Interview Q/A Routing &Switching

How will request send by http browser from workstation to any website goes through router and come back to workstation and open web page. Explain how packets travel in term of ip protocols. ?

Once a web site is entered in the address bar and enter key pressed the following process happens.

1. The system performs a check on the dns cache to check if there is any entry for the concerned domain name if yes goes to step 3.
2. The system performs a dns query for converting the domain name into ip by sending a query to the DNS server.
3. Once a dns response for the query is received the initiates a 3 way handshake with destination ip on port 80 or 443.
4. Client sends a sync to server, server sends a sync+ack to client. the client sends a ack to sync from the server.
5. Once a 3 way hand shake is completed the browser presents the request to server.
6. Based on the received request the server responds with requested file/data.
7. Once the transmitting of data is completed the server sends a fin packet, then client sends a fin packet and connection gets terminated.

Switching Interview Q/A

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Q1. PC1 and PC2 are connected to port 1 and port 2 of an ethernet switch using cat 5e cables. How many mac addresses would the switch have learnt before PC1 sends data to PC2?

Ans.The switch would not have learnt any mac-address in its mac-address table. Switches learn mac-addresses only after receipt of frame on the respective ports. In the above scenario, PC 1 has not yet send data, which is ultimately transported using ethernet frames.

Q2. How long would a switch retain a mac-address in its mac-address table?

Ans.Assume that a PC is connected to a switch port. When the PC sends a frame to the switch port, the source mac-address in the frame is learnt and added to the table. Switches have mac-age-time which are configurable as well as set with a default value. When there is inactivity from the PC (Data/frames are not send) for the mac-age-time value, the mac-address is removed.

Q3. What do switches do with the source and destination mac-address in a frame?
Ans. Switches would check the source mac-address in the frame and verify if it is available in the mac-address table. If not, the source mac-address would be added to the table. Switches would check if the destination mac-address is available in the mac-address table. If not, the frame would be flooded to all ports on the switch.Once a response is received from the intended recipient, the corresponding port would be updated with the mac-address entry (destination mac-address). In this way switch populates the mac-address table.

Q4. What do switches do if the source and destination mac-address on a frame is the same?

Ans. When the source and destination mac-address in the frame is the same, the switch would not add the entry to it’s mac-address table or send it to the required destination.The frame will be filtered by the switch and discarded.

Q5. How do hubs handle unicast and broadcast frames

Ans. Hubs work at Layer 1 of the OSI model. They do not understand frames. Irrespective of the type of the frame, unicast or broadcast, the signal carrying the frame would be send to all ports on the hub.

Q6. How are broadcast and unicast frames differentiated by switches

Ans. Broadcast frames are identified by the value in the destination mac-address of the frame as FF-FF-FF-FF-FF-FF .Unicast frames contains the specific value of the mac-address of the intended recipient.

Q7. What are the changes that would be applied on the mac-address table of a switch if a PC which is connected to port 1 is disconnected and connected back to port 2.Ans.When the PC is disconnected, from switch port1, the mac-address entry for the PC would be removed from the table since the link is broken. When it is connected back to switch port 2, the mac-address table would still not have the entry since there has been not data which is initiated yet from the PC.

Q8. Can a switch have an operating systemAns. Yes, switches which have operating system are termed as manageable switch. Switches which does not have an operating system, are unmanageable switch

Q9. A PC has two network cards. How many mac-addresses would it have

Ans.Every network card has a unique mac-address. So a PC with two network cards would have two mac-addresses

Q10. Can two devices on same network can have the same mac-address.

Ans. It is not possible. Every device has a different network card. Each network card has a unique mac-address.

Q11. What is size of a mac-addressAns.The size of mac-address is 48 bits.

  

Q12.PC1 wishes to communicate with PC2. PC1 is aware of the IP address of PC2. Would the mac-address of PC2 be required, for data communication between PC1 and PC2 or would the knowledge of IP address suffice

Ans. The IP address information of the peer would be sufficient from a users perspective. But for data to be sent on the physical network, computers need to be aware of the mac-address of the device with which it needs to communicate.

Q13. What is a trunk port used for

Ans. trunk port is used for carrying traffic traffic from different VLANS. Take an example of two switches connected using a cable on two ports. VLAN 2 and VLAN 3 are configured on both the switches. For VLAN communication to be successful between the switches, the ports should be configure as trunk ports.

Q14. What is the impact of an Ethernet frame in a cut-through and store and forward switching

Ans. Preamble and SFD is checked when a frame arrives on a switch port. This is for clock synchronisation and the SFD informs the switch port the beginning of a new frame. The next field is the destination mac-address. In cut-through switching as soon as the Destination mac-address is read, the frame is forwarded to the intended destination port. In store and forward switching, the entire frame is copied to the buffer, the CRC value of the frame calculated and then the frame is forwarded to the intended destination port. The destination would also calculate the CRC value of the frame.

