Friday, July 2, 2010

MPLS Questions Answers for CCNA and CCNP Engineers

Few interview MPLS questions I have added in my post MPLS Interview Questions for CCNA and CCNP Engineers. In this I am adding the quick answers to all the questions. If someone knows the best than the answers, please comment it.

1. 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. 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. MPLS uses TDP or LDP.
4. It works between layer 2 and layer 3.

Have you heard about Segment Routing? If not, look at the below given post. This is the next generation protocol which is already replacing LDP.
Segment Routing: Alternate of LDP and RSVP Segment Routing Traffic Engineering Basics Of Segment Routing Layer 3 MPLS VPN with Segment Routing - Nodal Segment Segment Routing Based MPLS Vs Classic MPLS Segment Routing Deployment - Control and Data Plane Segment Routing Control Plane - ISIS SR and LDP Interworking
5. 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. 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. One session
8. 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. Second last router performs the Penultimate Hop Popping function to remove the top most label.
10. See Aggregate Labels for this answer
11. Very easy
12. By adding route distinguisher
13. RD is not an extended community where as RT is an extended community.
14. RD is unique and local to router.
15. No
16. Yes
17. See this post (downstream on demand)
18. By using acl
19. 16 – 100000 is default range
20. Yes, need to develop full mesh BGP
21. See this post (Difference between VPNv4 and IPv4)
22. No, MP-iBGP is used because of the support of multi protocol which normal BGP doesn’t support
23. See MPLS Fundamentals
24. CEF is mandatory in Cisco routers for MPLS.
25. LDP is not configured in the path.
26. See this post (Implicit Null)
27. Refer MPLS Fundamental
28. IGP is required for IP Reachability
29. At another end MPLS IP is not configured.
30. Route id is transport address
31. 3031
32. Because it supports almost each and every protocol.
33. Very Easy
34. TDP is Cisco proprietary
35. Yes it supports
36. Yes we can use
37. Answer already given
38. Answer already given
39. Answer already given
40. No, IGP will work as it is but MPLS customer traffic forwarding will stop

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Anonymous said...

This is indeed helpful

Amitabh said...

hello Shivlu,

Recently, I have encountered a question that, in any central services VPN scenario if 2 customers are advertising same network prefixes (overlapping) and want to access a central vrf then what will happen. Is the return traffic knows where forward packet??Some people say that if the RD is unique(per VPN and per PE) then it can not be a problem. I'm little confused ....what actually will happen and how?

interview questions said...

The provided info was very interesting & informative!

julieanderson said...

I have been across many questions but these are different and very useful one!