Split Horizon and Route Poisoning - Layer 3 Routing Loops

Most of the time, I always be a part of layer 2 routing loop discussion but never heard about the engineers talking about layer 3 routing loop problem. This is being experienced during my recent visit in some company where all engineers knows why to use Spanning Tree Protocol but none of them knew why to use split horizon and route poisoning.
In layer 3 networks, there might chance of routing loops and split horizon, hold down timers and route poisoning are the techniques which help to prevent the layer 3 routing loop. Below depicted Figure 1 is showing the converge network. Let’s assume in case of failure of 10.4.0.0 network, router C will forward the update to router B and router B will forward the update to router A and router C as well. By doing this the same kind of the update which is being generated by C is received and C might think that he is getting the information of 10.4.0.0 network from B but In fact, 10.4.0.0 network is directly attached to router C. This situation can arise in smaller networks too.

The first work around is the split horizon technique which says not to send the updates to the interfaces from it has been received. It looks like send update information (Number of interfaces – Receiving Interface Updates).
Next one is route poisoning, when the router detects link down, the attached router sends the update to its neighbors. But in this case, the receiving router can send back the received information to the same interface from where it received by setting the route metric to maximum. Definitely this is the violation of split horizon rule but it helps router to understand about that particular network is down or inaccessible which actually help the convergence of routing. Now 10.4.0.0 is poisoned route which is having the maximum metric assigned as the route is not reachable. When the neighbor send the route back to the originator, it becomes reverse poisoned.

What does route poisoning do?

1. Set the hop count to an unreachable state as soon as the failed network is detected
2. Route remains poisoned until the hold-down timer expires.
3. Hold timer depends on the routing protocol; Every protocol is having different hold-down timer.
4. Only uni direction traffic flow.
5. If the route is not back up during the hold down time period expires, that route is removed from the routing table and added in the garbage table.

The last one is Hold Down timers. What does hold-timers do?
1. A router receives an update from a neighbor indicating that a network that previously was accessible is now no longer accessible.

2. The receiving router marks that route possibly down and starts the hold-down timer.

3. If an update with a better metric for that network is received from any neighboring router during the hold-down period, the network is reinstated and the hold-down timer is removed.

4. If an update from any other neighbor is received during the hold-down period with the same or worse metric for that network, that update is ignored. Thus, more time is allowed for the information about the change to be propagated.

5. Routers still forward packets to destination networks that are marked as possibly down. This allows the router to overcome any issues associated with intermittent connectivity. If the destination network truly is unavailable and the packets are forwarded, black hole routing is created and lasts until the hold-down timer expires. (Very Important Point). This could be the reason, administrators look forward to reduce the hold-down timers to increase the convergence time. Definitely if the network is not stable these timers generates lot of messages.

As per section 2.2.2, RFC 1058 explicitly says that “Split horizon with poisoned reverse will prevent any routing loops that involve only two gateways. However, it is still possible to end up with patterns in which three gateways are engaged in mutual deception.” Definitely this could be the case of broadcast of multi-access networks.

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Qos Interview Questions?

1. What is QOS and why it is required?
2. What is layer2 qos and layer3 qos?
3. What is tail drop?
4. Describe methods of QOS?
5. What is hardware QOS and Software QOS?
6. Difference between a policer and a shaper?
7. What is token bucket algorithm?
8. Where to define the markings?
9. Does QOS increase the load of the equipment?
10. What is TOS and IP DSCP?
11. what are the different classes available?
12. How to calculate the decimal value of classess?
13. What is the difference between priority and bandwidth command?
14. What is low latecy queueing?
15. what is class based weighted fair queuing?
16. What is first in first out queue (FIFO)?
17. What is fair queue?
18. If I give teh ip precendence five to data traffic, what will happen?

Other Useful Questions
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Do you think interviewer can judge technical knowledge of candidate in short duration of interview?

