Router A uses label 42 to forward a packet toward its destination. When it forwards the packet to router B, router B receives the packet with label 17. What happened to label 42?
ALabel 42 was corrupted in transit and router B corrected it to 17
BRouter A swapped label 42 for label 17 before forwarding, because labels have only local significance on each link
CRouter B relabeled the packet because it disagrees with router A's classification
DThe original label 42 is preserved inside the packet; label 17 is a new outer label added to a stack
MPLS labels have local significance — a label is meaningful only on the specific link between two adjacent routers. Router A and router B negotiate (via LDP or RSVP-TE) which label router B expects for traffic going to a given destination. Router A swaps its incoming label (42) for the outgoing label that router B expects (17) before forwarding. This per-hop label swap is the defining operation of a label switch router (LSR) and is what makes MPLS a label-switching technology, not a label-tunneling one.
Question 2 Multiple Choice
A network engineer claims that MPLS replaces IP routing in the network core. Why is this statement incorrect?
AMPLS does replace IP routing, but only for traffic that has been classified and labeled at the ingress router
BMPLS replaces IP routing in the core but still requires IP at the edges where packets enter and leave
CMPLS runs alongside IP routing — it uses IP routing to establish label-switched paths, and label forwarding operates on top of that infrastructure
DMPLS is being phased out, so the statement is only incorrect because MPLS no longer operates in real networks
MPLS does not replace IP routing — it depends on it. The Label Distribution Protocol (LDP) and RSVP-TE that distribute labels and establish Label Switched Paths use IP routing to communicate between routers. The ingress LER uses the IP routing table to decide which LSP to assign a packet to. MPLS provides an alternative forwarding mechanism inside the network, but the IP routing infrastructure remains essential for path establishment and label management. Interior LSRs skip the per-packet IP lookup, but IP routing underlies the entire system.
Question 3 True / False
In an MPLS network, interior label switch routers (LSRs) should examine the IP header of nearly every packet to determine where to forward it.
TTrue
FFalse
Answer: False
This is the key forwarding advantage of MPLS. Interior LSRs read only the MPLS label — a 20-bit value — and look it up in a flat label forwarding table. This is a direct index operation, far faster than longest-prefix IP matching against potentially hundreds of thousands of routes. Only the ingress LER (at the network edge) examines the IP header to assign the initial label. Once inside the MPLS network, IP-level forwarding is bypassed entirely until the egress LER pops the label and delivers the packet as a normal IP datagram.
Question 4 True / False
MPLS can carry traffic from multiple different Layer 3 protocols (IPv4, IPv6, and others) because label forwarding does not depend on the contents of the encapsulated header.
TTrue
FFalse
Answer: True
The 'Multiprotocol' in MPLS is meaningful: because interior LSRs forward based on the label alone and never inspect the Layer 3 payload, any Layer 3 protocol can be tunneled through the same MPLS infrastructure. An LSR does not need to understand IPv4, IPv6, or any other protocol in the payload — it simply reads the label, swaps it, and forwards. This protocol-agnosticism is one of the reasons MPLS became the backbone technology for carrier networks supporting diverse customer traffic types.
Question 5 Short Answer
Why do MPLS labels have 'local significance only,' and what coordination mechanism makes this work across an entire network?
Think about your answer, then reveal below.
Model answer: An MPLS label is an arbitrary number meaningful only on a single link between two adjacent routers — router A might use label 42 to mean 'traffic headed for destination X,' but router B uses label 17 for the same traffic on the next link. There is no global label namespace. This works because adjacent routers negotiate label bindings before traffic flows: protocols like LDP or RSVP-TE cause each router to advertise which label it expects to receive for each destination, and its upstream neighbor learns to swap its own label to match. The complete sequence of per-hop label assignments from ingress to egress defines a Label Switched Path (LSP).
The local-significance design allows labels to be small (20-bit) without risk of global collision — each router independently manages its own label space. The per-hop swap operation is what makes MPLS a switching technology rather than a tunneling technology: rather than encapsulating the original packet in a fixed header stripped at the far end, each intermediate router actively rewrites the label. This also enables label stacking, where multiple labels are stacked to support VPN separation and traffic engineering simultaneously on shared infrastructure.