Cisco ENCOR (MPLS) 5 — Questions and Answers
Question 1: Which Cisco IOS command enables MPLS forwarding on a specific interface?
- mpls ip (Correct answer)
- label-switching
- mpls enable
- ip mpls forward
Correct answer: mpls ip
The interface-level command 'mpls ip' enables MPLS forwarding and LDP label distribution on that interface.
Question 2: What is the MPLS EXP (experimental) field in a label entry now officially renamed to in RFC 5462?
- Priority bits
- Traffic Class (TC) bits (Correct answer)
- CoS bits
- DiffServ Code Point (DSCP)
Correct answer: Traffic Class (TC) bits
RFC 5462 renamed the MPLS EXP field to Traffic Class (TC) bits, reflecting its primary use for QoS marking in MPLS networks.
Question 3: In MPLS L2VPN, what is an 'attachment circuit' (AC)?
- The BGP session between PE routers for VPN route exchange
- The physical or logical interface connecting a CE to a PE (Correct answer)
- The MPLS LSP across the provider core
- The LDP session used to signal pseudowires
Correct answer: The physical or logical interface connecting a CE to a PE
An attachment circuit is the physical or logical link between a CE device and the PE router, carrying customer L2 traffic into the MPLS network.
Question 4: Which protocol is used to signal pseudowires in MPLS-based L2VPN services (AToM)?
- RSVP-TE
- LDP (targeted) (Correct answer)
- BGP with EVPN
- OSPF-TE
Correct answer: LDP (targeted)
AToM (Any Transport over MPLS) uses targeted LDP sessions between PEs to signal and establish pseudowires for L2VPN services.
Question 5: What does the MPLS 'TTL' field control in a label entry?
- The maximum number of labels in the stack
- Hop count to prevent routing loops, decremented at each LSR (Correct answer)
- The time a label binding remains valid in LDP
- The QoS priority for the labeled packet
Correct answer: Hop count to prevent routing loops, decremented at each LSR
The 8-bit TTL field in each MPLS label is decremented at each LSR to prevent forwarding loops, similar to the IP TTL field.
Question 6: In MPLS SR (Segment Routing), what is an Adjacency SID used for?
- Identifying a specific router node globally in the SR domain
- Forwarding traffic over a specific link between two routers (Correct answer)
- Defining the SRGB range on a router
- Distributing VPN labels between PE routers
Correct answer: Forwarding traffic over a specific link between two routers
An Adjacency SID identifies a specific link (adjacency) between two routers, allowing traffic to be steered over that exact interface.
Question 7: Which show command displays the MPLS label forwarding information base (LFIB) on a Cisco router?
- show ip route mpls
- show mpls ldp neighbor
- show mpls forwarding-table (Correct answer)
- show mpls traffic-eng topology
Correct answer: show mpls forwarding-table
The 'show mpls forwarding-table' command displays the LFIB, showing incoming labels and their corresponding outgoing labels and interfaces.
Cisco ENCOR (MPLS) 5
Which Cisco IOS command enables MPLS forwarding on a specific interface?