Last month I spent lot of time for interviewing the new candidates. The interview time was not more than 20 – 30 minutes for each candidate. The question comes; it’s very difficult to examine the candidate during that time period. Then how to get the best candidate for the applied position. I always look forward for the below mentioned point to judge the candidate:-
1. Always look for the basics.
2. Never jump into deep dive; Try to understand the answer from the candidate point of view.
3. If candidate is having good experience, then try to drill down his experience and listen to the problem faced during that time period and solutions proposed by candidate.
4. During the last minutes, I always try to put pressure on candidate and judge his analytical skills.
5. In the last 5 minutes; Give fast questions to candidate that will judge their thinking capabilities.
6. Where do you see your self after 5 years?
The same type of discussion is going on a linkedin.com and I captured few of the comments from the experts.

Most of the time interview is about selling your self, a good interviewer is expected to understand approach and process the person adopted in resolving the problem in past. During that cycle has he evaluated certain other approaches or will he do things differently once the problem is resolved.
This provide a complete insight in person and allow you to take the judgement on candidate to be selected.
Again we also need to evaluate if person is adaptable for new environment, which will allow him to perform.
Again we are only talking of experienced resources?
(Uday Gokhle - Consultant at Tata Consultancy Services)
Understanding the techincal knowledge to some extent can be done but the bigger question here is: that is the intervewer unbaist and is ready to understand the view point of the interviewee or his approach to resolve an issue or address the concern??

Or the interviewee is evaluated based on what a particular interviewer wants to hear or the replies of the candiadtes are more in sync with the interviewers experience then only it gets some weightage
(Preeti Jhangiani – HR Professional)

IT is very difficult to judge a person based on short term interview. But good interviewer can judge the the person's fairness about his job knowledge and exeprience by his body language and answer of tricky questions.
(Rajesh Aggarwal – Manager IT at Jindal)

The interviewer can gauge the technical ability of the person during the short time of interview based on some basic technical questions. But that would not be very accurate. Also sometimes this type of discussions on technical issues lead to Ego satisfaction of the interviewer hence the decision can be biased. Still in certain jobs like Technical Manager etc. it becomes necessary to see the depth of his technical knowledge before arriving to any decision.
Nandan D. Pradhan - DY. GENERAL MANAGER at FAIRMATE CHEMICALS )
According to Nandan, the interview becomes tough according to the applied position.

Talking of the Interview porcess in India ... Im 98% Sure I..... interviewers are not able to judge the candidate correctly Leave aside Technicality, its tru for persoanl traits too...

Simple how can anyone tell in 20 min or so what a person has done in 20 years.

Mos interviews are prefixed - either on strategy or by mindset of person who is talking to the candidate.

This process has to be stopped and a new mechanism needs to be evolved based on mathematical data & logic
Arvind Verma - DGM - QA at Wilo Pumps Limited
I completely agree to Arvind


I have lot of expr. in interviewing the candidates. I allocate lot of time for a candidate. I make them comfortable with me, make a friendly approach. -) An interview need not be of a question and answer type!!, it should be a technical discussion. It should start from basics things and slowly moving to in- depth in his skill areas. -) most of the candidates dont remember/recollect what they have done -) It is difficult to get candidates according to our requirement. what a candidate has done? and what he can do? how we can make him fit for the position? is important!!. -) instead of putting questions continuously, make him feel low.......give him a chance!!

It needs a min. one hour to do everything......
Veny M


Its not possible to judge the person in short term duration but some of the analytical skills could be judged during that time. I have lot of experience in interviewing the candidates, I normally check the basic fundamentals rather going to the deep technical dive. According to me, if the candidate is having good command on basics then he can do anything.
Shivlu Jain (http://www.mplsvpn.info)


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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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Is LDP required for VPNv4 Labels?

