SWITCH ARCHITECTURE AND DOMAINS

Question 1

What is an ingress and egress port?

Answer 1

– Ingress port -describes the port where a frame enters the device.
– Egress port- describes the port that frames use when leaving the device

Question 2

Which of the following does a LAN switch consider during frame forwarding: ingress port, egress port or destination MAC address. Explain

Answer 2

• A LAN switch forwards traffic based on the ingress port and the destination MAC address of an Ethernet frame. It maintains a table that is referenced
when forwarding traffic.
• There is only one master switching table that describes a strict association between MAC addresses and ports; therefore, an Ethernet frame with a given destination address always exits the same egress port, regardless of the ingress port it enters.

Question 3

What are the different switch frame forwarding methods?

Answer 3

1. Store and forward
2. Cut-through

Question 4

Describe the store and forward method

Answer 4

• The store-and-forward method makes a forwarding decision on a frame after it has received the entire frame and checked the frame for errors using a mathematical error-checking mechanism known as a cyclic redundancy check (CRC).

Question 5

What are the two main characteristics that distinguish store and forward method from the cut through method? Include diagrams

Answer 5

1. Error Checking :
   A switch using store-and-forward switching performs an error check on an incoming frame. After receiving the entire frame on the ingress port, the switch compares the frame-check-sequence (FCS) value in
   the last field of the datagram against its own FCS calculations. If the frame is error-free, the switch forwards the frame. Otherwise the frame is dropped.
2. Automatic Buffering:
   The ingress port buffering process used by store-and forward switches provides the flexibility to support any mix of Ethernet speeds. For example, handling an incoming frame traveling into a 100 Mb/s Ethernet port that must be sent out a 1 Gb/s interface would require
   using the store-and-forward

*See Netacad for diagrams

Question 6

Describe the cut-through method

Answer 6

A cut-through switch begins the forwarding process after the destination MAC address of an incoming frame and the egress port has been determined. I.e. it forwards the frame before it is entirely received

Question 7

Give an advantage and disadvantage of the cut-through method over the store-and-forward

Answer 7

An advantage to cut-through switching is the ability of the switch to start forwarding a frame earlier than store-and-forward switching making it faster.
A disadvantage is if there is a high error rate (invalid frames) in the network, cut-through switching can have a negative impact on bandwidth; thus, clogging up
bandwidth with damaged and invalid frames.

Question 8

What are the two primary characteristics of the cut-through method?

Answer 8

1. Rapid Frame Forwarding:
   A switch using the cut-through method can make
   a forwarding decision as soon as it has looked up the destination MAC address of the frame in its MAC address table. The switch does not have to
   wait for the rest of the frame to enter the ingress port before making its forwarding decision.
2. Fragment Free:
   Fragment free switching is a modified form of cut-through switching in which the switch waits for the collision window (64 bytes) to pass before forwarding the frame. This means each frame will be checked
   into the data field to make sure no fragmentation has occurred

Question 9

What is the advantage of fragment free forwarding

Answer 9

Fragment free switching provides better error checking than cut-through, with practically no increase in latency.

Question 10

In what situations is cut-through frame forwarding the best option?

Answer 10

The lower latency speed of cut-through switching makes it more appropriate for extremely demanding, high-performance computing (HPC) applications that require process-to-process latencies of 10 µs or less.

Question 11

Briefly describe CAM and how it works

Answer 11

CAM (Content Addressable Memory) is special type of memory used by Cisco switches. In the case of ordinary RAM the IOS uses a memory address to get the data stored at this memory location, while with CAM the IOS does the inverse; It uses the data and the CAM returns the address where the data is stored

Question 12

Why is the CAM considered to be faster than the RAM?

Answer 12

In CAM, returning the address is done in a single CPU cycle vs. the traditional programming of searching through a table, which will cost many CPU cycles. Thus the CAM is considered to be faster than the RAM since the CAM searches the entire memory in one operation.

Question 13

What is the CAM table and what is its function?

Answer 13

-The CAM table, also variably referred to as switch, switching table or MAC table, is used by the Ethernet switch logic to store information such as MAC addresses available on physical ports with their associated VLAN Parameters).
-Switch filtering and forwarding are done with a CAM
table.
-The CAM table is present in all switches for layer 2 switching allowing the switches to facilitate communications between connected nodes at high speed and in full-duplex regardless of how many devices are connected.

Question 14

How is the CAM table used to make Layer 2 forwarding decisions

Answer 14

In the case of Layer 2 switching tables, the switch
must find an exact match to a destination MAC address or the switch floods the packet out all ports in the VLAN. A CAM tables provide only two results: 0 (true) or 1 (false).

Question 15

How is a CAM table built? Include a topology and a
CAM table to Illustrate.

