Which four pieces of information are stored for each prefix in the LFIB? (Choose four.)
A.
local label
B.
outgoing label
C.
next-hop IP address
D.
outgoing interface
E.
incoming interface
F.
Layer 2 header rewrite information
Explanation:
Forwarding Labeled Packets
LSR forwards the packet based on:
Top Label value of the received packet
Corresponding entry in LFIB (LABEL <=> INTERFACE)
#show mpls forwarding-table – will show:
local label
outgoing label
prefix (network)
outgoing interface
next-hop
LSR expects packet to come with “top” label being “Local” (from show mpls forwarding-table).
If Outgoing label is “Aggregate”, then that means that this is a summary route and more specific
lookup is performed.
If LSR cannot find label/interface mapping in LFIB, then it drops the packet.
There are several “RESERVED” labels numbered from 0 to 15:
0 – explicit NULL – is used to preserver QoS info through EXP bits. It copies ‘ip prec’ or DiffServ.
1 – Router alert label – forces LSR to software switch the packet.
3 – Implicit NULL – this label is used for “connected” or “summary” routes. This way LSR signals its
neighbors to execute “POP label” operation on “connected” routes. It is called PHP, Penultimate Hop
Popping, and is used to make sure that LSR does not perform 2 lookups (label + ip).
14 – OEM alert label – is used for monitoring purpose.
In Cisco IOS, the default range is 16 through 100,000, but can be expanded by using “mpls label
range” command.
