Cisco ENCOR (MPLS) 2 — Questions and Answers
Question 1: Which MPLS label operation replaces an existing label with a new label?
- Push
- Pop
- Swap (Correct answer)
- Merge
Correct answer: Swap
The Swap operation replaces the top label in the MPLS stack with a new label at each transit LSR.
Question 2: What is the default MPLS label value reserved for the IPv4 explicit null label?
- 0 (Correct answer)
- 1
- 2
- 3
Correct answer: 0
Label value 0 is the IPv4 Explicit Null label, signaling the egress PE to pop the label and forward based on the IPv4 header.
Question 3: In an MPLS network, what does the term 'LSP' stand for?
- Label Switching Protocol
- Label Switched Path (Correct answer)
- Link State Propagation
- Label Stack Pointer
Correct answer: Label Switched Path
LSP stands for Label Switched Path, which is the unidirectional path from ingress to egress LSR that packets follow.
Question 4: Which protocol can be used to signal MPLS LSPs and also carries QoS traffic engineering constraints?
- LDP
- RSVP-TE (Correct answer)
- BGP
- OSPF
Correct answer: RSVP-TE
RSVP-TE (Resource Reservation Protocol - Traffic Engineering) signals LSPs and carries bandwidth and constraint information for TE tunnels.
Question 5: What is the role of a P (Provider) router in an MPLS VPN architecture?
- Connects to customer CE routers
- Performs label swap operations in the core (Correct answer)
- Maintains VRF routing tables
- Exchanges VPNv4 routes with BGP
Correct answer: Performs label swap operations in the core
P routers are core MPLS routers that only perform label swap operations and have no knowledge of VPN or customer routes.
Question 6: Which label is used in MPLS OAM to perform a router-alert function similar to IP router-alert?
- Label 0
- Label 1 (Correct answer)
- Label 2
- Label 14
Correct answer: Label 1
Label 1 is the Router Alert label, which causes each transit LSR to examine the packet more closely rather than just switching it.
Question 7: In MPLS, what does 'penultimate hop popping' (PHP) accomplish?
- Adds an extra label at the ingress PE
- Removes the top label one hop before the egress (Correct answer)
- Swaps all labels at the penultimate router
- Merges two LSPs at the second-to-last hop
Correct answer: Removes the top label one hop before the egress
PHP removes the outermost MPLS label at the penultimate LSR, reducing the egress PE's processing burden to a single lookup.
Cisco ENCOR (MPLS) 2
Which MPLS label operation replaces an existing label with a new label?