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MPLS Fundamentals

Here you will find answers to MPLS Fundamentals Questions

Question 1

What is a benefit of CEF switching?

A. CEF supports IP source prefix-based switching using the FIB.
B. CEF uses less memory than fast switching uses.
C. CEF is less CPU intensive than fast switching is.
D. CEF provides Netflow statistics with minimum CPU overhead.
E. CEF allows multiple data planes to share a common control plane.

 

Answer: C

Explanation

CEF offers the following benefits:
* Improved performance – CEF is less CPU-intensive than fast switching route caching. More CPU processing power can be dedicated to Layer 3 services such as quality of service (QoS) and encryption.
* Scalability – CEF offers full switching capacity at each line card when distributed CEF (dCEF) mode is active.
* Resilience – CEF offers unprecedented level of switching consistency and stability in large dynamic networks. In dynamic networks, fast switching cache entries are frequently invalidated due to routing changes. These changes can cause traffic to be process switched using the routing table, rather than fast switched using the route cache. Because the Forwarding Information Base (FIB) lookup table contains all known routes that exist in the routing table, it eliminates route cache maintenance and the fast switch/process switch forwarding scenario. CEF can switch traffic more efficiently than typical demand caching schemes.

(Reference: http://www.cisco.com/en/US/docs/ios/12_1/switch/configuration/guide/xcdcef.html)

Question 2

What are two concerns when implementing CEF switching? (Choose two)

A. increased CPU utilization from maintaining the FIB table
B. increased memory requirement
C. the requirement to disable other IOS features such as NBAR and MQC
D. increased memory requirement on the VIP when implementing DCEF
E. configuration complexity
F. troubleshooting complexity because of the many tables that CEF maintains

 

Answer: B D

Explanation

If the router contains too many routing (for example 100,000 networks), enabling the CEF may consume excessive memory. Enabling the distributed CEF will also affect memory utilization on Versatile Interface Processor (VIP) modules or line cards, because the entire FIB table will be copied to all VIP modules or line cards.

Question 3

Using MPLS unicast IP forwarding, what will happen if an LSR receives an unlabeled incoming packet?

A. It will process switch the packet by doing a routing table lookup.
B. It will forward the packet using the LFIB.
C. It will forward the packet using the LIB.
D. It will forward the packet using the FIB.
E. It will drop the packet immediately.

 

Answer: D

Explanation

The FIB (in the data plane) is the database used to forward unlabeled IP packets. A forwarded packet is labeled if a next-hop label is available for a specific destination IP network. Otherwise, a forwarded packet is not labeled.

Note:

The LIB (in the control plane) is the database used by Label Distribution Protocol (LDP) where an IP prefix is assigned a locally significant label that is mapped to a next-hop label that has been learned from a downstream neighbor.

The LFIB (in the data plane) is the database used to forward labeled packets. Local labels, previously advertised to upstream neighbors, are mapped to next-hop labels, previously received from downstream neighbors.

Question 4

When using MPLS unicast IP forwarding, what will happen if an LSR receives an incoming labeled packet but the LSR can’t find that incoming label in its LFIB?

A. The packet will be forwarded using the FIB.
B. The packet will be forwarded using the LIB.
C. The packet will be process switched by performing a route lookup in the routing table.
D. The packet will be forwarded using the LFIB with an imp-null outgoing label.
E. The packet will be dropped even if the IP destination exists in the FIB.

 

Answer: E

Question 5

Which three fields must be included in MP-BGP updates between PE routers in an MPLS network? (Choose three)

A. VPNv4 address
B. label used for VPN packet forwarding
C. AS path
D. Site of Origin
E. TE path designator

Answer: A B C

Explanation

An MP-BGP update contains these elements:

  • VPNv4 address
  • Extended communities (route targets, optionally SOO)
  • Label used for VPN packet forwarding
  • Any other BGP attribute (for example, AS path, local preference, MED, standard community)

An MP-BGP update exchange between PE routers contains these elements:

  • VPNv4 address
  • Extended BGP communities (route targets [RTs] are required; Site of Origin [SOO] is optional)
  • Label used for VPN packet forwarding

Question 6

Which three statements correctly describe how labels are used in cell-mode MPLS? (Choose three)

A. The ATM header VPI/VCI field is used for forwarding decisions.
B. The 32-bit MPLS label is preserved in the frame but is not used in the ATM network.
C. The original MPLS label is present only in the first cell of a packet.
D. The 32-bit MPLS label is inserted between the ATM header and the ATM AAL5 header.
E. Labels in cell-mode MPLS are not a scarce resource like in frame-mode MPLS.
F. The TTL field in the MPLS label is copied into the ATM header TTL field by the ingress edge LSR.

