mediumMultiple Choice
300-410 Practice Question: Which statement about IPv6 uRPF loose mode is…
Which statement about IPv6 uRPF loose mode is true?
⚠ Common exam trap
Watch out — candidates often confuse loose mode with strict mode, assuming loose mode still requires interface-level reachability, when in fact it only checks for the source address's existence in the FIB.
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
✓
It only verifies that the source address exists in the FIB.
In IPv6 unicast Reverse Path Forwarding (uRPF) loose mode, the router checks the FIB (Forwarding Information Base) to verify that the source address of an incoming packet exists in the routing table. It does not require the source address to be reachable via the same interface, which is the key distinction from strict mode. This allows loose mode to be used in asymmetric routing scenarios where the return path may not match the ingress interface.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
It requires the source address to be reachable via the same interface.
Why it's wrong here
Loose mode checks only that the source is present in the IPv6 routing table, regardless of ingress interface; requiring reachability via the same interface describes strict mode. Loose mode suits multihomed or asymmetric topologies where return paths differ.
- ✓
It only verifies that the source address exists in the FIB.
Why this is correct
Loose mode checks only that the source address is present in the FIB, regardless of the incoming interface. It does not verify the return path points back through the receiving interface, which is the strict-mode behaviour, so reachability alone satisfies the check.
- ✗
It drops packets with link-local source addresses.
Why it's wrong here
Loose mode performs a routing-table lookup on the source and does not filter link-local addresses, which are valid on-link sources. Dropping them would break neighbour discovery and other link-scoped traffic, so no such behaviour exists.
- ✗
It is enabled by default on all interfaces.
Why it's wrong here
IPv6 uRPF, strict or loose, is disabled by default and must be configured per interface with the ipv6 verify unicast source reachable-via command. Enabling it globally by default would break asymmetric routing, so administrators opt in deliberately.
Quick reference
Asymmetric Encryption Algorithm Comparison
| Algorithm | Key Exchange | Signatures | Equivalent Security Key | Notes |
|---|---|---|---|---|
| RSA-3072 | Yes | Yes | 128-bit | Widely deployed; slow for bulk data |
| ECDSA P-256 | No | Yes | 128-bit | Fast signatures; standard TLS certs |
| ECDH / ECDHE | Yes | No | 128-bit | Perfect forward secrecy in TLS 1.3 |
| DH / DHE | Yes | No | 128-bit (3072-bit key) | Replaced by ECDHE in modern TLS |
| Ed25519 | No | Yes | ~128-bit | SSH keys, modern PKI |
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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