300-510 · Question #198
Refer to the exhibit. A network consultant is updating the company's MPLS solution and configuring several new PE devices to enable full communication among PE devices connected to the network. The…
The correct answer is D. Configure Cisco Express Forwarding on all routers in the MPLS network. MPLS label switching depends entirely on Cisco Express Forwarding (CEF) being enabled on every router in the MPLS domain. CEF builds the FIB (Forwarding Information Base) and adjacency tables that MPLS uses to perform label lookup and switching operations. Without CEF, the…
Question
Refer to the exhibit. A network consultant is updating the company's MPLS solution and configuring several new PE devices to enable full communication among PE devices connected to the network. The updates must ensure the redundancy and scalability of the network. The egress LSR is configured with mpls Idp explicit-null While configuring the given PE router to be adjacent to the egress LSR. The consultant notices that MPLS forwarding is not working in the network. Which action resolves the issue?
Options
- AConfigure MVRF on all routers in the MPLS network
- BOpen TCP and UDP ports 464
- COpen TCP and UDP ports 179
- DConfigure Cisco Express Forwarding on all routers in the MPLS network
How the community answered
(30 responses)- A3% (1)
- C3% (1)
- D93% (28)
Explanation
MPLS label switching depends entirely on Cisco Express Forwarding (CEF) being enabled on every router in the MPLS domain. CEF builds the FIB (Forwarding Information Base) and adjacency tables that MPLS uses to perform label lookup and switching operations. Without CEF, the router cannot perform the label imposition, swapping, or disposition that MPLS requires. The mpls ldp explicit-null configuration on the egress LSR simply changes the advertised label from implicit-null (pop) to an explicit null label (value 0 for IPv4), signaling upstream routers to keep the label intact until the very last hop. This is unrelated to the root cause. The fix is ensuring CEF is enabled globally with ip cef (and optionally ipv6 cef) on all routers. Options A (MVRF) relates to multicast VPNs, B (port 464) is for Kerberos password changes, and C (port 179) is for BGP - none of which affect base MPLS forwarding.
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