350-501 Architecture Practice Question
A service provider is deploying multicast over MPLS L3VPN. Which mechanism is used to provide multicast in the core between PE routers?
⚠ Common exam trap
Cisco often tests the misconception that PIM-SM is always required for multicast in any network, but in an MPLS L3VPN core, the correct mechanism is mLDP or RSVP-TE P2MP, as PIM is typically confined to the customer edge or the PE-CE link.
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
✓
mLDP
In an MPLS L3VPN multicast deployment, the core network between PE routers uses a label-based multicast mechanism rather than a traditional PIM protocol. mLDP (Multipoint LDP) is the correct choice because it establishes point-to-multipoint (P2MP) LSPs in the MPLS core, enabling efficient multicast transport without requiring PIM in the core. This approach is defined in RFC 6388 and is commonly used for MVPN (Multicast VPN) implementations.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
IGMP
Why it's wrong here
IGMP is for host membership, not core multicast.
- ✗
PIM-SM
Why it's wrong here
PIM-SM can be used in the core but mLDP is the standard for MPLS multicast.
- ✗
PIM-DM
Why it's wrong here
PIM-DM is dense mode, not typically used in MPLS core.
- ✓
mLDP
Why this is correct
mLDP is the multicast label distribution protocol for MPLS.
Quick reference
VPN Protocol Comparison
| Protocol | Port | Encryption | Authentication | Use Case |
|---|---|---|---|---|
| IKEv2 / IPsec | UDP 500 / 4500 | AES-256 | Certificates / PSK | Site-to-site & remote access |
| SSL / TLS VPN | TCP 443 | TLS 1.3 | Certificates / MFA | Clientless remote access |
| L2TP / IPsec | UDP 1701 | AES (IPsec) | PSK / Certificates | Legacy remote access |
| WireGuard | UDP 51820 | ChaCha20 | Public keys | Modern high-performance VPN |
| PPTP | TCP 1723 | MPPE (weak) | MS-CHAPv2 | Legacy — avoid in production |
PPTP is considered insecure. IKEv2/IPsec and SSL VPN are the current recommended options.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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