Thursday, July 22, 2010

How does router perform IP Lookup?

Router confirms routing table before forwarding of any packet to any destination. If route exists in the routing database, packet forwarding will happen otherwise packet gets dropped. Let assume destination address or prefix is available in the routing table but how does router search the prefix from the routing table. Does router search in the form of binary, hex decimal, octal or decimal?

Obviously, router understands digital which means only binary (zeros and ones). Route prefixes are stored in the form of TRIE which means a binary ordered tree which is used to store an array. 
Let’s see how router stores the given prefixes in the routing table. To make it short I am assuming that prefix length is not more than 4 bits.
1. 0
2. 1
3. 001
4. 011
5. 0100
6. 010  
The above 4 bit prefixes will store in the router as depicted in the diagram. In diagram, red color circled numbers are showing the prefix serial number like prefix 5 is 0100.
                                                                             Figure 1                                                                          
Now how the router will search the destination prefix 0110 which is not present in the routing table but the search will be based on the longest prefix match. So router will search first 0, second 1 and third 1 but after third search it will not find anything. So prefix number 4 will be the longest prefix match and traffic forwarding will happen on next hop of prefix 4.

                                                                            Figure 2                                                                              

The above shown binary tree is also known as TRIE. The programming of this TRIE is done in with the help of data structure.      

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Tuesday, July 20, 2010

Chrome and Webmynd Web Browsing Issue

Since morning, I am facing a weird issue about opening of sites in chrome explorer. If I perform any of the new search in google, I get the results but after clicking on the any of the webpage, chrome shows that "Oops! Google Chrome could not find". But at the same time internet explorer is working fine. I am thinking that google has signed up

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Monday, July 19, 2010

Ready Made Junos For Laptop

Ready made JUNOS OS for your laptop. During googling I found the rapidshare link to download ready made JUNOS. Download the given files and follow the steps:-

Here is the running junos 7.4 in Qemu.

download the files extract to one folder. and follow these steps.
1. Install TAP interface buy installing open vpn software.
2. open command prompt and goto the qemu folder
3. run the following commands.
jqemu.exe -L . -m 256 -hda Olive.img -serial telnet::1001,server -localtime -net nic,vlan=1,macaddr=00:aa:00:00:01:01,model=i82559er -net tap,vlan=1,ifname=TAP1

4. Open another command prompt, tellnet to port 1001

Loggin details of Running Junos.
Login id= root
Pass= juniper123

Download link:

Open VPN

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Data Center Cable Mess Up

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Sunday, July 18, 2010

Integrate JUNOS with GNS3

After JUNOS installation on QEMU, now the next step is to integrate JUNOS with GNS3. This is only possible by adding cloud in the GNS; add the TAP1 interface in cloud and connect that interface with cisco router.
See the depicted images:-

Image 1:- Configure cloud interface with TAP1 interface

Image 2:- Select the TAP1 interface

Image 3:- Connect TAP1 interface of cloud with Cisco GNS3 router.

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Friday, July 16, 2010

Free Online CCNA - OSPF Labs

This post is fully dedicated to CCNA beginners who want to learn but don't want to install GNS3. This is the online practice lab where students can access the online simulator and practice for their CCNA labs.
Online OSPF Labs
1. Simulator for single ospf area
2. Simulator for single ospf area 2
3. OSPF NBMA Part - 1
4. OSPF NBMA Part - 2
5. OSPF Multi Access
6. OSPF Stub Area
7. OSPF Total Stub Area
8. Not So Stubby Area
9. Virtual Link Part 1
10. Virtual Link Part 2

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Analyse your data files online

We normally use wireshark to analyse pcap files. Today, I found a good web tool which would help you to analyse your pcap traces online. Now you don't need to install wireshark on your laptop or personal computer. Use online version of cloud shark.

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Wednesday, July 14, 2010

Technorati Claim Token

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Inter Vlan Routing Without Layer 3 Device

It is always said that if single vlan wants to communicate with another vlan, inter vlan routing is required. But in the depicted scenario, vlan 10 is able to communicate with vlan 20 without any layer 3 device. The logic behind the working is very simple and it is totally based on tagged and untagged concept. Access ports always tagged the incoming traffic with vlan configured on port and make untagged the outgoing traffic. The same is happening in the given scenario, when the traffic is being forwarded by PC-1 and Switch A access port receives the traffic, mark that traffic with vlan 10 and look forwards for the vlan 10 ports. After that Switch A another port which is part of vlan 10 receives the traffic and forward it towards Switch B. During forwarding Switch A port untagged that vlan and forwards the traffic without any vlan tag. Switch B port receives the incoming traffic and tagged it with vlan 20 and forwards the traffic towards PC-2. This way end to end communication takes place without the use of router or any layer 3 device. So we can say inter vlan routing without any layer 3 device.

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Thursday, July 8, 2010

Installing QEMU for JUNOS Integration

This post will cover the installation of JUNOS on PC. For JUNOS simulation, I am using OLIVE. Below are the steps required to get the installation done:-
1.       Download QEMU and JQEMU for Windows and add JQEMU in QEMU folder. JQEMU is required because it provides windows driver for virtual intel nic card.
2.       Download Open VPN to create TAP interface. This is required because we cannot upload the JUNOS image directly to QEMU. To achieve the task we need open vpn.
3.        Download Free BSD image.
4.       Download JUNOS operating system. I am using Jinstall-7.4r3.4-Domestic-Signed.tgz. This could be downloaded from here.
5.       Download free 3CDaemon ftp server to upload JUNOS in QEMU.

