Refer to the exhibit. According to the output of the command show tag-switching forwarding-table,
which four of these statements are true? (Choose four.
A.
Packets to the IP address 10.10.10.5/32 will be tagged with “17” toward the next hop.
B.
Label “19” will be advertised to MPLS neighbors so that they can use this label to reach the IP
address 10.10.10.6/32.
C.
IP address 10.10.10.4/32 is directly connected to the neighbor router on serial 3/0.
D.
Packets arriving with label “17” will be forwarded without any label toward serial 4/0.
E.
Packets arriving with label “20” will be forwarded with label “21” after label-swapping.
F.
Label “20” is advertised to MPLS neighbors so that they can use this information to reach the
prefix 10.10.10.8/32.
Explanation:
Label stacking is the encapsulation of an MPLS packet inside another MPLS packet – that is,adding an MPLS header “on top of” (hence stacking) an existing MPLS header. The result of
stacking is the ability to tunnel one MPLS LSP inside another LSP.
The primary advantage of LDP is that is scales well. It signals LSPs hop-by-hop, and so routers
along the path do not have to maintain state for each LSP. Therefore LDP is useful in edge
applications such as VPNs where hundreds or thousands of LSPs are originated and terminated.
But LDP has no traffic engineering capabilities; it just follows the IGP shortest path to find LSP
end-points.
A central concept to MPLS is the Forwarding Equivalence Class (FEC), and it’s something many
people new to the technology struggle to understand. So in this post I’d like to discuss FECs and
their role in MPLS.
An FEC is a set of packets that a single router:
(1) Forwards to the same next hop;
(2) Out the same interface; and
(3) With the same treatment (such as queuing).
FECs are nothing new. Every router performing generic IP forwarding determines the next hop to
which the packet is to be forwarded, the interface out which the packet is sent to get to that next
hop, and how to queue the packet for that interface. But we don’t often hear those very basic
procedures presented as “determining what FEC a packet belongs to.”