18 BGP Best-Path Algorithm

BGP does not use a single metric. To choose the single best path to a destination, it evaluates a list of attributes in a fixed order, stopping at the first one that breaks the tie. This note is the reference for that ordered process; each attribute has its own note.

Preconditions

Before a route can even enter best-path comparison, its next-hop must be reachable. A route whose next-hop cannot be resolved is invalid and is not considered.

The ordered algorithm

Attributes are evaluated top to bottom. The first step that distinguishes the candidates decides the winner.

#	Attribute	Rule
1	Weight	Higher wins (Cisco-proprietary, local to the router)
2	Local Preference	Higher wins (AS-wide)
3	Locally originated	Prefer routes originated by this router (network / redistribute over aggregate)
4	AS-PATH	Shorter wins (fewer AS to cross)
5	Origin	IGP (i) > EGP (e) > Incomplete (?)
6	MED	Lower wins (by default only among routes from the same neighboring AS)
7	eBGP over iBGP	Prefer an eBGP-learned path over an iBGP-learned path
8	Next-hop IGP metric	Prefer the path with the lowest IGP metric to the BGP next-hop
9	Multipath	If multipath is configured, install multiple equal paths for load sharing
10	Age	Prefer the oldest (most stable) eBGP route
11	Router ID	Prefer the lowest Router ID (or lowest Originator ID for reflected routes)
12	Cluster List	Prefer the shortest Cluster List
13	Neighbor IP	Prefer the route from the neighbor with the lowest IP address

Reading the order sensibly

A few groupings make the list easier to remember:

• Steps 1 to 3 are about local and locally-originated preference: Weight (this router), Local Preference (this AS), and routes this router itself originated.
• Steps 4 to 6 are the classic path attributes: AS-PATH, Origin, MED.
• Step 7 prefers external paths over internal ones.
• Steps 8 onward are tiebreakers used when everything above is equal, ending in deterministic ties (Router ID, Cluster List, neighbor IP).

Key reminders tied to the order

• Weight comes before Local Preference, so a higher Weight on one router can override a better Local Preference.
• MED (step 6) is only compared among routes from the same neighboring AS by default (always-compare-med changes this).
• Origin (step 5) prefers i over e over ?.
• The final steps guarantee a single deterministic winner even when all real attributes tie.

Self-check

Q1 — What is the first attribute checked in best-path?

A) Local Preference
B) Weight
C) AS-PATH
D) MED

Respuesta

B is correct. Weight is checked first (higher wins). Because it is evaluated before Local Preference, a higher Weight on a router can override a better Local Preference.

• A) False — Local Preference is second, after Weight.
• C) False — AS-PATH is step 4.
• D) False — MED is step 6.

Q2 — In what order are Weight and Local Preference evaluated, and which value wins each?

A) Local Preference first, then Weight; lower wins both
B) Weight first (higher wins), then Local Preference (higher wins)
C) Weight first (lower wins), then Local Preference (lower wins)
D) They are evaluated together

Respuesta

B is correct. Weight is step 1 (higher wins), Local Preference is step 2 (higher wins). Weight being first is why it can override Local Preference.

• A) False — Weight is first, and higher (not lower) wins both.
• C) False — higher wins for both, not lower.
• D) False — they are separate, ordered steps.

Q3 — At the Origin step, which is most preferred?

A) Incomplete (?)
B) EGP (e)
C) IGP (i)
D) All equal

Respuesta

C is correct. Origin prefers IGP (i) over EGP (e) over Incomplete (?).

• A) False — Incomplete is the least preferred.
• B) False — EGP sits between IGP and Incomplete, not first.
• D) False — they are ranked.

Q4 — At step 7, which path is preferred?

A) iBGP over eBGP
B) eBGP over iBGP
C) The path with the higher MED
D) The path with the longer AS-PATH

Respuesta

B is correct. Step 7 prefers an eBGP-learned path over an iBGP-learned path.

• A) False — it is the other way around.
• C) False — MED (lower wins) is step 6, already passed.
• D) False — shorter AS-PATH is preferred, and that was step 4.

Q5 — What guarantees a single deterministic winner when all real attributes tie?

A) The route is dropped
B) The final tiebreakers: lowest Router ID / Originator ID, shortest Cluster List, lowest neighbor IP
C) Both routes are installed automatically
D) The oldest route is always chosen first

Respuesta

B is correct. The final steps (Router ID / Originator ID, Cluster List, neighbor IP) deterministically break any remaining tie.

• A) False — a winner is chosen, not dropped.
• C) False — installing multiple requires multipath (step 9), not a tie.
• D) False — Age (step 10) comes before the ID tiebreakers, but the ID steps are what guarantee determinism.