In case of layer 3 VPN, two labels are normally carried by packet. But the differentiation between the labels is ipv4 and vpnv4. Ipv4 label is used for IGP and vpnv4 label is used for customer route. Normally a question comes in mind, “Is LDP responsible for both the labels”. The answer is no because LDP is only responsible for the top most label is IGP label and MP-iBGP is responsible for vpnv4 label which is present under beneath of IGP label. Even if the core network is not running LDP, but MP-iBGP is enabled from PE to PE, we can easily see the vpnv4 labels exchange. But the problem is that traffic forwarding will not happen because the core network doesn’t understand the labels.

How BGP does forward the vpnv4 label?

In the given logs, we can see that from 100.100.100.1 to 100.100.100.3 MP-iBGP tunnels are created. In the update message, under Border Gateway Protocol, we have update and path attribute messages. Path attribute is again having six different BGP attributes:-

1. Origin
2. AS_Path
3. MED
4. Local Preference
5. Extended Communities
6. MP Reach NLRI (Network Layer Reach ability Information)

In the depicted image, Carried Extended Communities are the route target values which we define in vrf. In this we have defined 65500:1 as route target value.

Another attribute is MP_REACH_NLRI which plays vital role in MP-iBGP. In this first value is Address Family: IPv4(1); 1 is reserved for IPv4 only. For more see RFC 1700. According to RFC 2858 Address Family Identifier is defined as 

“

         This field carries the identity of the Network Layer protocol

         associated with the Network Address that follows. Presently

         defined values for this field are specified in RFC 1700 (see

         the Address Family Numbers section).”

According to RFC 2858 SAFI is defined as “Subsequent Address Family Identifier:

         This field provides additional information about the type of

         the Network Layer Reachability Information carried in the

         attribute.”

In the given figure SAFI value is 128 because from 128 to 255 values are not being reserved by IANA. These are for private use. It means, if some vendor wants IOT with CISCO MP-iBGP then they might need to use the same SAFI (Not Sure About It).According to draft draft-ietf-l3vpn-rfc2547bis-03 section 4.3.4 SAFI 128 is used for labeled VPN IPv4 addresses.

Next field is next hop network address which is carrying the information of next hop address i.e. router id of advertising router and CISCO is using route distinguisher RD 0:0 for IGP routes. So it means by default global routing is also a part of a vrf which could be called global vrf(not sure).

  

Last field is Network Layer Reachability Information which is actually carrying VPNv4 label 19 also known as bottom label, route distinguisher and ipv4 prefix of vrf.   

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How Does Label Distribution Protocol(LDP) Initializes?

Highest loopback id starts the Label Distribution Protocol initialization process by sending common session parameter TLV which includes a sub TLV of parameters containing session protocol version, session keepalive time, advertisement method, loop detection and session path vector. In the given diagram, I have started MPLS LDP firstly on 100.100.100.1 (R1) and 100.100.100.2(R2). So in this case R2 send a first initialization message to R1 by adding all the above TLV parameters.

Structure of LDP initialization message sent from R2 to R1.

Figure 1 (Click on image to see in new window)

Being R2 is having highest LDP router id, so it start sending the label information to R1. We can say R2 is working as downstream LSR to R1. R2 sending Address message and label mapping message which are sub part of label distribution protocol.

Figure 2(Click on image to see in new window)

Address message is only containing the directly connected interface ip address of R2 which are 1.1.1.2, 2.2.2.2 and 100.100.100.2.

Figure 3(Click on image to see in new window)

Along with this R2 is also sending label mapping message which is containing the information about route, label and address family. Address family means whether it is ipv4 route or vpnv4 route. R2 is sending a information about 1.1.1.0 prefix by including label 3 which is used for PHP (Penultimate Hop Popping).

Figure 4(Click on image to see in new window)

The same way R2 is sending a label 16 to R1 for 100.100.100.1 which is loopback address of R1 itself and label 16 becomes as local label in R2 forwarding table.

Figure 5(Click on image to see in new window)

Figure 6(Click on image to see in new window)

Once this is done then R1 initiates a label mapping process by sending the label and FEC information to R2 ( R2 is upstream LSR to R1).

