The following are the top protocol testing interview questions and answers
1. What happens when the source and destination mac-address in a frame is the same
When the source and destination mac-address in the frame is the same, the frame is filtered and discarded.
2.A switch receives a frame with the address FF-FF-FF-FF-FF-FF for the 1st time. Will the frame be flooded or broadcasted.
Frames which have the address FF-FF-FF-FF-FF-FF are broadcast frames and would be broadcasted.
3.There are two PC’s connected to a switch. PC1 pings PC2. How many mac-addresses would be available on the cam table of the switch after PC1 pings PC2.
There would be two mac-address entries in the table. The switch would learn the mac-address of PC1 and PC2 and add the same in the cam table.
4.How many broadcast domains are there in an 8 port hub and switch respectively.
Both devices have 1 broadcast domain
5.You have been asked to test the root bridge election process of spanning tree functionality. Explain a basic test case and how you would design the lab for testing.
Spanning tree protocol election takes place by exchanging spanning tree BPDU packets. The switch which has the lowest priority value would be elected as the root bridge. For testing, connect 3 switches. Configure one of the switches with priority value lesser than the other switches. Wait for some time and check using appropriate commands on the configured switch to verify if it has been elected as the root bridge.
6. Explain the steps which you would follow to test mac-address learning functionality on a switch.
Connect two PC’s, PC1 and PC2 to different ports on the switch. Ping PC1 from PC2 or vice versa. Check the mac-address table of switch using appropriate commands. The switch should have learned the mac-address of both the PC’s.
7.Two PC’s which are on the same subnet are connected to ports which are on different VLAN. PC1 pings PC2. Will the ARP request triggered from PC1 reach PC2.
ARP request is a broadcast packet. Broadcast packets from one VLAN will not reach other VLAN.
8.A port on the switch is configured a tagged port. The port on the switch receives a tagged frame. How does the switch know the corresponding VLAN to which the frame belongs to
When the switch receives the frame it looks into the VLAN ID in the frame. In a tagged frame, the VLAN header would be inserted in the frame which would contain the details related to the VLAN ID.
9.The root bridge in a spanning tree topology has 3 links on 3 different ports which are connected to three other switches. How many ports on the root bridge can forward frames.
All the ports on the root bridge are designated ports. All designated ports can forward frames. In the above scenario, all the 3 ports on the root bridge can forward frames.
10.The BPDU frame consists of a field called as “Protocol Identifier”. Which protocol does this refer to
It refers to the spanning tree protocol
11.During which state in a STP election process would a switch begin to populate it’s mac-address table
The mac-address table is populated during the learning state.
12. Explain the concept of RIP passive interface and how you would test the feature on a router.
RIP passive interface is a feature which is used to disable RIP packets to be sent on the interface configured as passive. Connect the interface on the router to a PC. Setup the PC with protocol analyser software like wireshark. Configure RIP on the router and check with the protocol analyser to verify RIP packets are being sent. Once verified, configure the interface as passive interface. RIP packets should not be visible on the analyser.
13. Why is a DR/BDR not elected in a point to point OSPF network type.
A DR/BDR is elected in a scenario where there are multiple routers on the network. In a point to point OSPF network type, there are only two routers, due to which it is not required.