- A
The router forwards the packet normally.
Why wrong: IPv6 does not allow multicast source addresses.
- B
The router drops the packet silently.
Per RFC 2460, a packet with a multicast source address is invalid and must be discarded.
- C
The router sends an ICMPv6 error message back to the source.
Why wrong: No ICMP error is generated for invalid source addresses; the packet is simply dropped.
- D
The router rewrites the source address to the link-local address of the interface.
Why wrong: Routers do not modify source addresses; they drop invalid packets.
Question 427 of 2,152
IPv6 Multicast Source Address Handling
This 300-410 practice question tests your understanding of ipv6 traffic filtering and urpf. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
According to RFC 2460, what is the correct behavior when an IPv6 router receives a packet with a source address that is a multicast address?
Quick Answer
The answer is that the router drops the packet silently. This behavior is mandated by RFC 2460, which explicitly states that an IPv6 multicast address must never appear as the source address in any packet, as multicast addresses identify a group of receivers, not a single sender. When a router encounters such a packet, it discards it without sending any ICMP error message, because generating a reply to a multicast source would be both pointless and potentially harmful. On the Cisco CCNP ENARSI 300-410 exam, this concept tests your understanding of fundamental IPv6 addressing rules and packet validation, often appearing in questions about multicast forwarding or troubleshooting. A common trap is assuming the router will send an ICMPv6 error or forward the packet to the multicast group; remember that the source field is checked first, and a multicast source is an immediate, silent drop. Memory tip: “Multicast sources are never sources—silent discard enforces the course.”
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 router drops the packet silently.
RFC 2460 specifies that an IPv6 router must silently drop any packet with a multicast source address, as multicast addresses are only valid as destination addresses. This behavior prevents loops and misuse of multicast addressing in the network layer.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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 router forwards the packet normally.
Why it's wrong here
IPv6 does not allow multicast source addresses.
- ✓
The router drops the packet silently.
Why this is correct
Per RFC 2460, a packet with a multicast source address is invalid and must be discarded.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
The router sends an ICMPv6 error message back to the source.
Why it's wrong here
No ICMP error is generated for invalid source addresses; the packet is simply dropped.
- ✗
The router rewrites the source address to the link-local address of the interface.
Why it's wrong here
Routers do not modify source addresses; they drop invalid packets.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often confuse multicast source address rules with broadcast or unicast reverse path forwarding (uRPF) checks, assuming an ICMP error is sent or that the packet is forwarded normally.
Detailed technical explanation
How to think about this question
Under the hood, IPv6 routers perform a source address validation check early in the forwarding path; if the source is a multicast address (FF00::/8), the packet is discarded without any ICMPv6 notification. This behavior is critical in multicast-enabled networks to prevent a rogue device from injecting packets with a multicast source, which could disrupt multicast routing protocols like PIM or MLD.
KKey Concepts to Remember
- Read the scenario before looking for a memorised answer.
- Find the constraint that changes the correct option.
- Eliminate answers that are true in general but not in this case.
TExam Day Tips
- Watch for words such as best, first, most likely and least administrative effort.
- Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A practitioner preparing for the 300-410 exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
Quick reference
OSI Model Reference
| Layer | Name | PDU | Key Protocols / Devices |
|---|---|---|---|
| 7 | Application | Data | HTTP, HTTPS, DNS, SMTP, FTP, SSH |
| 6 | Presentation | Data | TLS / SSL, JPEG, ASCII encoding |
| 5 | Session | Data | NetBIOS, RPC, SIP |
| 4 | Transport | Segment / Datagram | TCP, UDP |
| 3 | Network | Packet | IP, ICMP, OSPF — Routers |
| 2 | Data Link | Frame | Ethernet, Wi-Fi, PPP — Switches, Bridges |
| 1 | Physical | Bits | Cables, NICs, Hubs, Repeaters |
What to study next
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FAQ
Questions learners often ask
What does this 300-410 question test?
IPv6 Traffic Filtering and uRPF — This question tests IPv6 Traffic Filtering and uRPF — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The router drops the packet silently. — RFC 2460 specifies that an IPv6 router must silently drop any packet with a multicast source address, as multicast addresses are only valid as destination addresses. This behavior prevents loops and misuse of multicast addressing in the network layer.
What should I do if I get this 300-410 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Last reviewed: Jul 4, 2026
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