Day 33 --- AS-Path Prepending as Inbound Traffic Engineering

1. Opening

AS-path prepending makes one advertisement appear longer by repeating the local ASN. It can influence remote path selection, but only after higher-priority policy such as remote LOCAL_PREF. It is therefore a signal, not a guarantee.

The operational objective is not simply to make BGP choose a route. It is to make the intended behavior predictable during the normal state, degraded state, and rollback state.

AS-Path Prepending as Inbound Traffic Engineering

2. Concept and standards behavior

RFC 4271 uses AS_PATH both for loop detection and as an input to path selection. Cisco route policy can prepend the local AS on selected exports. Remote networks may ignore the intended preference because of LOCAL_PREF, peering relationships, more-specific routes or their own policy.

BGP remains a policy protocol. RFC 4271 provides the protocol framework, while several practical traffic-engineering controls are Cisco or operator-policy mechanisms. Evaluate each design at three layers: protocol behavior, IOS XE implementation, and neighboring-AS policy.

Separate route eligibility, route selection, and route export. Eligibility asks whether a path is usable. Selection determines the local best path. Export policy determines what a neighbor is permitted to learn.

3. Scenario

AS 65010 is dual-homed to ISP-A AS 65020 and ISP-B AS 65030. The enterprise service prefix is 203.0.113.0/24. Transit links use 192.0.2.0/30 and 198.51.100.0/30.

Success criteria 1. Primary and backup behavior is explicit. 2. No route is exported merely because it exists locally. 3. Failure behavior is observable in BGP and advertised-route evidence. 4. Rollback is independent of session redesign. 5. A negative test proves unintended advertisement is absent.

4. Topology

ISP-A AS65020 --- 192.0.2.0/30 --- EDGE1/AS65010
                                      |
                               203.0.113.0/24
                                      |
ISP-B AS65030 --- 198.51.100.0/30 --- EDGE2/AS65010

5. Prerequisites

• IOS XE 17.18.x documentation baseline.
• IPv4 unicast activated for both eBGP peers.
• Service prefix valid for origination.
• Independent management access.
• Pre-change capture of BGP state and advertised routes.

6. Baseline configuration

Topic-specific configuration excerpt --- not a complete device configuration.

ip prefix-list PL-SERVICE permit 203.0.113.0/24
route-map PREPEND-ISP-B permit 10
 match ip address prefix-list PL-SERVICE
 set as-path prepend 65010 65010 65010

router bgp 65010
 address-family ipv4
  neighbor 198.51.100.1 route-map PREPEND-ISP-B out
 exit-address-family

7. Verification before modification

show bgp ipv4 unicast
show bgp ipv4 unicast 203.0.113.0/24
show bgp ipv4 unicast neighbors
show bgp ipv4 unicast neighbors 192.0.2.1 advertised-routes
show bgp ipv4 unicast neighbors 198.51.100.1 advertised-routes
show route-map
show ip prefix-list

The question is not whether the policy object exists; it is whether the intended route is selected, matched and actually exported to the intended neighbor.

8. Controlled modification

Apply three additional copies of AS65010 only toward ISP-B. Verify the local advertised path. Use an authorized external looking glass or lab remote AS to test whether ISP-A becomes preferred.

Predict Adj-RIB-Out behavior before applying the change. Re-evaluate policy using the least disruptive supported mechanism.

9. Fault injection

Illustrative lab — not a real incident.

Set a high LOCAL_PREF inside a simulated remote AS for the ISP-B path. The remote AS should still prefer that path despite the longer AS_PATH, demonstrating why prepending is not deterministic.

Change one variable only, capture the changed state, and compare it with the baseline.

10. Troubleshooting

1. Confirm affected prefix and traffic direction.
2. Confirm eBGP sessions are Established.
3. Confirm local route eligibility/origination.
4. Inspect selected BGP route.
5. Inspect match objects.
6. Inspect route-map sequence and implicit deny.
7. Inspect advertised routes per provider.
8. Confirm policy direction.
9. Determine whether route refresh is required.
10. Run positive traffic test.
11. Run negative/containment test.
12. Correct the smallest proven cause and repeat the same evidence set.

11. Root cause and correction

The mistaken assumption is that shortest AS_PATH is globally the first decision. Correct the design expectation: remote business policy may precede AS-path length.

12. Post-fix verification

Verify session state, route acceptance, selected path, exported attributes, advertised routes, forwarding and the negative test. Local advertisement does not prove remote selection.

13. Rollback

Revert only the new policy action, re-evaluate policy with the least disruptive supported method, and confirm both providers return to baseline. Roll back immediately for loss of all reachability, unintended transit, unapproved deaggregation or export outside the approved prefix set.

14. Production lessons

Prepending is most defensible when measured, limited to specific prefixes, and paired with failure testing. Excessive prepends add path noise without creating contractual control.

15. Knowledge check

map?

policy?

leaking?

1. Why is advertised-routes stronger evidence than displaying a route
2. Which parts are controlled locally and which depend on upstream
3. What negative test proves the restricted/backup advertisement is not

Answers

inbound selection are remote policy.

routes in the healthy state.

1. It validates resulting per-neighbor export state.
2. Local selection/export are local; remote LOCAL_PREF, propagation and
3. Verify the protected prefix is absent from the neighbor's advertised

16. Sources

• RFC 4271 --- AS_PATH selection/loop prevention
• Cisco IOS XE 17.x --- BGP path manipulation guidance