Answer 15

The table is built by recording the source address and inbound port of all frames. As frames arrive on switch ports, the source MAC addresses are learned and recorded in the CAM table. The port of arrival and the VLAN are both recorded in the table, along with a timestamp.

*See Netacad for illustrations

Question 16

How is the CAM table updated if: a MAC address learned on one switch port has moved to a different port or if a MAC address is already present in the table for the correct arrival port

Answer 16

If a MAC address learned on one switch port has moved to a different port, the MAC address and timestamp are recorded for the most recent arrival
port and the previous entry deleted. If a MAC address is already present in the table for the correct arrival port, only its timestamp is updated.

Question 17

What is the difference between the CAM table and ARP table?

Answer 17

• A CAM table maps Ethernet hardware addresses to switch ports. There is only one CAM table found in the switch.
• An ARP table maps IP addresses to Ethernet hardware addresses. Each host (TCP/IP client) connected to the network has its own ARP Table.

Question 18

What is a collision domain?

Answer 18

In hub-based Ethernet segments, network devices compete for the medium, because devices must take turns when transmitting. The network segments that share the same bandwidth between devices are known as collision domains.

Question 19

When do collisions occur?

Answer 19

When two or more devices within that the same collision domain try to communicate at the same time, a collision will occur.

Question 20

Discuss collision domains in both half and full duplex

Answer 20

If an Ethernet switch port is operating in half duplex, each segment is in its own collision domain. However, Ethernet switch ports operating in full duplex eliminate collisions; therefore, there is no collision domain.

Question 21

What will a switch do if: a)both devices can operate in full duplex and b) only one can operate in full duplex

Answer 21

By default, Ethernet switch ports will autonegotiate full duplex when the adjacent device can also operate in full duplex. Their highest common bandwidth is also used
If the switch port is connected to a device operating in half-duplex, such as a legacy hub, then
the switch port will operate in half duplex. In the case of half duplex, the switch port will be part of a collision domain.

Question 22

What is a broadcast domain?

Answer 22

A collection of interconnected switches forms a single broadcast domain. Only a network layer device, such as a router, can divide a Layer 2 broadcast domain. Routers are used to segment broadcast domains, but will also segment a collision domain

Question 23

What happens when a switch receives a broadcast frame?

Answer 23

When a switch receives a broadcast frame, it forwards the frame out each of its ports, except the ingress port where the broadcast frame was received. Each device connected to the switch receives a copy of the broadcast frame and processes it

Question 24

What are the advantages of broadcast domains?

Answer 24

• Broadcasts are sometimes necessary for initially locating other devices and network services.
• Fast and reliable communication for offices in different locations.

Question 25

What are the disadvantages of broadcast domains?

Answer 25

• They reduce network efficiency as network bandwidth is used to propagate the broadcast traffic.
• Too many broadcasts and a heavy traffic load on a network can result in congestion, which slows down network performance.
• E.g. tendency to drop Web data signals after reaching network router interface borders.

Question 26

Illustrate broadcast and collision domains

Answer 26

See https://elearning.strathmore.edu/pluginfile.php/240105/mod_resource/content/2/1.3%20-%20Switch%20Architecture%20%20Domains.pdf

slide 9 for illustration

Question 27

Explain some ways in which switches can alleviate congestion

Answer 27

By default, interconnected switch ports attempt to establish a link in full duplex, therefore eliminating
collision domains.
Each full duplex port of the switch provides the full bandwidth to the device or devices that are connected to that port.
Switches use a table of MAC addresses to determine the segment to which the frame is to be sent, and can lessen or eliminate collisions entirely.

Question 28

What are some important characteristics of switches for alleviating Network Congestion

Answer 28

• High port density - Switches have high-port densities: 24- and 48-port switches are often just a single rack unit and operate at speeds of 100 Mb/s, 1 Gb/s, and 10 Gb/s. Large enterprise switches may support many
hundreds of ports.
• Large frame buffers - The ability to store more received frames before having to start dropping them is useful, particularly when there may be congested ports to servers or other parts of the network.
• Port speed - Depending on the cost of a switch, it may be possible to support a mixture of speeds. Ports of 100 Mb/s, and 1 or 10 Gb/s are common (100 Gb/s is also possible).
• Fast internal switching - Having fast internal forwarding capabilities allows high performance. The method that is used may be a fast internal bus or shared memory, which affects the overall performance of the switch.
• Low per-port cost - Switches provide high-port density at a lower cost.

Question 29

When two switches with 3 clients each are connected to each other, what does that increase, the broadcast domain or the collision domain?

Answer 29

When two switches are connected together, the broadcast domain is increased

Question 30

Can a hub create a collision domain? Why?

Answer 30

No. The reason being, it neither breaks a collision domain nor a broadcast domain, i.e a hub is neither a collision domain separator nor a broadcast domain separator. All the devices connected to a hub are in a single collision and single broadcast domain. Remember, hubs do not segment a network, they just connect network segments.