300-410 Layer 3 Technologies Practice Question
A network engineer is deploying BGP on a Cisco IOS XE router. The router must advertise the network 10.10.0.0/16 to an external peer, but the engineer notices that the prefix is not being advertised even though the network command is configured. The routing table shows that 10.10.0.0/16 is present as two separate /24 routes via OSPF. What is the most likely cause?
⚠ Common exam trap
The trap here is assuming that a BGP network statement will automatically summarize or advertise a supernet even when only more specific routes exist in the routing table.
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
✓
The network command requires an exact match in the routing table.
BGP network statements require an exact match in the routing table to originate a prefix. The routing table contains only the more specific /24 routes, so the /16 is not advertised. To resolve this, the engineer must create a matching route, such as a static route to Null0, or use the aggregate-address command with the summary-only option. The other options do not address the fundamental requirement of an exact match.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
The BGP router ID is not reachable, preventing advertisement.
Why it's wrong here
BGP router ID reachability is not required for advertising networks. The router ID is used for identification and tie-breaking, but it does not affect whether a network statement installs a prefix into BGP. Even if the router ID is unreachable, BGP can still advertise prefixes as long as they exist in the routing table. Thus, this is not the cause of the missing advertisement.
- ✗
The BGP neighbor is configured with a send-community attribute that filters the prefix.
Why it's wrong here
The send-community attribute controls whether community values are propagated to a neighbor; it does not filter prefixes. Even if communities were stripped, the prefix would still be advertised. Prefix filtering is done via distribute-lists, prefix-lists, or route-maps. Since the prefix is not even in the BGP table, community settings are irrelevant. Thus, this option is incorrect.
- ✗
The OSPF routes are not being redistributed into BGP.
Why it's wrong here
The network command does not rely on redistribution. It injects a prefix into BGP if an exact match exists in the routing table, regardless of the source protocol. While redistribution could advertise the /24s, it would not create the /16. The issue is the missing exact match for the /16, not the lack of redistribution. Therefore, redistribution is not the root cause.
- ✓
The network command requires an exact match in the routing table.
Why this is correct
BGP network statements only advertise a prefix if there is an exact matching route in the IP routing table. Here, only 10.10.1.0/24 and 10.10.2.0/24 exist, not 10.10.0.0/16, so BGP will not originate the aggregate. To advertise the /16, the engineer must either create a summary route (e.g., a static route to Null0) or use the aggregate-address command.
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 and reviewed by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
Last reviewed September 2026 · checked against the official Cisco exam blueprint
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