Installation Process:-
1.       Put the free BSD mini ISO image in QEMU directory.
2.       Run depicted command from command prompt to create OLIVE image.
C:\GNS-JUNOS\qemu-0.9.0-windows>qemu-img.exe create olive.img -f qcow2 4G (4G means allocating 4Gb memory)

3.       Run depicted command to start the QEMU installation process.
C:\GNS-JUNOS\qemu-0.9.0-windows>qemu.exe -L . -m 256 -hda olive.img -cdrom 4.4-mini.iso -boot d -localtime 
4.       Qemu installation window will pop up and start with the standard installation.

Figure 1

Skip kernal configuration and go with the standard one.

                               Figure 2

                                 Figure 3

In the next windows, simply select option A for all and press Q.

                           Figure 4

After pressing Q, the next window will come and select Standard (Install a standard MBR(no boot manager))

                          Figure 5

                      Figure 6

The next step is the utmost step which will define the partition type.We have to define partition like given below:-
1. 1024M for / partition and select FS(File System)
2. 1024M which is swap partition.
3. 12M for /config and select FS(File System)
4. 2035M for /var and select FS(File System)

Make sure to follow the same steps as depicted in the images. 

                       Figure 7

Now press C to create first partition which is "/"

                     Figure 8

                      Figure 9

                   Figure 10

Now create the second partition which is swap.

                 Figure 11

                 Figure 12

Create the third partition which is /config.

                  Figure 13

                     Figure 14

Now move to the last partition

                 Figure 15

                   Figure 16

After complete partition, it will like below:-

                   Figure 17

Then select no for the setting ((Ethernet/SLIP config, gateway, inetd, FTP, NFS, security, console, linux compatibility, set time, etc), only configure root password.

                    Figure 18

Select user and then press X.

Select no for user confirmation request.

                       Figure 19

Select CD/DVD and press enter

                    Figure 20

Press Yes in the next window and you will find congratulations window for the installation.

                        Figure 21

After this press ALT+Control+2 and type quit to come out. In the next post, I will use this installation to install JUNOS in PC.

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Wednesday, July 7, 2010

How To Install Junos In GNS3

Click here to see the Qemu Installation
After spending 3 continuous late nights; Finally I am able to install JUNOS on my PC. The procedure is bit cumbersome but not impossible. I had faced lot of issues during installation but where there is will there is way. Now next step is to integrate with GNS3. I am preparing document for the installation process with all the snap shots. Sooner will upload it.

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Tuesday, July 6, 2010

Symbolic Analysis of Relay and Switching Circuits

Zero and One also fascinates me and could not able to understand why Digital Circuits are always linked with Mathematics. Claude Shannon father of Information Theory, was the one who gave us that concept. Fortunately during googling, I got the thesis published by Claude Shannon in 1940 which tells all about the building block of using mathematics in digital circuits. Click here to access his thesis of “A symbolic Analysis Of Relay and Switching Circuits".
A Symbolic Analysis of Relay and Switching Circuits, Claude Elwood Shannon proved that Boolean algebra and binary arithmetic could be used to simplify the arrangement of the electromechanical relays then used in telephone routing switches, then turned the concept upside down and also proved that it should be possible to use arrangements of relays to solve Boolean algebra problems.

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Monday, July 5, 2010

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 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.
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 (

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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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DoT Plans To Bar Chinese and Israeli Equipments

The government has drawn up a comprehensive list of Chinese and Israeli telecom gear makers who will be temporarily barred from supplying equipment to mobile service companies in India till the new network security norms are in place.

The list, which has been prepared by the Intelligence Bureau (IB), has the names of 26 companies, including top Chinese Original Equipment Manufacturers(OEMs), Lenovo, Huawei Technologies, ZTE, Sunsea Telecoms, UT Starcom, Tongyu Communications, Wuhan Fibrehome International, Shenzhen Grentech, Maipu Communications, and one Israeli telecom gear maker, Comverse.

Source ET

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Basics of ATM (Asynchronous Transmission Mode)

ATM packet is also known as CELL. It has a fixed size of 53 bytes, which consist 48 bytes of payload and 5 bytes of header. CELL header is further divided into types of format:-
1. UNI Cell Format
2. NNI Cell Format
UNI Cell Format aka User Network Interface, which is directly connected to any ATM switch and responsible to send and receive atm cells.

NNI Cell Format aka Network to Network Interface; the interface between any two ATM switches.

ATM is based on pure circuit and packet switching technique. ATM connection is having VPI and VCI concept. VPI stands for Virtual Path Identifier and VCI stands for Virtual Connection Identifier. The VPI/VCI field is 24 bits long in UNI 28 bits in NNI. VPI field is 8 bits long in UNI and 12 bits long in NNI. The VCI field is 16 bits long in UNI as well as in NNI. It means we can have only 2^8=256 VPI connections in UNI and 2^12=4096 connections in NNI. Each UNI and NNI VPI is having 65536 VCI connections.
VPI connections can be assigned from any number but some limitation is there for assigning VCI connections. VCI 0 – 15 are reserved by ITU-T, 16 – 31 are reserved by ATM forum and 32 to 65535 are free and could be used by users.
Virtual connection is the combination of both VPI and VCI. VPI and VCI values are always local to the ATM devices and have nothing to do with the remote. Intermediate ATM switches translate the incoming and outgoing VPI and VCI values along with their ingress and egress ports. This operation is referred as label swapping.

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