Chapter 2 Switching Concepts

What is FCS

is an error checking process that helps to ensure that the frame is free of physical and data-link errors.

What are the 2 main characteristics of Store-and-forward switching?

1. Error checking 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. With any mismatch in speeds between the ingress and egress ports, the switch stores the entire frame in a buffer, computes the FCS check, forwards it to the egress port buffer and then sends it. For example, handling an incoming frame traveling into a 100 Mbps Ethernet port that must be sent out a 1 Gbps interface would require using the store-and-forward method.

Which switch characteristic helps keep traffic local and alleviates network congestion? A: Large frame buffers B: fast internal switching C: high port density D: fast port speed

C: high port density

How long does a switch keep an entry in its MAC address table

most switches, about 5 minutes

If there are too many broadcasts and a heavy traffic load on the network what may happen?

network performance is degraded

Which command displays information about the auto-MDIX setting for a specific interface?​

show controllers

What are broadcast domains?

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. When a device sends a Layer 2 broadcast, the destination MAC address in the frame is set to all binary ones. (FF:FF:FF:FF:FF:FF) The Layer 2 broadcast domain is referred to as the MAC broadcast domain. The MAC broadcast domain consists of all devices on the LAN that receive broadcast frames from a host. 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.

What are 4 characteristics of switches that alleviate network congestion? Explain them too.

Fast port speeds - Switches with faster port speeds cost more but can reduce congestion. Fast internal switching - Switches use a fast internal bus or shared memory to provide high performance. Large frame buffers - Switches use large memory buffers to temporarily store more received frames before having to start dropping them. - This enables ingress traffic from a faster port (e.g., 1 Gbps) to be forwarded to a slower (e.g., 100 Mbps) egress port without losing frames. High port density - High port density switches also help keep traffic local, which helps alleviate congestion. - high port density switch lowers overall costs because it reduces the number of switches required. For instance, if 96 access ports were required, it would be less expensive to buy two 48-port switches instead of four 24-port switches.

Explain what cut-through switching is.

With cut-through switching there is no FCS check performed, therefore this method may forward invalid frames Because of no FCS it can perform rapid frame switching — meaning the switch 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. IMPORTANT: If there is a high error rate (invalid frames) in the network, cut-through switching can have a negative impact on bandwidth, thereby clogging up bandwidth with damaged and invalid frames.

What are Collision Domains

are network segments that share the same bandwidth between devices. When two or more devices within the same collision domain try to communicate at the same time, a collision will occur. If an Ethernet switch port is operating in half-duplex, each segment is in its own collision domain. By default, Ethernet switch ports will autonegotiate full-duplex when the adjacent device can also operate in full-duplex. - half duplex is determined then the switch port will be added as part of a collision domain

What is fragment free switching?

is a modified form of cut-through switching in which the switch only starts forwarding the frame after it has read the Type field. Fragment free switching provides better error checking than cut-through, with practically no increase in latency.

Egress

is used to describe the port that frames will use when leaving the device

Ingress

is used to describe the port where a frame enters the device

Explain what Store-and-forward switching is.

This 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)

Describe the 2 step process performed on every Ethernet frame that enters a switch

Step 1. - learn Examining the Source MAC Address If the source MAC address does not exist in the MAC address table, the MAC address and incoming port number are added to the table. If the source MAC address does exist, the switch updates the refresh timer for that entry. By default, most Ethernet switches keep an entry in the table for five minutes. If the source MAC address does exist in the table but on a different port, the switch treats this as a new entry. — The entry is replaced using the same MAC address, but with the more current port number. Step 2. - Forward Examining the Destination MAC Address If the destination MAC address is a unicast address, the switch will look for a match between the destination MAC address of the frame and an entry in its MAC address table If the destination MAC address is in the table, it will forward the frame out of the specified port If the destination MAC address is not in the table, the switch will forward the frame out of all ports except the incoming port. — This is called an unknown unicast If the destination MAC address is a broadcast or a multicast, the gram is also flooded out of all ports except the incoming port.