 

Answer: A B C

Explanation

Cell-mode MPLS is MPLS using ATM Layer 2 encapsulation, where the ATM switch is participating as an LSR. In cell-mode MPLS, a label cannot be inserted on every cell; therefore, the virtual path identifier/virtual channel identifier (VPI/VCI) fields in the ATM header are used as a label.

Cell-mode MPLS uses the ATM header’s VPI/VCI field for forwarding decisions while the 32-bit label is still preserved in the frame but not used in the ATM network. The original label is only present in the first cell of a packet.

MPLS_Frame_mode_Cell_mode.jpg

Question 7

Refer to the exhibit. It shows an MPLS network with four routers as well as two customer routers. Router E is advertising the network 192.168.12.0/24. Rather than propagating the 192.168.12.0/24 network, Router D aggregates the 192.168.12.0/24 route into 192.16.0.0/16. Which route or routes will Router B receive?

MPLS_path_selection.jpgA. only 192.168.0.0/16
B. only 192.160.12.0/24
C. both 192.168.0.0/16 and 192.160.12.0/24
D. neither of the routes-the LSP is broken

 

Answer: A

Question 8

Refer to the exhibit. A diagram of a router connected to an MPLS-enabled ATM switch via an LC-ATM MPLS interface, and a partial configuration for the MPLS-enabled ATM switch and router are shown. Which statement describes what is incorrect about the configuration shown?

MPLS_ATM.jpg

A. CEF has not been enabled on the router.
B. The VPI range of 2-3 is invalid.
C. The control VPI/VCI has not been set to 0/32 on the router.
D. VC-merge has not been enabled on the ATM switch interface.
E. The router has not been configured to specifically use LDP.

 

Answer: A

Question 9

Refer to the exhibit. A router connected to an MPLS-enabled ATM switch via an LC-ATM MPLS interface, and a partial configuration for the MPLS-enabled ATM switch and the router are shown. Which statement describes what is incorrect about the configurations shown?

MPLS_ATM_subinterface.jpg

A. The router subinterface is not specified as an MPLS cell-mode subinterface.
B. CEF has not been enabled on the ATM interface of the MPLS-enabled ATM switch.
C. The control VPI/VCI has not been set to 0/32.
D. VC-merge has not been enabled on the ATM interface of the MPLS-enabled ATM switch.
E. The router has not been configured to specifically use LDP.
F. The ATM interface of the MPLS-enabled ATM switch has not been configured for cell-mode MPLS operations.

 

Answer: A

Categories: BGP + MPLS 642-691 Exam Tags:
  1. jude
    July 1st, 2011 at 08:46 | #1

    please rectify the answer in question 17 A. only 192.168.0.0/16 to reflect 192.16.0.0/16.Might be confusing to some people

  2. Tamiru
    November 18th, 2011 at 13:20 | #2

    Anybody on Question 7 Please,
    because the “summary-only” command is not published. How could it advertise only the prefix, How about the the specific prefix 192.168.12.0/24?

  3. sharklover
    January 2nd, 2012 at 18:31 | #3

    regarding question-7:
    i think there is typo mistake how could router D aggregates 192.168/24 route into 192.16/16
    i think i should b 192.168/16.

  4. Chapangara
    January 11th, 2012 at 18:47 | #4

    Thats a typo on number 7.

  5. ski
    March 7th, 2012 at 13:32 | #5

    Hi all,

    regarding question 7. The routers will get the /24 as well the /16 route, due to the BGP aggregation does not include the summary-only feature. So in this case the answer C, also both is the right answer.

  6. Sreekanth
    May 7th, 2012 at 09:35 | #6

    I think they never mentioned about the routing protocol as BGP for summarizing the routes in Question no 7 , so by a normal IGP , a neighbor only get a summary address .

    Hence option A should be correct

  7. Peter
    May 12th, 2012 at 00:26 | #7

    @Sreekanth, we normally don’t call aggregate for IGP summary, so I believe @ski also correct.

  8. ki
    May 15th, 2012 at 14:51 | #8

    Well also normally BGP is not used for core routing. By the way how do IGP link state protocols like OSPF, do summary which are normally used internal routing unless there is area boundary.

  9. chris
    July 26th, 2012 at 11:56 | #9

    For question 7, router D will only advertise the more specific routes along with the aggregate route if it’s running BGP for core routing. normally, the MPLS core uses an IGP like OSPF, which does not advertise more specific routes along with the aggregate.

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