Inspect rejected UPDATE attributes, local AS number and AS_PATH on each advertised prefix. This check would diagnose BGP AS_PATH loop detection, but would not first isolate the described iBGP next-hop reachability. Begin with: Check BGP NEXT_HOP, recursive route resolution and underlay reachability on the receiver.

Inspect candidate AS origins, MED values, higher-priority attributes and effective route-selection rules. This check would diagnose BGP MED same-neighbor-AS comparison, but would not first isolate the described iBGP next-hop reachability. Begin with: Check BGP NEXT_HOP, recursive route resolution and underlay reachability on the receiver.

Review ingress marks, traffic policy counters, queue occupancy and congestion test results. This check would diagnose QoS DSCP remarking and trust, but would not first isolate the described iBGP next-hop reachability. Begin with: Check BGP NEXT_HOP, recursive route resolution and underlay reachability on the receiver.

Configure supported RR-client relationships and confirm intended reflection rules between client and nonclient sessions. This is a legitimate response to the different issue of BGP route-reflector client scoping, but it does not correct the specific iBGP split-horizon route propagation condition reported. The relevant technical distinction is: An ordinary iBGP speaker does not re-advertise iBGP-learned routes to other iBGP peers.

Correct the IS-IS area identities or redesign the intended Level-2 boundary before forming the peer. This is a legitimate response to the different issue of IS-IS level-1 area adjacency, but it does not correct the specific iBGP split-horizon route propagation condition reported. The relevant technical distinction is: An ordinary iBGP speaker does not re-advertise iBGP-learned routes to other iBGP peers.

Add supported PBR next-hop reachability tracking or failover logic consistent with service requirements. This is a legitimate response to the different issue of PBR failed-next-hop fallback, but it does not correct the specific iBGP split-horizon route propagation condition reported. The relevant technical distinction is: An ordinary iBGP speaker does not re-advertise iBGP-learned routes to other iBGP peers.

Reflected routes reach intended clients without repeated originator or cluster loops. That observation would support a separate case of BGP route reflector cluster loop prevention, not verification of the reported iBGP split-horizon route propagation. The relevant observable outcome here is: All intended iBGP peers learn the eligible prefix without loops or unintended repeated announcements.

A test prohibited prefix is rejected while authorized matching routes retain expected attributes. That observation would support a separate case of Route-policy permit and deny node order, not verification of the reported iBGP split-horizon route propagation. The relevant observable outcome here is: All intended iBGP peers learn the eligible prefix without loops or unintended repeated announcements.

The valid corrected route is accepted and routes still containing the receiver AS are denied. That observation would support a separate case of BGP AS_PATH loop detection, not verification of the reported iBGP split-horizon route propagation. The relevant observable outcome here is: All intended iBGP peers learn the eligible prefix without loops or unintended repeated announcements.

BGP best-path and route installation require reachable NEXT_HOP; iBGP advertising does not always rewrite it. This explains iBGP next-hop reachability, but not why the issue in this scenario occurs. The specific mechanism is: An ordinary iBGP speaker does not re-advertise iBGP-learned routes to other iBGP peers.

Ordered route-policy matching stops at an applicable node and its permit or deny behavior. This explains Route-policy permit and deny node order, but not why the issue in this scenario occurs. The specific mechanism is: An ordinary iBGP speaker does not re-advertise iBGP-learned routes to other iBGP peers.

BGP ORIGIN ranks IGP ahead of EGP and Incomplete after preceding path-selection rules. This explains BGP origin attribute tie break, but not why the issue in this scenario occurs. The specific mechanism is: An ordinary iBGP speaker does not re-advertise iBGP-learned routes to other iBGP peers.

Mismatched metric styles can hide prefixes or distort shortest-path selection. This consequence is associated more directly with IS-IS metric-style consistency. For the stated conditions the immediate exposure is: Ignoring iBGP split horizon can silently prevent remote clients from learning routes.

Stripping a no-export marker can leak intended internal reachability to other networks. This consequence is associated more directly with BGP route policy community NO_EXPORT. For the stated conditions the immediate exposure is: Ignoring iBGP split horizon can silently prevent remote clients from learning routes.

An Established session with received prefixes can still leave routes unusable if next hops are unresolved. This consequence is associated more directly with iBGP next-hop reachability. For the stated conditions the immediate exposure is: Ignoring iBGP split horizon can silently prevent remote clients from learning routes.

Inspect peer addresses, source interfaces, loopback routes and TCP endpoint addresses. This check would diagnose Loopback-based iBGP update source, but would not first isolate the described iBGP split-horizon route propagation. Begin with: Inspect adjacency graph, route source type and advertisement history on the transit router.

