642-887 全真模擬試験 DEMO

問題1: Which four pieces of information are stored for each prefix in the LFIB? (Choose four.)
A. incoming interface
B. next-hop IP address
C. outgoing interface
D. Layer 2 header rewrite information
E. outgoing label
F. local label
正解: B,C,E,F
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 it
s 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.