350-501 Architecture Practice Question
Which two protocols are commonly used for label exchange in an MPLS network? (Choose two.)
⚠ Common exam trap
Cisco often tests the misconception that IGPs like OSPF or IS-IS can be used for MPLS label exchange because they are commonly associated with MPLS traffic engineering extensions, but those extensions (e.g., OSPF-TE) only carry link attributes, not labels.
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
✓
LDP
LDP (Label Distribution Protocol) is a standardized protocol (RFC 5036) specifically designed for distributing MPLS labels between routers to establish Label Switched Paths (LSPs) without requiring an IGP to carry label information. BGP (Border Gateway Protocol) can also exchange MPLS labels, particularly in MPLS VPN and Inter-AS scenarios, where it carries VPNv4 routes with MPLS labels using the BGP label-mapping capability (RFC 3107).
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
OSPF
Why it's wrong here
OSPF does not exchange MPLS labels.
- ✗
PIM
Why it's wrong here
PIM is for multicast, not label exchange.
- ✓
LDP
Why this is correct
LDP exchanges labels for IGP prefixes.
- ✓
BGP
Why this is correct
BGP exchanges labels for VPN prefixes.
- ✗
IS-IS
Why it's wrong here
IS-IS does not exchange MPLS labels.
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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