Cisco ENCOR (MPLS) 3 — Questions and Answers
Question 1: In MPLS VPN, what address family does the PE router use to distribute VPN routes between PEs via BGP?
- IPv4 unicast
- IPv6 unicast
- VPNv4 (Correct answer)
- L2VPN EVPN
Correct answer: VPNv4
PEs use the VPNv4 address family (RFC 4364) which prepends a Route Distinguisher to IPv4 prefixes to make them globally unique.
Question 2: What is the purpose of a Route Distinguisher (RD) in MPLS L3VPN?
- Controls which routes are imported into a VRF
- Makes overlapping customer IP prefixes unique in the BGP table (Correct answer)
- Encrypts VPN routes between PE routers
- Defines the MPLS label range per VPN
Correct answer: Makes overlapping customer IP prefixes unique in the BGP table
The RD is prepended to customer prefixes to create unique VPNv4 routes, allowing overlapping IP space from different customers to coexist in the BGP table.
Question 3: What is the function of a Route Target (RT) in MPLS L3VPN?
- Uniquely identifies each VPN route in BGP
- Controls import and export of routes into VRFs (Correct answer)
- Assigns MPLS labels to VPN prefixes
- Determines the BGP next-hop for VPN routes
Correct answer: Controls import and export of routes into VRFs
Route Targets are BGP extended communities used to control which routes are exported from and imported into each VRF.
Question 4: In MPLS L3VPN, how many labels are typically imposed on a packet as it enters the MPLS network?
- One label only
- Two labels — VPN label and transport label (Correct answer)
- Three labels — RD, RT, and transport
- No labels — only the IP header is used
Correct answer: Two labels — VPN label and transport label
Two labels are used: the inner VPN label (assigned by the egress PE to identify the VRF/CE) and the outer transport label (for forwarding across the MPLS core).
Question 5: Which command verifies the MPLS label bindings learned via LDP on a Cisco router?
- show mpls forwarding-table
- show mpls ldp bindings (Correct answer)
- show mpls traffic-eng tunnels
- show ip bgp vpnv4 all
Correct answer: show mpls ldp bindings
The 'show mpls ldp bindings' command displays the label bindings that LDP has distributed and received for each prefix.
Question 6: What happens when an MPLS router receives a packet with label value 3 (Implicit Null)?
- Drops the packet
- Forwards the packet with the label unchanged
- Pops the label and forwards based on the next header (Correct answer)
- Swaps the label with a new transport label
Correct answer: Pops the label and forwards based on the next header
Label 3 (Implicit Null) signals the penultimate hop to pop the label stack, enabling PHP so the egress router only performs an IP lookup.
Question 7: In MPLS Traffic Engineering, what is the purpose of the CSPF (Constrained Shortest Path First) algorithm?
- Distributes LDP labels across the network
- Calculates the best path meeting bandwidth and TE constraints (Correct answer)
- Builds the MPLS forwarding table from IGP routes
- Establishes BGP sessions between PE routers
Correct answer: Calculates the best path meeting bandwidth and TE constraints
CSPF runs on the Traffic Engineering Database (TED) to find the best path satisfying bandwidth, affinity, and other TE constraints before signaling with RSVP-TE.
Cisco ENCOR (MPLS) 3
In MPLS VPN, what address family does the PE router use to distribute VPN routes between PEs via BGP?