CCNP MPLS and VPN Technologies 1 — Questions and Answers
Question 1: What is the primary function of a Label Edge Router (LER) in an MPLS network?
- Forward packets based solely on IP routing tables
- Push or pop MPLS labels at the boundary of the MPLS domain (Correct answer)
- Assign BGP community attributes to MPLS paths
- Perform RSVP signaling between core routers
Correct answer: Push or pop MPLS labels at the boundary of the MPLS domain
An LER (also called PE router) sits at the edge of the MPLS domain and is responsible for pushing labels onto incoming IP packets or popping labels from outgoing packets.
Question 2: In MPLS LDP, what is the default label distribution mode on Cisco routers?
- Ordered control with downstream on demand
- Independent control with downstream unsolicited (Correct answer)
- Ordered control with upstream on demand
- Independent control with downstream on demand
Correct answer: Independent control with downstream unsolicited
Cisco IOS uses independent control mode with downstream unsolicited distribution by default, meaning each LSR independently assigns and advertises labels without waiting for downstream peers.
Question 3: Which MPLS label value is reserved to indicate the Explicit Null label for IPv4?
- 0 (Correct answer)
- 1
- 2
- 3
Correct answer: 0
Label value 0 is the IPv4 Explicit Null label, which signals that the label must be popped but the packet should still be forwarded based on its IP header.
Question 4: What mechanism does Penultimate Hop Popping (PHP) use to reduce processing on the egress LSR?
- The ingress PE router pops the label before forwarding
- The second-to-last LSR pops the outermost label before the final hop (Correct answer)
- The egress PE applies a swap operation instead of a pop
- BGP next-hop self is enabled on the penultimate router
Correct answer: The second-to-last LSR pops the outermost label before the final hop
PHP allows the penultimate (second-to-last) LSR to remove the top MPLS label so the egress LSR receives a plain IP packet or inner label, reducing its lookup overhead.
Question 5: In an MPLS L3VPN, which protocol is used to exchange VPN routing information between PE routers?
- OSPF with route redistribution
- MP-BGP with VPNv4 address family (Correct answer)
- IS-IS with extended TLVs
- EIGRP with named mode
Correct answer: MP-BGP with VPNv4 address family
Multiprotocol BGP (MP-BGP) with the VPNv4 address family carries customer VPN prefixes between PE routers using Route Distinguishers and Route Targets to separate and import/export routes.
Question 6: What is the purpose of a Route Distinguisher (RD) in MPLS L3VPN?
- It selects which VRF imports a prefix from BGP
- It makes overlapping customer IP prefixes unique in the BGP table (Correct answer)
- It defines the MPLS label stack depth for VPN packets
- It authenticates BGP sessions between PE routers
Correct answer: It makes overlapping customer IP prefixes unique in the BGP table
An RD is prepended to customer prefixes to create a unique 96-bit VPNv4 address, allowing two customers with overlapping IP address spaces to coexist in the same MP-BGP table.
Question 7: Which command verifies the MPLS label forwarding table on a Cisco router?
- show ip route mpls
- show mpls forwarding-table (Correct answer)
- show mpls ldp bindings detail
- show mpls traffic-eng tunnels
Correct answer: show mpls forwarding-table
The 'show mpls forwarding-table' command displays the Label Forwarding Information Base (LFIB), showing incoming labels, outgoing labels, and next-hop interfaces.
CCNP MPLS and VPN Technologies 1
What is the primary function of a Label Edge Router (LER) in an MPLS network?