Routing Interview Q/A

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Q1. Can routers check ethernet frames?
Ans. Routers look for information inside the IP packet header for routing the packets. But the packets itself would be encapsulated inside frames like ethernet, PPP etc. For example, if the IP packet is encapsulated inside an ethernet frame, the router would first need to check the destination mac-address inside the frame and verify if it is intended for itself and only then be able to process the IP packet header, which would be the data inside the packet.

Q2. How do routers populate routing entry for a directly connected network?

Ans. A directly connected network refers to the network address corresponding to the IP address and subnet mask configured on the specific network interface card on the router. The IP address and subnet mask would be “And” ed to derive the network address.

Q3. How do routers forward IP packets?

Ans. When a router receives an IP packet, it would check the destination IP address in the packet. The router would then performing an “And “ operation with the subnet mask of the different route table entries with the destination IP address and verify if the corresponding network address in the routing table matches. On a match, the packet would be forwarded to the interface to which it the route entry is associated with.

Q4. What are the parameters that are provided in a static route

Ans. A static entry would typically contain the network address, network mask and the default gateway. Let’s table an example where a router has two directly connected networks. The hosts within the router, needs to access a network which is not a directly connected network. The network is one hop away from the router and is reachable via the default gateway of the router. So the network address value would be the one which the hosts would want to reach via the router. This network address along with the network mask and the gateway address through which the network can be reached is configured as a static route.

Q5. How does a PC know as to whether the destination IP address is on the same or different network

Ans. Assume that a PC is configured with an IP address 192.168.2.1 and subnet mask 255.255.255.0. If the user on the PC issues a ping to the IP address 192.168.1.1, the PC would check in its routing table for matching network address and decide if it is on the same or different network.

Q6. An ADSL router is configured for internet connection. A switch is connected to the router to which the hosts are connected. What type of route entry should be configured on the router for the hosts to access the internet

Ans .A default route entry should be configured on the router. IP addresses which are internet based, cannot be configured explicitly on the routers since they would not be known beforehand. Examples are IP addresses of websites which the user tries to access. The default route once configured would take care of destination networks which the router is unaware

Q7. In which type of addressing (class full or classless) is the subnet mask information carried in the IP header.

Ans. Whatever the type of addressing, the subnet mask is never carried in an IP header

Q8. What is the difference between class full and classless addressing

Ans.In classful addressing, based on the first octet value of the IP address, the class of the network (A,B,C) is decided. In classless addressing, the octet value is not considered. Rather, the subnet mask is used to derive the network address.

Q9. What type of IP address (Private or Public ) is used when a computer is connected to the internet directly

Ans. Public IP address

Q10. On which hardware component on a computer is the IP address configured

Ans. The IP address is configured on the network card.

  

Q11. What route would be populated in the routing table of a PC when the default gateway is configured on it

Ans. A default route (0.0.0.0) would be populated when the default gateway is configured. The default route is required to route packets to unknown networks , like packets bounded for internet.

Q12. How many default route entries would be available in the routing table of a PC with two gateways

Ans. Every default gateway would correspond with a default route. So two default entries would be available on the routing table of a PC with two gateways

13. How router advertise control and data packet.:

Flow of Routing Information

Routing information is the information about routes learned by the routing protocols from a router’s neighbors. This information is stored in routing tables. The routing protocols advertise active routes only from the routing tables. An active route is a route that is chosen from all routes in the routing table to reach a destination.

To control which routes the routing protocols place in the routing tables and which routes the routing protocols advertise from the routing tables, you can configure routing policies, which are sets of rules that the policy framework uses to preempt default routing policies.

The Routing Engine, which runs the router's control plane software, handles the flow of routing information between the routing protocols and the routing tables and between the routing tables and the forwarding table. The Routing Engine runs the Junos OS and routing policies and stores the active router configuration, the master routing table, and the master forwarding table,

Flow of Data Packets

Data packets are chunks of data that transit the router as they are being forwarded from a source to a destination. When a router receives a data packet on an interface, it determines where to forward the packet by looking in the forwarding table for the best route to a destination. The router then forwards the data packet toward its destination through the appropriate interface.

The Packet Forwarding Engine, which is the central processing element of the router’s forwarding plane, handles the flow of data packets in and out of the router’s physical interfaces. Although the Packet Forwarding Engine contains Layer 3 and Layer 4 header information, it does not contain the packet data itself (the packet's payload).

14. What is difference between L3 Switch and Router?

  - L3 Switch do switching at layer 3 by preserving the source and destination mac and preserving the TTL value of the IP header of the 1st routed packet,

    so  the first packet is routed using normal routing lookup, but after that all packet are switched.

 - router do normal routing lookup, but by introducing fast switching and CEF, packets are also now switched on a router.

 - Switches doesnt support some QoS features.

 - Switches doesnt support NAT.

 - The forwarding on switches is done on ASIC (Application Specific Integrated Circuits) which is done in hardware rather than a software.

 - Forwarding on routers are done in a software.

 - router supports different WAN technologies (modules) unlike switches.