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Switch Questions

November 20th, 2018 Go to comments

Quick review of switch function:

In brief, the basic switching function at Layer 2 adheres to these rules for determining forwarding responsibility:
+ If the destination MAC address is found in the CAM table, the switch sends the frame out the port that is associated with that destination MAC address in the CAM table. This process is called forwarding.
+ If the associated port to send the frame out is the same port that the frame originally came in on, there is no need to send the frame back out that same port, and the frame is ignored. This process is called filtering.
+ If the destination MAC address is not in the CAM table (that is, unknown unicast), the switch sends the frame out all other ports that are in the same VLAN as the received frame. This is called flooding. It does not flood the frame out the same port on which the frame was received.
+ If the destination MAC address of the received frame is the broadcast address (FFFF.FFFF.FFFF), the frame is sent out all ports that are in the same VLAN as the received frame. This is also called flooding. The only exception is the frame is not sent out the same port on which the frame was received.

Reference: http://www.ciscopress.com/articles/article.asp?p=2348264

Question 1

Explanation

In cut-through switching, the switch copies into its memory only the destination MAC address (first six bytes of the frame) of the frame. After processing these first six bytes, the switch had enough information to make a forwarding decision and move the frame to the appropriate switchport. This switching method is faster than store-and-forward switching method.

In store-and-forward switching, the switch copies each complete Ethernet frame into the switch memory and computes a Cyclic Redundancy Check (CRC) for errors. If a CRC error is found, the Ethernet frame is dropped. If no CRC error is found then that frame is forwarded.

Question 2

Explanation

From the output we notice that the administrator has just shut down Interface Vlan1, which is the default VLAN so no one can access it remotely (like telnet) -> B is correct.

Answer A is not correct as STP calculation does not depend on which port comes up first or last. STP recalculates when there is a change in the network.

A normal switch can operate without VLAN -> C is not correct.

This IOS does support VLAN because it has VLAN 1 on it -> D is not correct.

Question 3

Explanation

Host A (172.19.1.1) and the management IP address of the Switch (172.19.1.250) are in the same subnet so telnet from host A to the switch can be successful even if a default gateway is not set on host A.

But host B (172.19.32.2) and the management IP address of the Switch (172.19.1.250) are not in the same subnet. Therefore packets from host B must reach the router Fa0/0.32 interface before forwarding to the switch. But when the switch replies, it does not know how to send packets so an appropriate default gateway must be assigned on the switch (to Fa0/0.32 – 172.19.32.254).

Answer A is not correct because even when host B & the switch are in the same subnet, they cannot communicate because of different VLANs.

Answer C is not correct as host B can ping other two hosts.

Answer D is not correct because host B always belongs to VLAN 32 so assigning an IP address in VLAN 1 does not solve the problem.

Question 4

Question 5

Question 6

Explanation

In the configuration above, the “no shutdown” command was missing so interface Vlan 1 is still inactive. Notice that only the loopback command does not need the “no shutdown” command to work.

Question 7

Explanation

A VLAN does not need to be assigned to any port -> B is not correct.

An access port can only receive traffic from one VLAN -> C is not correct.

If not assigned to a specific VLAN, an access port carries traffic for VLAN 1 by default -> D is not correct.

An access port will drop packets with 802.1Q tags -> A is correct. Notice that 802.1Q tags are used to packets moving on trunk links.

Question 8

Explanation

Carrier Sense Multiple Access with Collision Detection (CSMA/CD) is the LAN access method used in Ethernet. When a device wants to gain access to the network, it checks to see if the network is free. If the network is not free, the device waits a random amount of time before retrying. If the network is free and two devices access the line at exactly the same time, their signals collide. When the collision is detected, they both back off and wait a random amount of time before retrying.

CSMA/CD is used with devices operating in half-duplex mode only. CSMA/CD helps devices connecting to half-duplex switch ports operate correctly.

Question 9

Explanation

The “!” is a special character which is not allowed in the hostname of Cisco device.

Question 10

Question 11

Explanation

All ports on a Layer 2 switch are in the same broadcast domain. Only router ports separate broadcast domains.

Comments (7) Comments
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    September 16th, 2019

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    September 17th, 2019

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