Which two fundamental modifications, related to traffic forwarding, does MPLS introduce?
(Choose two.)
A.
IP lookup is performed on every hop within the MPLS core.
B.
IP destination routing is reduced to label lookup within the MPLS network.
C.
For unicast routing, labels are assigned to FECs (in other words, IP prefixes).
D.
For multicast routing, labels are assigned to IP multicast groups.
Explanation:
MPLS works by tagging packets with an identifier (a label) to distinguish the LSPs. When a packet
is received, the router uses this label (and sometimes also the link over which it was received) to
identify the LSP. It then looks up the LSP in its own forwarding table to determine the best link
over which to forward the packet, and the label to use on this next hop.
A different label is used for each hop, and it is chosen by the router or switch performing the
forwarding operation. This allows the use of very fast and simple forwarding engines, as the router
can select the label to minimize processing.
Ingress routers at the edge of the MPLS network use the packet’s destination address to
determine which LSP to use. Inside the network, the MPLS routers use only the LSP labels to
forward the packet to the egress router.
In the diagram above, LSR (Label Switched Router) A uses the destination IP address on each
packet to select the LSP, which determines the next hop and initial label for each packet (21 and
17). When LSR B receives the packets, it uses these labels to identify the LSPs, from which it
determines the next hops (LSRs D and C) and labels (47 and 11). The egress routers (LSRs D
and C) strip off the final label and route the packet out of the network.
As MPLS uses only the label to forward packets, it is protocol-independent, hence the term “MultiProtocol” in MPLS. Packet forwarding has been defined for all types of layer-2 link technologies,
with a different label encoding used in each case.