After this, R3 is started which is having loopback address of 100.100.100.3. In this case R3 will start the initialization process of LDP being R3 is having highest LDP router id.

Figure 7(Click on image to see in new window)

Now R3 becomes downstream LSR to R2 and R2 becomes upstream LSR to R3. Once R3 will exchange all the label mappings then R2 will initiate the process and send the information to R3.

Now how the updates of new route will flood. To test this, a new route 100.100.100.100 is installed in R3. After this we saw a label mapping message was sent from 100.100.100.2 to 100.100.100.1 and vice versa but the communication was being started by 100.100.100.2

Figure 8(Click on image to see in new window)

How does LDP sends new updates to its peer?

LDP uses label mapping message to send the partial label updates to its peer.

Label mapping message is containing the information of new route 100.100.100.100 with label 18.

Figure 9(Click on image to see in new window)

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What is Inter Packet Gap or Inter Frame Gap?

Let's assume if a Gigi port which could transfer raw data up to 125,000,000 bytes per second with minimum frame size of 64 bytes. Device must need some time to process one packet after another. The difference of time to process from one packet to another is known as inter-frame or inter-packet gap. The minimum inter frame gap period is 96 bits or 12 bytes which amounts to 96 nano seconds + 7 byte of preamble and 1 byte of delimiter consequence 84 bytes.

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Advantages of MPLS

A interview always start with the question called "Advantages of MPLS" and most of the time students don't know much about it and start fumbling with not good answers. I am giving few quick answers about MPLS advantages, if some is having other than that please share.

1. No propagation of routes in the core of service provider.
2. In legacy GRE customer is responsible for the management but in case of MPLS SP is responsible.
3. Customers can use the same ip address which is not possible in case of GRE.
4. MPLS saves cost as compared to ATM or frame relay network.
5. MPLS increase the response time.
6. Customers can have the QOS according to their requirements.
7. Fast reroute features by using traffic engineering.
8. MVPN support which saves lot of bandwidth.

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Multicast Interview Questions For Service Provides Candidates

Multicast Interview Questions For Service Provides Candidates

1. Why multicast is required in MPLS SP cloud?
2. Does MPLS understand multicast?
3. What is Assert message in multicast and when these are required?
4. What is the difference between dense mode, sparse mode and sparse dense mode?
5. Can we use dense mode by using sparse mode? If yes, then what is the use of sparse dense mode?
6. What is the difference between (*,G) and (S,G) entry?

7. In sparse dense mode if we don’t use RP what will be the effect?
8. In SSM( Source Specific Multicast) how to advertise RP?
9. Which type of entries created in Auto RP, BSR, MSDP and SSM mode?
10. What is the difference between ip igmp join group and ip igmp static join group?
11. SSM requires IGMPv3 but in MPLS SP cloud why we don’t use IGMPv3?
12. What is the advantage of using SSM in MPLS SP cloud?
13. How to maintain S,G entry for a long time?
14. How to select Multicast Group? Is there any type of overlapping happens, if yes then what’s its advantage and disadvantage?
15. In MPLS cloud, if we define two loopbacks in multicast for creating hierarchical cloud, what will happen?
16. I am receiving RPF failure message at PE end and don’t have access of CE router, how to solve the problem without having CE access?
17. Difference between Shortest path tree and shared path tree?
18. Difference between data and default mdt?
19. Does default mdt works without data mdt and vice versa?
20. Can MPLS SP cloud have SSM and MSDP at same time? If yes, then what’s the reason for specifying two protocols?
21. What is difference between address-family ipv4, vpnv4 and mdt?
22. If SP cloud doesn’t use address-family mdt what will happen?
23. If you ask to design a MPLS based multicast network, what will you suggest?
24. What is MDT tunnel?
25. Can we modify MDT tunnel?
26. What is the multicast source address and group address in MPLS SP cloud?


