Cisco ENCOR (MPLS) 4 — Questions and Answers
Question 1: Which MPLS feature allows a single physical interface to carry traffic for multiple VPNs without sub-interfaces?
- MPLS TE Fast Reroute
- VRF-Lite (Correct answer)
- MPLS over GRE
- Segment Routing
Correct answer: VRF-Lite
VRF-Lite associates a physical or logical interface directly to a VRF, allowing multiple VPN instances on one router without MPLS label switching in the core.
Question 2: In MPLS Segment Routing (SR-MPLS), what does a Node Segment ID (SID) represent?
- A dynamic LDP label for a specific FEC
- A globally unique MPLS label identifying a specific router (Correct answer)
- A local label assigned per BGP VPN prefix
- An RSVP-TE tunnel identifier
Correct answer: A globally unique MPLS label identifying a specific router
A Node SID is a globally significant MPLS label (from the SRGB) that uniquely identifies a node in the SR domain and instructs transit routers to forward toward that node.
Question 3: What is the Segment Routing Global Block (SRGB)?
- A reserved range of MPLS labels used for Segment Routing SIDs (Correct answer)
- The BGP address family for SR routes
- A group of routers participating in SR-TE
- The OSPF area designated for SR distribution
Correct answer: A reserved range of MPLS labels used for Segment Routing SIDs
The SRGB is the range of MPLS label values reserved on a router for Segment Routing, from which Node and Prefix SIDs are allocated.
Question 4: In MPLS TE Fast Reroute (FRR), what is a 'backup tunnel'?
- A secondary BGP path for VPN traffic
- A pre-signaled LSP used to reroute traffic around a failure (Correct answer)
- An LDP session backup for label distribution
- A standby VRF for failover routing
Correct answer: A pre-signaled LSP used to reroute traffic around a failure
FRR backup tunnels are pre-established RSVP-TE LSPs that provide sub-50ms rerouting around a protected link or node failure.
Question 5: Which LDP message type is used by a router to advertise its label bindings to neighbors?
- Hello message
- Initialization message
- Label Mapping message (Correct answer)
- Notification message
Correct answer: Label Mapping message
LDP Label Mapping messages carry FEC-to-label bindings that a router distributes to its LDP peers.
Question 6: What is 'targeted LDP' used for in MPLS networks?
- Establishing LDP sessions between non-directly-connected routers (Correct answer)
- Distributing labels only within an OSPF area
- Enabling LDP on loopback interfaces only
- Advertising TE constraints via LDP
Correct answer: Establishing LDP sessions between non-directly-connected routers
Targeted LDP establishes LDP sessions between routers that are not directly connected, commonly used for AToM (Any Transport over MPLS) pseudowire signaling.
Question 7: In MPLS, what does the S-bit (Stack bit) set to 1 in a label entry indicate?
- The label is a special reserved label
- This label is the bottom of the label stack (Correct answer)
- The packet requires explicit-null treatment
- The label was assigned by LDP, not RSVP
Correct answer: This label is the bottom of the label stack
The S-bit (Bottom of Stack bit) set to 1 indicates that this label entry is the last (innermost) label in the MPLS label stack.
Cisco ENCOR (MPLS) 4
Which MPLS feature allows a single physical interface to carry traffic for multiple VPNs without sub-interfaces?