MPLS is a forwarding mechanism that allows routers to forward packets based on labels rather than IP addresses. It is called multi-protocol label switching because it supports various protocols and layer 2.5 technology because it places the MPLS header between the layer 2 and layer 3 headers. MPLS provides advantages like decreasing forwarding overhead, supporting applications like VPNs, TE, and QoS, and supporting non-IP protocols. It uses protocols like LDP and TDP and has default label ranges of 16-100000.

2. Question - What is MPLS?
MPLS is a forwarding mechanism which allows the
router to forwarding packets based upon labels instead
of IP Address.
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3. Question - Why MPLS is called as Multi Protocol Label
Switching?
MPLS is called as Multi Protocol because it supports
all the protocols like Ethernet, Frame-Relay , X.25 and
forwards packets based upon Labels.
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4. Question - Why MPLS is called as Layer 2.5
Technology ?
MPLS is called as Layer 2.5 Technology because the
router inserts MPLS Header in between Layer 2 and
Layer 3 Header.
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5. Question - What are the applications of MPLS?
MPLS supports following applications,
– Unicast and multicast IP routing
– VPN
– TE
– QoS
– AToM
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6. Question - What are the advantages of MPLS?
MPLS provides following advantages-
 MPLS decreases the forwarding overhead on the core
routers.
 MPLS supports multiple useful applications such as
VPN,TE,QOS.
 MPLS supports the forwarding of non-IP protocols, because
MPLS technologies are applicable to any network layer
protocol.
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7. Question - What is the protocol used by MPLS?
MPLS uses TDP or LDP.
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8. Question - What is the port number used by LDP?
LDP runs on port number 646 ( UDP port number for
discovery of LDP neighbors and TCP port for
exchanging the label information.
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9. Question - What is LDP?
LDP is a Labelling Protocol used to share label
information with other routers and create label
forwarding table.
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10. Question - What are the prerequisites to run MPLS?
IP CEF
IGP Protocol
LDP router-id must be reachable
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11. Question - What is the default range of MPLS labels in
Cisco routers?
16 – 100000 is default range
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12. Question - What is the difference between P and PE
router?
P router doesn’t have Customer network routes where
in PE router is having customer network routes.
Another reason is P router doesn’t require MP-iBGP
but for PE it is must.
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13. Question - What are the functions done by MPLS?
PUSH ( Adding the Label)
POP ( Removing the Label)
SWAP ( Changing the Label)
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14. Question - Can I make my PE router as P?
To make your PE router as P, you need to remove the
BGP configurations and after that it will not participate
with customer network.
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15. Question - What is Penultimate Hop Popping?
Penultimate hop popping is a method of reducing
label lookups on egress router. It is basically done by
the one hop before the egress router.
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16. Question - What is TTL Propagation in MPLS?
TTL Propagation is a method of copying the TTL value
from IP Header to MPLS header.
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17. Question - What is the difference between Per
Platform Label Space and Per Interface Label Space?
Per Platform label space indicates the labels assigned
based upon the destination network and Per Interface
label space indicates the labels assigned based upon
the destination network and interface.
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18. Question - What is LDP IGP Synchronization?
LDP IGP Synchronization allows the router not to
forward the packets on the link where ldp is down but
IGP is enabled.
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19. Question - Is summarization is recommended in
MPLS?
No
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20. Question - What is the difference between Implicit Null
and Explicit Null?
Implicit Null ( Label 3) is used to indicate the other
router to remove label before sending the packets (
PHP).
Explicit Null ( Label 0) is used for QOS and disables
PHP behavior.
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21. Question - What is the difference between VPN and
MPLS?
VPN is generally Virtual Private Network which could
be configured by using GRE tunnels. In that if you
want a full mesh than administrator need to setup n*n-
1 tunnels. But in case of MPLS VPN, by default CPE
works in full mesh form because of route target.
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22. Question - Two routers are having 4 equal cost links,
how many ldp sessions will be established?
One session
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23. Question - What are the different types of labels?
Implicit Null, Explicit Null, Aggregate Label etc.,
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24. Question - My LDP router id, OSPF router id and BGP
router id is different, will it work to forward the traffic of
customers or not?
LDP router id and BGP router-id should be same if SP
is using labels only for loopbacks. If labels are
generated for each and every route then no problem at
all.
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25. Question - Without route reflector can I implement
MPLS?
Yes, need to develop full mesh BGP
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26. Question - Does LDP require OSPF, IS-IS or
BGP?
Yes, IGP is required for IP Reachability
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