AS Path Prepending: Traffic Engineering with BGP

What Is AS Path Prepending?

AS path prepending is a BGP traffic engineering technique where you artificially lengthen the AS path for a route announcement. Since BGP prefers shorter AS paths, prepending makes a route less attractive to remote networks, steering traffic toward your other links.

Without prepending:
  Route via Provider A: [64512] 203.0.113.0/24  (path length: 1)
  Route via Provider B: [64512] 203.0.113.0/24  (path length: 1)

With prepending on Provider B:
  Route via Provider A: [64512] 203.0.113.0/24          (path length: 1)
  Route via Provider B: [64512 64512 64512] 203.0.113.0/24  (path length: 3)

Result: Most networks prefer Provider A (shorter path).

Configuration Examples

# Cisco IOS: Prepend 2x on announcements to Provider B
route-map PREPEND-TO-PROVIDER-B permit 10
  set as-path prepend 64512 64512

router bgp 64512
  neighbor 10.0.0.2 route-map PREPEND-TO-PROVIDER-B out

# Junos: Prepend 3x
policy-statement PREPEND-3X {
    then {
        as-path-prepend "64512 64512 64512";
    }
}
protocols bgp group PROVIDER-B export PREPEND-3X

# BIRD: Prepend 2x
filter export_provider_b {
    bgp_path.prepend(64512);
    bgp_path.prepend(64512);
    accept;
}

How Many Times to Prepend?

Never prepend more than 3 times. Beyond that, there is no measurable benefit, and excessively long AS paths can trigger route filtering (some networks reject paths longer than 50 ASes).

Limitations of Prepending

Prepending is a blunt instrument with significant limitations:

• Local preference wins -- If a remote network has set local-preference for your routes, prepending is ignored (local-pref is evaluated before AS path length).
• Affects all traffic -- You cannot selectively prepend for specific source networks.
• Global impact -- Prepending to one provider affects how all networks worldwide see your routes through that provider.
• Not deterministic -- You cannot guarantee where traffic will enter your network.

Better Alternatives

Using Communities Instead

Most transit providers support action communities that set local preference within their network:

# Instead of prepending, use community to lower preference at Provider B
route-map DEPREF-AT-B permit 10
  set community 3356:90    # Lumen: set local-pref to 90

# At Provider A, set high preference
route-map PREFER-AT-A permit 10
  set community 174:120    # Cogent: set local-pref to 120

Communities are more effective than prepending because they influence local-preference, which is evaluated before AS path length.

When Prepending Makes Sense

Despite its limitations, prepending is appropriate when:

• Your upstream does not support communities
• You need a quick, temporary traffic shift
• You want a simple backup/primary configuration
• You are at an IXP with no community support