hardMultiple ChoiceObjective-mapped
300-410 Practice Question: An engineer configures MPLS forwarding on a router
An engineer configures MPLS forwarding on a router. The MPLS LDP neighbors are established, but some prefixes are not being assigned labels. The engineer checks the 'show mpls ldp bindings' and sees that the prefixes are missing. What is the most likely explanation?
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 prefix is learned via BGP, and LDP does not assign labels to BGP routes by default.
MPLS LDP assigns labels to prefixes that are in the routing table and are IGP routes. If a prefix is learned via BGP, LDP will not assign a label to it by default unless the 'mpls ldp autoconfig' or 'label-distribution' is configured for BGP. Additionally, if the prefix is a connected route or a static route, LDP may not assign a label unless explicitly configured. The edge case is that the prefix is a BGP route, and LDP does not label BGP routes by default.
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 prefix is learned via BGP, and LDP does not assign labels to BGP routes by default.
Why this is correct
LDP by default only assigns labels to IGP routes (OSPF, EIGRP, IS-IS). BGP routes are not labeled unless 'mpls ldp autoconfig bgp' or 'label-distribution' is configured for BGP.
- ✗
The 'mpls ldp advertise-labels' command is configured to deny the prefix.
Why it's wrong here
If the prefix is denied by an advertise-labels filter, it would not appear in bindings. However, the question states some prefixes are missing, and this is a possible cause but less likely than the default behavior for BGP routes.
- ✗
The 'mpls ldp label' command is missing on the interface.
Why it's wrong here
The 'mpls ldp label' command is not a valid command; LDP is enabled per interface with 'mpls ip'.
- ✗
The 'tag-switching' is not enabled globally.
Why it's wrong here
MPLS forwarding requires 'mpls ip' globally and per interface. If not enabled, LDP neighbors would not form, but the question states neighbors are established.
Go deeper
Related to this question
Learn chapter
OSPF Route Summarization and Filtering
Key term
LDP Protocol
LDP, or Label Distribution Protocol, is a protocol that routers use to automatically exchange labels that enable MPLS (Multiprotocol Label Switching) to create fast, efficient paths for data packets across a network.
Key term
MPLS Label Distribution
MPLS Label Distribution is the process by which routers exchange labels that tell them how to forward packets across a network without looking at the IP address each time.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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