LAYER-2 SWITCHING - INTERVIEW QUESTIONS

 

Question - 1 Which type address used by layer 2 switching ?

Layer 2 switching uses Hardware Address (MAC Address) of devices. Media Access Control (MAC) address burned into each and every Ethernet network interface card (NIC). The MAC, or hardware, address is a 48-bit (6-byte) address written in a hexadecimal format.

 

Question - 2 How to bridges and switches build and maintain the filter table ?

Bridges use software to create and manage a filter table, switches use application-specific integrated circuits (ASICs) to build and maintain their filter tables.

 

Question - 3 Why layer 2 switches and bridging are faster than router ? 

Layer 2 switches and bridges are faster than routers because they don’t take up time looking at the Network layer header information. Instead, they look at the frame’s hardware addresses before deciding to either forward, flood, or drop the frame.

 

Question - 4 Which type features are provided by layer 2 switching ?

• Hardware-based bridging (ASICs)
• Wire speed
• Low latency
• Low cost
• Switch is also known as multiport bridge and by default switches breakup collision domain.

 

Question - 5 What makes layer 2 switches so efficient ?

• In switching no modification to the data packet takes place. The device only reads the frame encapsulating the packet, which makes the switching process considerably faster and less error prone than routing processes are.

• And if you use layer 2 switching for both work-group connectivity and network segmentation (breaking up collision domains), you can create more network segments than you can with traditional routed networks.
• Plus, layer 2 switching increases bandwidth for each user because each connection (interface) into the switch is its own collision domain.
• Switches create private, dedicated collision domains and provide independent bandwidth on each port, unlike hubs.

 

Question -6 What is collision domain ?

Collision domain is the network segment with the two or more host sharing the same bandwidth.

 

Question - 7 What is broadcast domain ?

Broadcast domain refers to a group of devices on a network segment that hear all broadcast sent on that network segment.

 

Question - 8 What is Latency ?

Latency is the time measured from when a frame enters a port to when it exits a port.

 

Question - 9 What is bandwidth ?

Bandwidth is the bit rate of available or consumed information capacity expressed typically in metric multiple of bit per second.

 

Question - 10 What are limitations of layer 2 switching ?

• Neither layer 2 switches nor bridges break up broadcast domains by default—something that not only limits your network’s size and growth potential, but also can reduce its overall performance.
• Broadcasts and multicasts, along with the slow convergence time of legacy spanning trees, can give you some major grief as your network grows. These are the big reasons layer 2 switches cannot completely replace routers (layer 3 devices) in the internetwork. 

 

Question - 11 What are differences between Bridging and LAN switching ?

• Bridges are software based, while switches are hardware based because they use ASIC chips to help make filtering decisions.
• A switch can be viewed as a multiport bridge.
• There can be only one spanning-tree instance per bridge, while switches can have many. 
• Most switches have a higher number of ports than most bridges.
• Both bridges and switches flood layer 2 broadcasts.
• Bridges and switches learn MAC addresses by examining the source address of each frame received.
• Both bridges and switches make forwarding decisions based on layer 2 addresses.

 

Question - 12 Explain the function of layer 2 switching ? 

Address learning -
Layer 2 switches and bridges remember the source hardware address of each frame received on an interface, and they enter this information into a MAC database called a forward/filter table.

Forward/filter decisions -  
Step - 1 When a frame arrives at a switch interface, the destination hardware address is compared to the forward/filter MAC database. If the destination hardware address is known and listed in the database, the frame is only sent out the correct exit interface. The switch doesn’t transmit the frame out any interface except for the destination interface. This preserves bandwidth on the other network segments and is called frame filtering.
Step - 2 But if the destination hardware address is not listed in the MAC database, then the frame is flooded out all active interfaces except the interface the frame was received on. If a device answers the flooded frame, the MAC database is updated with the device’s location (interface). If a host or server sends a broadcast on the LAN, the switch will flood the frame out all active ports except the source port by default.

Loop avoidance - 
If multiple connections between switches are created for redundancy purposes, network loops can occur. Spanning Tree Protocol (STP) is used to stop network loops while still permitting redundancy

 

Question - 13 How cab we see the mac address forward-filter table of switches ?

Show mac address-table - The command show mac address-table will show you the forward/filter table used on the LAN switch.

 

Question -14 How a forward/filter table is populated when Host 1 is communicating with Host 2 in given scenario? 

Figure : Empty Mac Forward Filter Table on Switch

Mac Forward Filter Table

                                                             F0/0:
                                                             F0/1:
                                                             F0/2:
                                                             F0/3:

Figure : How switches learn host's locations

Mac Forward Filter Table

                                                             F0/0: 0000.8c01.000A  Step - 2

                                                             F0/1: 0000.8c01.000B  Step - 4

                                                             F0/2:

                                                             F0/3:

In this figure four hosts attached to a switch. When the switch is powered on, it has nothing in its MAC address forward/filter table, But when the hosts start communicating, the switch places the source hardware address of each frame in the table along with the port that the frame’s source address corresponds to.

Step 1. Host 1 sends a frame to Host 2. Host 1’s MAC address is 0000.8c01.000A; Host 2’s MAC address is 0000.8c01.000B.
Step 2. The switch receives the frame on the F0/0 interface and places the source address in the MAC address table.
Step 3. Since the destination address is not in the MAC database, the frame is forwarded out all interfaces—except the source port.
Step 4. Host 2 receives the frame and responds to Host 1. The switch receives this frame on interface F0/1 and places the source hardware address in the MAC database.
Step 5. Host 1 and Host 2 can now make a point-to-point connection and only the two devices will receive the frames. Hosts 3 and 4 will not see the frames, nor are their MAC addresses found in the database because they haven’t yet sent a frame to the switch.
If Host 1 and Host 2 don’t communicate to the switch again within a certain amount of time, the switch will flush their entries from the database to keep it as current as possible.

