BGP Best-Path Selection: The Full 13-Step Order

Why this order matters

BGP is a policy-based routing protocol: the “best path” is whatever the operator says it is. Cisco’s implementation walks a fixed decision tree of attributes so that the operator can predict, exactly, which path a change will select. If you can’t recite the order, you can’t design predictable inbound/outbound policy.

The full 13-step order

Cisco IOS / IOS-XE / NX-OS BGP compares two paths in this order. Whichever step first produces a clear winner ends the comparison.

The mnemonic

Most textbooks use variants of:

N W L L A O M E I O R N

Next-hop · Weight · Local pref · Locally originated · AS-path · Origin · MED · eBGP > iBGP · IGP metric · Oldest · Router-ID · Neighbor

Or a shorter version most engineers actually remember for the top 4:

W L A M: Weight, Local pref, AS-path, MED.

If you can order those four and remember eBGP > iBGP sits between them and the tie-breakers, the rest falls into place.

The three attributes you’ll see 80% of the time

1. Weight (higher wins)

Cisco-only. Set per-router with a route-map inbound. Not advertised anywhere. Use it when you want this router to prefer a path without affecting the rest of the AS.

route-map PREFER-ISP1 permit 10
 set weight 200
!
router bgp 65001
 neighbor 203.0.113.1 route-map PREFER-ISP1 in

2. Local Preference (higher wins)

Advertised to iBGP neighbors, so the whole AS agrees on the exit. Use for outbound policy, “the whole company should exit via ISP1 unless ISP1 is down.”

route-map OUT-VIA-ISP1 permit 10
 set local-preference 300
!
router bgp 65001
 neighbor 203.0.113.1 route-map OUT-VIA-ISP1 in

3. AS_PATH prepending (longer loses)

Add your AS number multiple times to make a path look worse to neighbors, so they prefer someone else. Use for inbound policy, “please, ISP2, don’t send me traffic on this prefix.”

route-map PREPEND-OUT permit 10
 set as-path prepend 65001 65001 65001
!
router bgp 65001
 neighbor 203.0.113.2 route-map PREPEND-OUT out

Worked example

R1 has three eBGP paths to 10.0.0.0/24:

Walk the algorithm:

1. Weight: ISP-C wins (100 vs 0). Comparison ends. Winner: ISP-C.

If Weight had been equal, we’d have compared Local Pref (ISP-A and ISP-B tie at 200 > ISP-C at 100 → ISP-C drops). Then AS-path: ISP-B (length 1) beats ISP-A (length 2). Winner would have been ISP-B.

Understand: weight is checked FIRST, so a locally-set weight overrides an AS-wide local-preference decision. This is often surprising.

Common exam / real-world mistakes

1. Confusing weight and local-preference direction. Weight = local (never leaves the router). Local pref = AS-wide (advertised iBGP). MED = “hint to neighbor AS” (advertised eBGP outbound but not further).
2. Setting MED when AS_PATH varies. MED is only compared if the prefix is coming from the same neighbor AS. Otherwise BGP skips step 6.
3. Assuming lower LP wins. Higher LP wins. Trap for exam-day nerves.
4. Forgetting the next-hop reachability check. If the next-hop advertised in BGP isn’t in your IGP, the route is invalid regardless of any attribute. Common iBGP mistake with route-reflectors.
5. Never using bgp bestpath as-path multipath-relax when you want ECMP over eBGP paths from different upstream ASes.

Cheat strip

Order  N W L L A O M E I O R N
       Next-hop reach  →  Weight (hi)  →  Local pref (hi)
       →  Locally originated  →  AS-path (short)  →  Origin (i > e > ?)
       →  MED (lo, same-AS only)  →  eBGP > iBGP  →  IGP metric (lo)
       →  Oldest eBGP  →  Router-ID (lo)  →  Neighbor (lo)

Top 3:  Weight (local, hi wins)
        Local pref (AS-wide, hi wins)
        AS-path length (short wins)

Verify:  show bgp ipv4 unicast | show ip bgp <prefix>
         show ip bgp neighbors <ip> advertised-routes