Inspect ip-prefix sequence, mask-length qualifiers and test candidate routes. This check would diagnose Prefix-list network and mask matching, but would not first isolate the described iBGP split-horizon route propagation. Begin with: Inspect adjacency graph, route source type and advertisement history on the transit router.

Inspect BGP communities on receipt and advertisement, send-community settings and outbound policy. This check would diagnose BGP route policy community NO_EXPORT, but would not first isolate the described iBGP split-horizon route propagation. Begin with: Inspect adjacency graph, route source type and advertisement history on the transit router.

Use MED as a lower-is-better hint only under the supported path-comparison policy and verify competing attributes. This is a legitimate response to the different issue of BGP MED same-neighbor-AS comparison, but it does not correct the specific iBGP split-horizon route propagation condition reported. The relevant technical distinction is: An ordinary iBGP speaker does not re-advertise iBGP-learned routes to other iBGP peers.

Correct the route propagation or erroneous AS_PATH modification while retaining normal loop prevention. This is a legitimate response to the different issue of BGP AS_PATH loop detection, but it does not correct the specific iBGP split-horizon route propagation condition reported. The relevant technical distinction is: An ordinary iBGP speaker does not re-advertise iBGP-learned routes to other iBGP peers.

Correct the IS-IS area identities or redesign the intended Level-2 boundary before forming the peer. This is a legitimate response to the different issue of IS-IS level-1 area adjacency, but it does not correct the specific iBGP split-horizon route propagation condition reported. The relevant technical distinction is: An ordinary iBGP speaker does not re-advertise iBGP-learned routes to other iBGP peers.

Every prefix has a stable authoritative origin and route import does not recursively reappear. That observation would support a separate case of Redistribution route tag loop avoidance, not verification of the reported iBGP split-horizon route propagation. The relevant observable outcome here is: All intended iBGP peers learn the eligible prefix without loops or unintended repeated announcements.

The peer sees the approved aggregate and every allocated component remains reachable under tested failures. That observation would support a separate case of BGP prefix aggregation and specifics, not verification of the reported iBGP split-horizon route propagation. The relevant observable outcome here is: All intended iBGP peers learn the eligible prefix without loops or unintended repeated announcements.

Every device interprets intended link costs and SPF selects the documented shortest paths. That observation would support a separate case of IS-IS metric-style consistency, not verification of the reported iBGP split-horizon route propagation. The relevant observable outcome here is: All intended iBGP peers learn the eligible prefix without loops or unintended repeated announcements.

BGP MED is a nontransitive hint often compared among paths learned from the same neighboring AS. This explains BGP MED same-neighbor-AS comparison, but not why the issue in this scenario occurs. The specific mechanism is: An ordinary iBGP speaker does not re-advertise iBGP-learned routes to other iBGP peers.

An RR's advertisement rules depend on whether the source and target peers are clients. This explains BGP route-reflector client scoping, but not why the issue in this scenario occurs. The specific mechanism is: An ordinary iBGP speaker does not re-advertise iBGP-learned routes to other iBGP peers.

OSPF SPF computes paths from the link-state topology using accumulated interface costs. This explains OSPF SPF path-cost comparison, but not why the issue in this scenario occurs. The specific mechanism is: An ordinary iBGP speaker does not re-advertise iBGP-learned routes to other iBGP peers.

Misunderstanding origin can lead to misleading troubleshooting changes to later attributes. This consequence is associated more directly with BGP origin attribute tie break. For the stated conditions the immediate exposure is: Ignoring iBGP split horizon can silently prevent remote clients from learning routes.

Defining a classifier without applying the intended policy gives a false impression of active QoS. This consequence is associated more directly with MQC classifier and behavior association. For the stated conditions the immediate exposure is: Ignoring iBGP split horizon can silently prevent remote clients from learning routes.

Any-source joins cannot always represent source-specific subscriptions and may allow unwanted streams. This consequence is associated more directly with IGMPv3 source-specific membership. For the stated conditions the immediate exposure is: Ignoring iBGP split horizon can silently prevent remote clients from learning routes.

Check metric-style configuration, TLVs, interface costs and selected SPF paths. This check would diagnose IS-IS metric-style consistency, but would not first isolate the described iBGP split-horizon route propagation. Begin with: Inspect adjacency graph, route source type and advertisement history on the transit router.

Audit security policy order, matching zones and addresses, and permitted-versus-denied test flows. This check would diagnose Default-deny network segmentation, but would not first isolate the described iBGP split-horizon route propagation. Begin with: Inspect adjacency graph, route source type and advertisement history on the transit router.

Inspect configured peer AS, received OPEN messages and BGP neighbor reset reasons. This check would diagnose BGP peer autonomous-system mismatch, but would not first isolate the described iBGP split-horizon route propagation. Begin with: Inspect adjacency graph, route source type and advertisement history on the transit router.