 

Question - 15 What is the advantage of redundant link between switches ?

Redundant links between switches are a good idea because they help prevent irrecoverable network failures in the event one link stops working. 

 

Question - 16 How do you stop someone from simply plugging a host into one of your switch ports—or worse, adding a hub, switch, or access point into the Ethernet jack in their office? 

 You can stop them in their tracks by using port security.

Switch#config t
Switch(config)#int f0/1
Switch(config-if)#switchport mode access →[Change the port from desirable mode to access port]

Switch(config-if)#switchport port-security ?
aging                       Port-security aging commands
mac-address            Secure mac address
maximum               Max secure addresses
violation                 Security violation mode
Switch(config-if)#switchport port-security maximum 1 → [Allow only one host per port means only one mac address can be used on that port]
Switch(config-if)#switchport port-security violation shutdown →[Shutdown the port if rule is violated or user try to add another host on that segment]

Switch(config-if)#switchport port-security mac-address sticky
Switch(config-if)#switchport port-security maximum 2
Switch(config-if)#switchport port-security violation shutdown

 

Question - 17 What are difference between Protect Mode and Restrict Mode in Security Violation ?

                                 Switch(config-if)#switchport port-security violation ?

                                 protect                              Security violation protect mode

                                 restrict                              Security violation restrict mode

                                 shutdown                          Security violation shutdown mode

Protect Mode -  Protect mode means that another host can connect but its frames will just be dropped.
Restrict Mode - It alerts you via SNMP that a violation has occurred on a port. You can then call the
abuser and give the warning.

 

Question - 18 What is use of Sticky in port-security ?

Note -  If the port shuts down, all dynamically learned addresses are removed. 

•  You can use "switchport port-security mac-address sticky" command to enable sticky learning. It convert the dynamic MAC addresses to sticky secure MAC addresses and to add them to the running configuration by enabling sticky learning.  
• When you enter this command, the interface converts all the dynamic secure MAC addresses, including those that were dynamically learned before sticky learning was enabled, to sticky secure MAC addresses.
• If you save the sticky secure MAC addresses in the configuration file, when the switch restarts, the interface does not need to relearn these addresses. If you do not save the configuration, they are lost.
• If sticky learning is disabled, the sticky secure MAC addresses are converted to dynamic secure addresses and are removed from the running configuration.

 

Question - 19 What is aging in port-security ?

By port security aging you can set the aging time and aging type for all secure addresses on a port. Aging use to remove and add hosts on a secure port without manually deleting the existing secure MAC addresses while still limiting the number of secure addresses on a port.
    
Switch(config-if)# switchport port-security [ aging {static | time aging_time | type {absolute | inactivity} ]

• Static keyword enables aging for statically configured secure addresses on this port. 
• Time aging_time keyword specifies the aging time for this port. Valid range for aging_time is from 0 to 1440 minutes. If the time is equal to 0, aging is disabled for this port. 
• Type keyword sets the aging type as absolute or inactive. 
• If you are using absolute aging, then all the secure addresses on this port ago out exactly after the time (minutes) specified and are removed from the secure address list. 
• If you are using inactive aging, then secure addresses on this port ago out only if there is no data traffic from the secure source address for the specified time period

  

Question - 20 What is desirable mode ?

If port is in desirable mode then port desires to trunk if it senses another switch just connected.

 

Question - 21 You must know these following basic thing, when you are configuring a layer 2 switch.

•  There is no ip address configure on the switch's physical interface, the ip address is configured under a logical interface, called a management domain or vlan.
• All port on the switch are enabled by default.
• You can apply securities and other features on more than one interface at a time through the int range command.

Switch#config t
Switch(config)#int f0/1-4             →   It means you are taking F0/1, F0/2, F0/3 and F0/4
Switch(config-if-range)#switchport mode access

Switch(config-if-range)#switchport port-security ?
Switch(config-if-range)#switchport port-security maximum 1

Switch(config-if-range)#switchport port-security mac-address sticky
Switch(config-if-range)#switchport port-security violation shutdown

• You cannot set port security on a port while in desirable mode, so initially you will have to change interface desirable mode to access port through the Switchport mode access command.

 

Question - 22 Why did I get only four pings to work instead of five ?  

First ping didn’t work because of the time that ARP takes to resolve the IP address to its corresponding hardware MAC address.

Other Posts:-

OSI LAYERS AND FUNCTIONS
IP ROUTING BASIC
IP ROUTING SCENARIOS
RIP BASIC
RIP TROUBLESHOOTING
NETWORK COMMAND ISSUE
INTERFACE ISSUE
VERSION MISMATCH ISSUE
PASSIVE INTERFACE ISSUE
MAXIMUM HOP COUNT ISSUE
RIP OFFSET LIST CONFIGURATION
BGP - INTERVIEW QUESTIONS (Part - 1)
LAYER-2 SWITCHING

SHARE

Anubhav Upadhyay

Hello and welcome to networktopic Blog. My name is Anubhav. I am a Senior Network Egineer. I have created this blog specially to serve interview questions and answer on Network Routing and Switching, I will try my best to serve correct and updated networking knowledge for you as per my corporate experience.

• 
• 
• 
• 
•