Day 32 --- Selective Advertisement and More-Specific Traffic Engineering

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1. Opening

Selective advertisement controls which prefixes each provider receives. Advertising a more-specific route can attract traffic because forwarding follows longest-prefix match before BGP attributes are compared. That power makes deaggregation effective---and potentially harmful if used casually.

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.


Selective Advertisement and More-Specific Traffic Engineering
Selective Advertisement and More-Specific Traffic Engineering


2. Concept and standards behavior

An aggregate and a more-specific are different NLRI. If both are globally visible, routers normally forward destinations covered by the more-specific toward that route regardless of the aggregate's AS-path. The design must therefore distinguish intentional TE from unnecessary contribution to global routing-table growth.

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


Selective Advertisement and More-Specific Traffic Engineering
Selective Advertisement and More-Specific Traffic Engineering

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-AGG permit 203.0.113.0/24
ip prefix-list PL-MORE-SPEC permit 203.0.113.0/25

route-map TO-ISP-A permit 10
 match ip address prefix-list PL-AGG
route-map TO-ISP-A permit 20
 match ip address prefix-list PL-MORE-SPEC

route-map TO-ISP-B permit 10
 match ip address prefix-list PL-AGG

router bgp 65010
 address-family ipv4
  neighbor 192.0.2.1 route-map TO-ISP-A out
  neighbor 198.51.100.1 route-map TO-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

Advertise the /25 only to ISP-A while retaining the /24 to both providers. Predict that destinations in the /25 can prefer ISP-A because of longest-prefix match, assuming upstream acceptance.

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.

Accidentally permit the /25 toward both providers. Observe that the intended directional signal disappears because both upstreams now receive the same more-specific.

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 root cause is export-policy scope, not best-path selection. Correct the ISP-B prefix list so only the approved aggregate is exported.

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

Deaggregation is a routing-table and operational cost, not a free TE knob. Use it only with prefix-ownership, RPKI/IRR, provider prefix-length acceptance and rollback implications understood.

15. Knowledge check


  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 --- export policy and NLRI
  • Cisco IOS XE 17.x --- BGP configuration and policy guidance
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