350-401 Infrastructure Practice Question
Exhibit
Refer to the exhibit. R1# show ip route | include 10.1.1.0 O E2 10.1.1.0/24 [110/20] via 10.0.1.2, 00:00:34, GigabitEthernet0/0 O E2 10.1.1.0/24 [110/20] via 10.0.2.2, 00:00:34, GigabitEthernet0/1 R1# show ip ospf interface GigabitEthernet0/0 | include Cost Process ID 1, Router ID 1.1.1.1, Network Type BROADCAST, Cost: 10 R1# show ip ospf interface GigabitEthernet0/1 | include Cost Process ID 1, Router ID 1.1.1.1, Network Type BROADCAST, Cost: 100 R1# traceroute 10.1.1.1 source Loopback0 Type escape sequence to abort. Tracing the route to 10.1.1.1 1 10.0.1.2 4 msec 4 msec 4 msec 2 10.0.3.2 8 msec 8 msec 8 msec
Refer to the exhibit. R1 has two equal-cost OSPF E2 routes to 10.1.1.0/24 via two different next hops. However, when tracing to 10.1.1.1, all traffic uses the path through 10.0.1.2. What is the most likely reason?
⚠ Common exam trap
Cisco often tests the subtle tie-breaking behavior for OSPF E2 routes, where candidates mistakenly assume that equal-cost E2 routes will always be load-balanced, ignoring the interface cost tie-breaker that Cisco IOS applies.
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
✓
OSPF E2 routes do not factor interface cost; but the router uses the interface cost as a tie-breaker for equal-cost routes.
OSPF E2 routes do not include the internal cost to the ASBR; the cost shown in the routing table is the external metric only. When two E2 routes have the same external metric, Cisco IOS uses the interface cost as a tie-breaker to select the best next hop. In this scenario, the interface to 10.0.1.2 has a lower cost than the interface to 10.0.2.2, so all traffic is forwarded via 10.0.1.2.
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 route has a higher administrative distance.
Why it's wrong here
Administrative distance is a per-protocol preference value; all OSPF routes, including external types like E2, have the same AD of 110 on Cisco devices. For two routes to be considered equal-cost candidates for the same prefix, they must have identical AD and equal metric. A difference in AD would cause the route with the lower AD to be installed and the other ignored, not shown as an alternative. Therefore, the exhibited equal-cost routes cannot be differentiated by administrative distance.
- ✗
A default route is overriding the specific route.
Why it's wrong here
A default route is a prefix of 0.0.0.0/0 that is only used when no more specific prefix matches the destination IP. In the routing table, a specific route to the destination always wins over a default route regardless of metric or administrative distance due to longest-prefix-match. The exhibit clearly lists two routes to a specific network, not a default route; there is no default route shown, and even if one existed, it would not influence the selection between two specific routes. Hence, a default route is not overriding the specific route.
- ✗
The route via 10.0.2.2 is an E1 route.
Why it's wrong here
The routing table in the exhibit explicitly labels both entries as E2, indicating OSPF external type 2 routes. E1 and E2 differ fundamentally: E1 metric includes the internal cost from the router to the ASBR, while E2 metric is the ASBR-advertised external cost alone. If one route were E1, its metric would be larger by the internal cost and it would not share the same metric value for equal-cost load balancing. Since the label and the equal shown metric confirm both are E2, the route via 10.0.2.2 cannot be an E1 route.
- ✓
OSPF E2 routes do not factor interface cost; but the router uses the interface cost as a tie-breaker for equal-cost routes.
Why this is correct
OSPF E2 routes advertise the external metric from the ASBR and intentionally discard the internal cost accumulated along the path, so two E2 routes with the same metric appear equal. However, Cisco IOS and many other implementations avoid a random choice by applying a tie-breaking rule: when the E2 metric and AD are identical, the router compares the OSPF interface cost (or the cost to the ASBR) and prefers the path with the lower cost. In the exhibit, the two E2 routes have equal external metrics, but the route via the interface with lower cost is installed, while the other is held as a candidate. Thus, the apparent equal-cost routes are not truly equal for forwarding because interface cost breaks the tie.
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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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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