Learning Objectives

• Order the BGP best-path selection algorithm steps
• Determine the best path given multiple path attributes
• Understand how LOCAL_PREF, AS_PATH, and MED influence path selection
• Apply BGP path selection in real-world multi-homed topologies

The BGP Best-Path Selection Algorithm

When a BGP router receives multiple paths to the same prefix, it must pick the single best path to install in the routing table. BGP evaluates paths sequentially, comparing attributes one at a time. The algorithm stops at the first attribute that breaks the tie.

The order of evaluation:

1. Highest weight (Cisco proprietary, local to the router)
2. Highest LOCAL_PREF (prefer routes with higher local preference)
3. Locally originated (routes originated by this router are preferred)
4. Shortest AS_PATH (fewest AS numbers traversed)
5. Lowest origin type (IGP < EGP < incomplete)
6. Lowest MED (multi-exit discriminator)
7. eBGP over iBGP (external paths preferred over internal)
8. Lowest IGP metric to NEXT_HOP (closest exit point)
9. Oldest route (for eBGP paths only)
10. Lowest neighbor RID

Understanding Each Attribute

Weight — A Cisco-proprietary, locally significant value (0-65535). Higher weight wins. Weight only affects the local router and is not advertised to peers. It is checked first, making it the most powerful attribute for local policy.

LOCAL_PREF — An integer (0-4294967295) exchanged within the AS via iBGP. Higher is better. Standard default is 100. This is the primary tool for influencing outbound traffic from the AS.

AS_PATH — The list of AS numbers a route traversed. Shorter is preferred. BGP operators can use as-path prepend to artificially lengthen the AS_PATH and make a path less preferred.

MED (Multi-Exit Discriminator) — An integer exchanged between ASes to influence inbound traffic. Lower MED is better. Unlike LOCAL_PREF, MED is non-transitive — it stays within the neighboring AS and is not propagated further.

Multi-Homing Scenarios

An organization connected to two ISPs (multi-homed) can use BGP attributes to engineer traffic:

Controlling outbound traffic (which ISP your traffic uses to reach the internet):

• Set higher LOCAL_PREF on routes received from the preferred ISP
• Or set lower LOCAL_PREF on routes from the backup ISP

Controlling inbound traffic (which ISP carries traffic coming into your network):

• Advertise shorter AS_PATH to preferred ISP
• Or advertise a longer AS_PATH (prepend) to make the backup ISP less attractive

Key Takeaways

• BGP evaluates 10 path attributes in a strict sequential order
• Weight and LOCAL_PREF control outbound traffic; MED controls inbound traffic
• AS_PATH prepend makes a path artificially longer and thus less preferred
• Multi-homed networks use BGP attributes for traffic engineering
• Understanding the order of evaluation is critical for correct BGP policy design