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Sunday, September 4, 2011

MPLS Interview Questions

1. 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.

2. What is MPLS and why it is being so popular in short time?

MPLS is multi protocol label switching mechanism which uses the label to forward the traffic to the next hop address. It is popular because it must be used for CPN (Converge Packet Network).

3. What is the protocol used by MPLS?

MPLS uses TDP or LDP.

4. MPLS works on which layer?

It works between layer 2 and layer 3.

5. 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.

6. 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.

7. Two routers are having 4 equal cost links, how many ldp sessions will be established?

One session

8. 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.

9. What is Penultimate Hop Popping and why it is required? Which router performs the PHP function?
Second last router performs the Penultimate Hop Popping function to remove the top most label.

11. What are the different types of labels?

Implicit Null, Explicit Null, Aggregate Label etc.,

12. How to make customer route unique?

By adding route distinguisher

13. What is the difference between RD and RT?

RD is not an extended community where as RT is an extended community.

14. Can I assign a same RD to two different customers?

RD is unique and local to router.

15. Is RD travels in route update?


16. My customer is having three branches and all are attached to three different PEs, In this case can I use the different vrf names?


17. What is downstream on demand?

Downstream router is the one which is responsible to advertise the label first to upstream router in case of downstream on demand method is selected.
Upstream router is the one which advertise the labels to its downstream router after receiving label bindings from it.

18. How to filter MPLS labels?

By using ACLs

19. What is the default range of MPLS labels in Cisco routers? How to extend that range?

16 – 100000 is default range

20. Without route reflector can I implement MPLS?

Yes, need to develop full mesh BGP

21. What is the difference between VPNv4 and IPv4 address family?

we always accept and forward ip packets to customers, for this we need to use ipv4 address-family. When the customers packets are being received by PE they become labeled one and to forward labeled packets to different PE/RR; address-family vpnv4 is required. In short we can say that ipv4 address-family is being used for customers and vpnv4 address-family is used by SP core.

22. What is MP-iBGP? Can we use normal BGP in lieu of MP-iBGP?

No, MP-iBGP is used because of the support of multi protocol which normal BGP doesn’t support

24. What is CEF and without enabling CEF, can we make MPLS work?

 CEF is mandatory in Cisco routers for MPLS.

25. I am receiving end to end customer routes on various PE but not able to ping those routes, what’s could be the problem?

LDP is not configured in the path.

27. Default timers of LDP?

28. Does LDP require OSPF, IS-IS or BGP?

 IGP is required for IP Reachability

29. In neighbor discovery command, I am receiving only xmit, what does it mean?

 At another end MPLS IP is not configured.

30. What is transport address?

Route id is transport address


  1. Good Stuff ...Short and direct answers:)

  2. good knowledge on MPLS. nice answers thank q very much

  3. nice! nice!! nice!!!

  4. For question
    15. Is RD travels in route update?
    the correct answer is : RD + address is the route. It does travel in route update. RD is used to make routes unique in mp bgp network.

  5. its just cramming and usless

  6. Thanks your work helped many users around the globe... Good Work , please keep it work

  7. yep answer to question 15 is incorrect

    1. RD is locally significant so it doesnt travel in the updates

    2. RD is locally significant per PE, but it makes the VPN routes unique, so it requires to be carried in the route updates.

      you can see the update packet in sniffer, RD is displayed in the NLRI part

    3. PE may have several VRFs, but it only has 1 BGP table.
      When a route is redistributed into BGP from the VRF (export), the RD is associated as part of the route, which makes the route unique within the entire BGP table.
      Adding the RD, also makes this a VPNv4 route, separate from the IPv4 routes in BGP.
      RT controls which routes will be imported on other PE, for different VRF's.

  8. Question 15 is WRONG,,
    An RD is carried along with a route via MP-BGP when exchanging VPN routes with other PE routers.

  9. Thank you but still not all the answers are correct and you need to correct the wrong answers

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  11. Mind blowing...supv stuff....

    Can you share ques and ans associated to JNCIA and JNCIS

  12. Mind blowing...supv stuff....

    Can you share ques and ans associated to JNCIA and JNCIS

  13. Hello,

    The 15th question's answer is wrong, RD ( Route distinguish ) is carried under route update when we are using VPNv4 address family between two iBGP peers on cisco **(using 15.3T platform)

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