Controlling traffic leaving your AS

Weight affects only one router, so it is useful when a single router has two providers. Local preference is the usual tool for the whole AS: set it on routes learned from your preferred provider and every internal router follows.

route-map FROM-ISP-A permit 10
 set local-preference 200
router bgp 64500
 neighbor 198.51.100.1 route-map FROM-ISP-A in

Influencing traffic entering your AS

You cannot directly control other networks' decisions, but you can make some paths look less attractive:

• AS_PATH prepending: add your own ASN several times to routes sent to the backup provider, so the path looks longer.
• MED: tell a neighbouring AS that has two links to you which link to prefer. Lower is better, and it only works with that one neighbour.
• Communities defined by your provider, and advertising more specific prefixes on the preferred link, are also common.

route-map TO-ISP-B permit 10
 set as-path prepend 64500 64500 64500
router bgp 64500
 neighbor 192.0.2.1 route-map TO-ISP-B out

Prepending more than three or four times rarely helps further, because many networks set local preference on customer routes, and that is checked before AS_PATH length.

Multipath

By default BGP installs one best path. maximum-paths 2 (eBGP) or maximum-paths ibgp 2 lets the router load-share across several paths, but only if they tie on weight, local preference, AS_PATH length, origin, MED and IGP metric. The best path is still the only one advertised to peers.

Real-world example: a hospital group had a fast fibre service from one provider and a cheaper backup from another. All outbound traffic used the backup, because its routes happened to have shorter AS paths. The engineer set local preference 200 on routes learned from the primary provider, which moved outbound traffic straight away. Inbound traffic still arrived via the backup, so he prepended the hospital's ASN three times on announcements to the backup provider. Within minutes most inbound traffic shifted to the primary link, leaving the backup idle until needed.