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Switching Interview Questions For CCNA and CCNP Engineers

After IGP, BGP and MPLS interview question, Now I am adding switching basic questions. Please read the below questions:-

1. Difference between hub, bridge and switch?
2. What is mac address and why it is required?
3. In layer 2 domain do we need ip address for communication?
4. What is arp and why it is required?
5. What is Spanning Tree Protocol aka STP?
6. What is the difference between STP, MSTP, PVST and RSTP?
7. Can we use the two same paths for same vlan?
8. What is the difference between broadcast and collision domain?
9. Define type of lan traffic.
10. What is destination address of broadcast frame?
11. Can we connect a switch to switch with straight cable?
12. Define functions of switch.
13. What is arp timeout?
14. What is aging process?
15. What is BPDU?
16. What is path cost?
17. Define selection criteria of STP root bridge.
18. How to non bridge decide which port will elect as root port?
19. If a nonroot bridge has two redundant ports with the same root path cost, how does the bridge choose which port will be the root port?
20. Port states of spanning tree protocol.
21. If the users face delay during initial login, what you will suggest to implement?
22. Why spanning tree BPDU filter is used?
23. Can I use BPDU filter on trunk ports?
24. What is port security?
25. I want to learn only a single mac from the port, what need to be configured?
26. Can we use spanning port-fast on trunk ports?
27. If management ip address is changed, will user’s traffic will be dropped?
28. Difference between trunk and access port?
29. What is UDLD and why it is required?
30. What is interface vlan on switch?
31. How to perform inter vlan routing without layer 3 device?
32. How to stop superior bpdu participating in switching domain?
33. How Vlan In Local Switching Domain is selected?
34. How to provide redundancy to MPLSVPN customer?
35. What are deep buffers in switching?
36. How to build Low Latency Data Center Switching Fabric?


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MPLS Interview Questions for CCNA and CCNP Engineers

After IGP and BGP questions, I prepared basic list of MPLS interview questions which could help you to clear your next level. In the up coming post, I will be covering switching. Below is the list of questions:-

1. What is the difference between VPN and MPLS?
2. What is MPLS and why it is being so popular in short time?
3. What is the protocol used by MPLS?
4. MPLS works on which layer?
5. What is the difference between P and PE router?
6. Can I make my PE router as P?
7. Two routers are having 4 equal cost links, how many ldp sessions will be established?
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?
9. What is Penultimate Hop Popping and why it is required? Which router performs the PHP function?
10. I am receiving aggregate label, what does it mean?
11. What are the different types of labels?
12. How to make customer route unique?
13. What is the difference between RD and RT?
14. Can I assign a same RD to two different customers?
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?
18. How to filter MPLS labels?
19. What is the default range of MPLS labels in Cisco routers? How to extend that range?
20. Without route reflector can I implement MPLS?
21. What is the difference between VPNv4 and IPv4 address family?
22. What is MP-iBGP? Can we use normal BGP in lieu of MP-iBGP?
23. What is LIB, LFIB?
24. What is CEF and without enabling CEF, can we make MPLS work?
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?
26. What is explicit null and implicit null?
27. Default timers of LDP?
28. Does LDP require OSPF, IS-IS or BGP?
29. In neighbor discovery command, I am receiving only xmit, what does it mean?
30. What is transport address?
31. What is the RFC of MPLS?
32. Why MPLS is called multi protocol?
33. What is the difference between MPLS, SSL and IPSec?
34. I am using different vendor products and want to implement TDP, what type of challenges will you face?
35. Does MPLS support IPv6?
36. Can I use the existing IPv4 MPLS backbone for IPv6?
37. Define various troubleshooting commands in MPLS?
38. What is forward equivalence class aka FEC?
39. What is adjacency table?
40. Difference between MPLS IP and MPLS Label Protocol LDP command?
41. If MPLS get disable, will it harm my IGP or IPv4 traffic?
Click here for answers
42. What is MPLS-TP?
43. What is downstream and upstream router in MPLS? 44. Difference between MPLS and MPLS-TP?
45. How Does LDP Initializes?
46. What is Cell Mode MPLS Over ATM?
47. Difference Between VC Based Multiplexing And Logical Link Control Encapsulation in ATM?
48. Basics of ATM?
49. ATM is packet or circuit switching?
50. Is LDP Required for VPNv4 Labels?
51. What will happen if you see your PE loopback in vpnv4 table?
52. What is Bidirectional Forwarding Detection?
53. Different types of PseudoWire?
54. Modes of EoMPLS (Ethernet over MPLS)
55. What is L2VPN Over Metro Ethernet?
56. What is E-VPN(Ethernet VPN)?
57. what is FEC in traditional IP routing and MPLS?
58. How does CEF understand overlapping of ip addresses ?
59. What is the difference between Optimum, Fast and CEF Switching?
60. Advantages of MPLS.
61. Define CEF FIB table entries.


