Troubleshooting ECMP Load Balancing Failure: Recursive Next-Hop Resolution
You are the network engineer for a mid-sized enterprise with a Juniper MX router running Junos. The router has two uplinks to the internet: one to ISP-A via ge-0/0/0 (10.0.0.1/30) and one to ISP-B via ge-0/0/1 (10.0.0.5/30). You have configured static default routes to both ISPs: one to 10.0.0.2 and one to 10.0.0.6. Both routes have the same preference (default 5) and metric. You enabled ECMP to load-balance outbound traffic. After testing, you notice that all traffic is being sent only to ISP-A, and none to ISP-B. You verify that both interfaces are up and that the next-hop addresses are reachable. You check the routing table and see both routes are active but with different next-hop counts. What is the most likely cause of the traffic not being load-balanced?
Quick Answer
The answer is that one of the static routes resolves to multiple next hops, causing an ECMP load balancing failure. In Junos, when both static default routes have equal preference and metric, they are installed in the routing table, but ECMP requires each route to resolve to a single next hop for proper load sharing. If the route to ISP-B (10.0.0.6) undergoes recursive next-hop resolution and finds multiple paths—perhaps due to a shared subnet or indirect route—the route with a single next hop (ISP-A) becomes the sole forwarding path, while the multi-next-hop route is ignored for load balancing. On the JNCIA-Junos exam, this scenario tests your understanding of how Junos handles recursive next-hop resolution and its impact on ECMP, a common trap where both routes appear active but traffic fails to distribute. Remember the memory tip: “One hop per route for ECMP to compute; multiple hops will make the traffic mute.”
⚠ Common exam trap
It's easy for candidates to assume both routes are equally valid for ECMP simply because they appear in the routing table, but they overlook how recursive resolution can cause one route to have multiple next hops, breaking the equal-cost multipath condition.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
One of the static routes resolves to multiple next hops, causing imbalance.
When both static default routes have the same preference and metric, Junos installs both in the routing table. However, ECMP load-balancing requires that each route resolve to a single next hop. If one static route resolves to multiple next hops (e.g., through a recursive lookup to a prefix with multiple paths), the route with a single next hop (ISP-A) becomes the sole active path for forwarding, while the multi-next-hop route (ISP-B) is not used for load balancing. This explains why all traffic goes to ISP-A despite both routes being present.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
One of the static routes resolves to multiple next hops, causing imbalance.
Why this is correct
If one route resolves to multiple next hops, it may appear as multiple routes, skewing load balancing.
- ✗
ECMP is not enabled in the forwarding-options.
Why it's wrong here
ECMP is enabled, but the issue is next-hop count.
- ✗
The route to ISP-B is not active.
Why it's wrong here
Both routes are active.
- ✗
Hash-based load balancing requires per-packet configuration.
Why it's wrong here
Hash-based is default; per-packet is not typical.
Visual reference
Quick reference
Routing Protocol Comparison
| Protocol | Metric | Max Hops | Algorithm | Type |
|---|---|---|---|---|
| RIP v2 | Hop count | 15 | Bellman-Ford | Distance vector |
| OSPF | Cost (bandwidth) | Unlimited | Dijkstra (SPF) | Link state |
| EIGRP | Composite metric | Unlimited | DUAL | Hybrid |
| IS-IS | Cost | Unlimited | Dijkstra | Link state |
| BGP | Policy / attributes | Unlimited | Path vector | Path vector |
RIP's 15-hop limit makes it unsuitable for large networks. OSPF and EIGRP dominate modern enterprise deployments.
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Same concept, more angles
1 more way this is tested on JN0-106
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. Refer to the exhibit. How many next hops are installed for the 10.1.1.0/24 route?
easy- A.4
- ✓ B.2
- C.3
- D.1
Why B: The route 10.1.1.0/24 has two next hops installed because the routing table shows two distinct next-hop addresses (e.g., 192.168.1.1 and 192.168.2.1) under the same route entry. In JUNOS, when equal-cost paths exist, the route is installed with multiple next hops, and the count reflects the number of unique next-hop entries, not the number of interfaces or paths in a load-balancing set.
JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This JN0-106 practice question is part of Courseiva's free Juniper Networks certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the JN0-106 exam.