The BGP Best-Path Algorithm, Top to Bottom

You have met the path attributes one at a time: WEIGHT, LOCAL_PREF, AS_PATH, ORIGIN, MED. Now you get to see how they fit together. For any given prefix a BGP router may know several candidate paths, but it installs exactly one of them as best. The way it chooses is not a vote and not a score. It is an ordered list of tie-breakers, and the router walks that list from top to bottom, stopping the instant a step produces a single winner.

That "stop at the first decider" behavior is the whole trick. If WEIGHT already separates two paths, nothing below WEIGHT ever gets looked at. AS_PATH length, MED, IGP cost: all irrelevant the moment something higher up has decided. So the order is not trivia. It is the priority of your levers.

The precondition: a usable NEXT_HOP

Before a path is even allowed into the contest, its NEXT_HOP must be reachable. If your routing table has no route to the NEXT_HOP address, BGP marks that path as not a candidate and skips it entirely. A perfectly attractive path with an unreachable next hop loses by default, because it never enters the comparison. Keep this in mind: a "missing" best path is very often a NEXT_HOP resolution problem, not an attribute problem.

The ordered list

Here is the sequence FRR and Cisco use. For each step, the rule and which direction wins.

1. Highest WEIGHT. Cisco/FRR local-only knob, never advertised. Higher wins. Pure local preference on this one router.
2. Highest LOCAL_PREF. Carried to every iBGP router, so it sets an AS-wide preference. Higher wins.
3. Locally originated. A path this router originated itself (via network, redistribute, or aggregation) beats a path learned from a peer. Local origination wins.
4. Shortest AS_PATH. Fewer ASes usually means a shorter path through the internet. Lower count wins.
5. Lowest ORIGIN. The origin type, ordered i (IGP) < e (EGP) < ? (incomplete). Lower wins, so IGP-origin beats incomplete.
6. Lowest MED. A neighbor's hint about which of its entry points to use. Lower wins, but only when the paths come from the same neighbor AS.
7. eBGP over iBGP. An externally learned path beats an internally learned one, because eBGP gets you out of your own AS sooner. eBGP wins.
8. Lowest IGP metric to the NEXT_HOP. Among remaining ties, prefer the path whose NEXT_HOP is closest by your IGP. This is "hot-potato" routing: hand the traffic off as cheaply as possible. Lower metric wins.
9. Final tie-breakers. If still tied: prefer the oldest eBGP route (it is the most stable), then the path from the lowest router-id, then the lowest neighbor IP. These are deterministic so two routers never disagree.

Which lever maps to what

Read the list again as a toolbox and it organizes itself:

• WEIGHT is your local override, scoped to one router.
• LOCAL_PREF is your AS-wide outbound preference, the standard way to say "leave through this provider."
• AS_PATH is the natural metric, and AS_PATH prepending is how you make a path look worse to the outside world.
• ORIGIN and MED are finer hints, MED being the way one AS suggests an entry point to its neighbor.
• The bottom steps (eBGP-over-iBGP, IGP metric, tie-breakers) are what the router falls back on when you have not expressed any preference at all.

The further up a step sits, the stronger your control. That is exactly why the next lesson reaches for LOCAL_PREF.

A worked example

Your router learns prefix 203.0.113.0/24 from two places, and assume both next hops resolve fine.

• Path A: AS_PATH 65010 65020 65030, LOCAL_PREF 100, learned via eBGP.
• Path B: AS_PATH 65040 65030, LOCAL_PREF 100, learned via eBGP.

Walk the ladder. Step 1, WEIGHT: both default to 0, tie. Step 2, LOCAL_PREF: both 100, tie. Step 3, locally originated: neither, tie. Step 4, shortest AS_PATH: Path A has three ASes, Path B has two. Path B wins right here. The router stops. It never inspects ORIGIN, MED, or anything below, because step 4 already produced a single winner.

Now change one thing: suppose you had set LOCAL_PREF 200 on Path A. Then step 2 decides, Path A wins, and the longer AS_PATH never even matters. Same two paths, a different and higher-priority lever, opposite outcome. That is the algorithm in one picture.

The decision ladder

What's next

You now know the full ladder and, more importantly, that the top of it is where your strongest controls live. WEIGHT only affects one router, so the first attribute you reach for to steer a whole AS is the second rung: LOCAL_PREF. Next, in bgp-influence-local-pref, you will set LOCAL_PREF on inbound routes and watch the entire AS agree to leave through the exit you chose.