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BGP Interview Questions For CCNA and CCNP Engineers

After IGP interview questions, I am posting BGP interview questions for CCNA and CCNP engineers. These questions are very basic and easy but during interview time no one knows how these will be asked by interviewer. So a good practice is to read it as much as we can so that we could provide the best answer to interviewer. Next post will cover about the basic questions of ip access-lists and prefix list. Read the questions below and in case any problem feel free to contact me.

1. BGP is IGP or EGP?
2. BGP is link state or distance vector protocol?
3. BGP uses which port?
4. When to use BGP?
5. Can I use BGP instead of any IGP?
6. Can I run two BGP process on single router?
7. What is Autonomous System?
8. Types of BGP routing table?
9. What is the BGP path selection criteria?
10. Define various BGP path attributes.
11. Why weight doesn’t fall under path attribute category?
12. What is confederation?
13. What is route reflector and why it is required?
14. What is no-synchronization rule?
15. Default BGP timers.
16. When does BGP use 0.0.0.0 router id?
17. Does route reflector come in actual path during traffic forwarding?
18. What is Site of origin aka SOO?
19. What is the cost of external and internal BGP routes?
20. Can we use local preference outside the autonomous system?
21. Does it require that BGP router-id should reachable in cloud?
22. What is recursive lookup in BGP and how it works?
23. What is the meaning of update source loopback?
24. If a static route is advertised in BGP without using update source what will be the next hop address in update?
25. Define various types of communities and why they are used?
26. If BGP neighbor state is showing idle what does it mean?
27. In Multihoming scenario if primary link gets fail, after how long traffic will be shifted to secondary link.
28. I am having two routes for remote destination but only single route is installing in routing table, what’s the reason for this?
29. How many links can be assigned for load balancing or sharing?
30. In eBGP I am establishing my neighbourship with loopback address but it’s not coming up. Please specify different reasons for not coming up.
31. Can we redistribute BGP in IGP? Please explain your answers.
32. What is cluster id?
33. I am receiving updates from eBGP peer, will the next hop change or not?
34. I am receiving updates from iBGP peer, will the next hop change or not?
35. A router is receiving same route from two different eBGP peers. The AS information contains in peer 1 is {65500, 65550, 65555} and in peer 2 is {65501, 65501}. But I want to make peer 1 preferred.
36. What is the difference between next-hop-self and update source loopback?
37. Define loop prevention mechanism in BGP.
38. What will happen if route reflector is not getting proper updates?
39. What will happen if route reflectors does not synchronize?
40. What is the advantage of using BGP AS Prepend?
41. Can we use BGP as backdoor link for customers instead of OSPF? If yes, please let us know what could the issues BGP create?
42. What is BGP PIC?
43. Use BGP as Link Protection in case of Dual PoP?
44. How to achieve Inter-AS Communication-MP-eBGP?
45. What can happen if Route Reflector(RR) is not getting proper route updates?
46. What is route reflector synchronization?
47. How to use BGP as PE-CE backdoor link?
48. What is Hierarchical FIB - BGP-PIC?
49. BGP Graceful Restart, NSR and NSF
50. BGP Redistribution Vs MPLS, which one you will select?
51. Best practices to define BGP Communities as per RFC 1998.
52. What is BGP RFC 3107 or What is BGP Label Update in IPv4?
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IGP Interview Questions For CCNA and CCNP Engineers

Market is on boom and almost every company has opened its door for new positions and everyone is looking for change to grab new positions with new challenges. So make sure that you have prepared the answers for the below questions before your interview. The questions are generic and will make very good impression on interviewer if you answer them in organised and structured manner. The depicted IGP interview questions are for CCNA and CCNP engineers.

