Thursday, 12 July 2012

Answer these common networking interview questions


These are some of the basic networking questions, but every time i ask these 90 % of the people give answers wrong. Try it and post in comments , lets see ...

1) What is destination mac address in ARP header ?

2) What is destination IP address for any DHCPDISCOVER packet ?

3) What tables/statistics/parameter bridge will modify/update when any packet it receives for the first time ?

4) If i have 390 switches in between PC1 and PC2 will they ping ? what will happen if i change switches with routers ?

5) What is maximum and minimum converge time for STP ( spanning tree protocol ) when they will come?

6) Your desktop PC is it a router or bridge ?

7) Why IP address is required for pure L2 switch, nno routing functionality ?

8) What will happen if you  ping from PC ? What tables and parameters is looked into before sending the packet out?

9) Lets say a single broadcast packet is received by a 4 port switch on one of its port, now how many packets it will generate and send to all ports?

10) How tftp protocol identifies if the file transfer is finished? Is there any bit in message exchange which signifies this?

Please post your answers....

6 comments:

  1. Let me give it a try. Please post the correct answers.

    1) Broadcast address proably all f's
    2) Whatever was configured as the DHCP Server.
    3) Source Dest MAC addresses?
    4) Will they ping each other? Don't think so unless STP
    etc is running.
    5) No idea.
    6) It can be made a router by enabling packet forwarding.
    7) For management of the switch.
    8) It will first check the ARP table to see if it has the MAC address for the destination. If not then it will do a ARP query. Else it will then check the routing table for the outgoing gateway to use for sending the ICMP packet.
    9) Should first generate 3 packets and send it to the other three ports. Problem arises if there is loop.
    10) No idea.

    Thanks for the nice questions. I really like your site and philosophy of sharing knowledge which is sadly missing in corporate India.

    -Bhaskar

    ReplyDelete
    Replies
    1. Nice try. Thanks for posting comments.

      Answers :

      1,2,3: Wrong,

      4,6,8: Partially correct

      7,9 : Right

      5, 10 : Not attempted

      Delete
    2. Answers :

      Answer 1) The ARP packet format is

      Ethernet 2 header + ARP header , in Ethernet 2 header the destination mac is FF:FF:FF:FF:FF:FF, but for ARP header the target mac address or the destination mac address will always be 0.0.0.0 and is filled in by the host , who gives ARP reply.


      Keep learning Keep sharing...

      Delete
  2. Question 2) What is destination IP address for any DHCPDISCOVER packet ?

    Answer : The destination IP address for such packets are 255.255.255.255, known as limited broadcast packets. For more info please go through this.

    http://www.embeddedsystemtesting.com/2011/11/what-is-difference-between-limited.html

    ReplyDelete
  3. Answers :

    3) What tables/statistics/parameter bridge will modify/update when any packet it receives for the first time ?

    Ans : It will update the bridging table or learning table with following parameters :ageing timer , source mac address, port no, vlan id


    4) If i have 390 switches in between PC1 and PC2 will they ping ? what will happen if i change switches with routers ?

    Ans : For bridge there is no limitation. They will ping. It needs only mac address, the packet will be forwarded to all port of other switches until it reaches the destination. For Routers the TTL limit is there for packets so packet cannot cross beyond TTL.

    5) What is maximum and minimum converge time for STP ( spanning tree protocol ) when they will come?

    Ans : Min 30 ( root bridge port gets down) MAX 50 (15 +15 +20)


    6) Your desktop PC is it a router or bridge ?

    Ans : Your PC can act as both bridge and router. As it has routing table which can be seen using command "route print" it has basic routing features. It can be made bridge also. Right click any network interface and see "Bridge connection" will be present. Using this you can make a bridge .

    7) Why IP address is required for pure L2 switch, no routing functionality ?

    And : Managing the switch.

    8) What will happen if you ping from PC ? What tables and parameters is looked into before sending the packet out?

    Ans : First destination IP address is looked into the routing table ( local /host specific / network specific route is present)

    if local -> ARP table-> entry not present-> send ARP request, entry present -> send ECHO request

    If not local-> check default gw -> not present-> drop the packet
    Default gw present-> check ARP table for gateway-> ARP present-> send ECHO Request


    9) Lets say a single broadcast packet is received by a 4 port switch on one of its port, now how many packets it will generate and send to all ports?

    Ans : It will generate 3 packets and send to the other three ports. Switch should not send the packet back to the same interface on which it receives the packet.

    10) How tftp protocol identifies if the file transfer is finished? Is there any bit in message exchange which signifies this?

    Ans : Any block which has less than 512 bytes is considered as the end of file. Read my another post on TFTP for details.

    ReplyDelete
  4. Genuinely good thanks, I do believe your trusty audience would probably want a great deal more blog posts of this nature maintain the good hard work.
    Software Testing Interview Questions

    ReplyDelete

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