Which of the following protocols uses label-switching routers and label-edge routers to forward traffic?
Correct Answer: D
Section: Network architecture
In an MPLS network, data packets are assigned labels. Packet-forwarding decisions are made solely on the contents of this label, without the need to examine the packet itself. MPLS works by prefixing packets with an MPLS header, containing one or more labels. An MPLS router that performs routing based only on the label is called a label switch router (LSR) or transit router. This is a type of router located in the middle of a MPLS network. It is responsible for switching the labels used to route packets. When an LSR receives a packet, it uses the label included in the packet header as an index to determine the next hop on the label-switched path (LSP) and a corresponding label for the packet from a lookup table. The old label is then removed from the header and replaced with the new label before the packet is routed forward. A label edge router (LER) is a router that operates at the edge of an MPLS network and acts as the entry and exit points for the network. LERs respectively, add an MPLS label onto an incoming packet and remove it off the outgoing packet. When forwarding IP datagrams into the MPLS domain, an LER uses routing information to determine appropriate labels to be affixed, labels the packet accordingly, and then forwards the labelled packets into the MPLS domain. Likewise, upon receiving a labelled packet which is destined to exit the MPLS domain, the LER strips off the label and forwards the resulting IP packet using normal IP forwarding rules.