1. Difference between RIPv1 and RIPv2?
2. How many number of routes carried by RIP packet?
3. Is OSPF link state or distance vector or path vector protocol?
4. What is the difference between OSPF and IS-IS and which one is preferred?
5. Can we use BGP instead of any IGP?
6. How many network types available in OSPF?
7. Different type of Link State Advertisements aka LSA?
8. LSA 3 and LSA 4 are generated by which router?
9. When to use Stub and Not So Stubby Area?
10. How to get the external routes without making area Not So Stubby?
11. What is the different type of route summarization available in OSPF?
12. What is the requirement of doing summarization?
13. A major network is advertised as summary in one area and few of the routes from that network is configured in another area. What will happen in that case?
14. If any of the OSPF area is not stabilized, does it impact another area?
15. What is the use of forwarding address in LSA 5 and LSA 7?
16. External routes are available in OSPF database but not installing in routing table?
17. If loopback is not configured, what will be the router-id selected by OSPF process?
18. Can we run multiple OSPF process in single router and what is the advantage of using it?
19. What are timers of OSPF?
20. Multicast address of used by OSPF.
21. OSPF works on which layer?
22. What is backbone area in OSPF?
23. Can we use OSPF without backbone area?
24. Is it required that OSPF router-id must reachable in IGP cloud?
25. After configuring new router-id, automatically it will be used or do we need to use some type of command to get it operational.
26. Why the secondary ip address of interface is not advertising in IGP cloud?
27. OSPF neighbourship is not coming up. Please tell the various steps to troubleshoot it.
28. One side MTU is 1500 and another side MTU is 1600. Does it affect neighbourship?
29. Provide process of DR and BDR election.
30. If DR is down and no BDR is configured what will happen?
31. What is the difference between a neighbor and adjacent neighbor?
32. My OSPF neighbourship is showing 2-way, what does it mean?
33. Define different type of OSPF neighbor states?
34. OSPF external routes are not redistributing?
35. What is Layer 3 routing loop?
36. OSPF LSA and Packet Format
37. How does OSPF Sham Link in different area work?
38. What is Link State Advertisement (LSA) - 1?
39. What is Link State Advertisement (LSA) - 2?
40. What is Link State Advertisement (LSA) - 3?
41. What is Link State Advertisement (LSA) - 4?
42. How to design OSPF Network or OSPF Design Consideration?
43. What to ask from customer if he demands OSPF as PE - CE Routing Protocol?
44. What is C and R in OSPF debug?
45. How does CPE Area 0 & PE Super backbone Communicate?
46. Why OSPF VPNv4 Routes Look As External Routes Instead Of Inter Area Routes?
47. How does ISP hack by using OSPF as PE-CE routing protocol?
48. OSPF High Availability with SSO,NSF and NSR
49. How does OSPF behave with SSO,NSF and NSR? 50. How does CISCO EIGRP DUAL Algorithm works for selecting successor?
51. Define various tools which participates in OSPF fast convergence
52. How does event propagation tool help OSPF to converge fast?
53. How does OSPF Fast Convergence Tools - Event Processing helps to reduce convergence time?
54. OSPF Fast Convergence Tools - Updating RIB 55. What is Discard Route or Null0 Route?
56. How does static routing behaves?
57. What are the fundamentals of route redistribution?
58. Which routing protocol is best between OSPF and EIGRP?
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