Read And Learn - Icnd1 Chapter 7 Lan Switching Concepts

Front 1

A switch’s effect of segmenting an Ethernet LAN into one collision domain per interface is sometimes called microsegmentation.

Back 1

The IEEE defines three general categories of Ethernet MAC addresses:

Unicast addresses: MAC addresses that identify a single LAN interface card.

Broadcast addresses: A frame sent with a destination address of the broadcast address (FFFF.FFFF.FFFF) implies that all devices on the LAN should receive and process the frame.

Multicast addresses: Multicast MAC addresses are used to allow a dynamic subset of devices on a LAN to communicate.

Front 2

The primary job of a LAN switch is to receive Ethernet frames and then make a decision: either forward the frame out some other port(s), or ignore the frame. To accomplish this primary mission, transparent bridges perform three actions:

Deciding when to forward a frame or when to filter (not forward) a frame, based on the destination MAC address.

Learning MAC addresses by examining the source MAC address of each frame received by the bridge

Creating a (Layer 2) loop-free environment with other bridges by using Spanning Tree Protocol (STP)

Back 2

To decide whether to forward a frame, a switch uses a dynamically built table that lists MAC addresses and outgoing interfaces. Switches compare the frame’s destination MAC address to this table to decide whether the switch should forward a frame or simply ignore it.

Front 3

unknown unicast frames frames whose destination MAC addresses are not yet in the bridging table

Back 3

Switches keep a timer for each entry in the MAC address table, called an inactivity timer. The switch sets the timer to 0 for new entries. Each time the switch receives another frame with that same source MAC address, the timer is reset to 0.

Front 4

Spanning Tree Protocol
Without STP, frames would loop for an indefinite period of time in Ethernet networks with physically redundant links.

Back 4

To prevent looping frames, STP blocks some ports from forwarding frames so that only one active path exists between any pair of LAN segments (collision domains). The result of STP is good: frames do not loop infinitely, which makes the LAN usable. However, although the network can use some redundant links in case of a failure, the LAN does not load-balance the traffic.

Front 5

Switching Methods:
Store-and-forward:
The switch fully receives all bits in the frame (store) before forwarding the frame (forward). This allows the switch to check the FCS before forwarding the frame.

Back 5

Switching Methods:
Cut-Through:
The switch forwards the frame as soon as it can. This reduces latency but does not allow the switch to discard frames that fail the FCS check.

Front 6

Switching Methods:
Fragment Free:
The switch forwards the frame after receiving the first 64 bytes of the frame, thereby avoiding forwarding frames that were errored due to a collision.

Back 6

Switches provide many additional features not offered by older LAN devices such as hubs and bridges. In particular, LAN switches provide the following benefits:

Switch ports connected to a single device microsegment the LAN, providing dedicated bandwidth to that single device.

Switches allow multiple simultaneous conversations between devices on different ports.

Switch ports connected to a single device support full duplex, in effect doubling the amount of bandwidth available to the device.

Switches support rate adaptation, which means that devices that use different Ethernet speeds can communicate through the switch (hubs cannot).

Front 7

Switches forward frames based on the destination address:

a. If the destination address is a broadcast, multicast, or unknown destination unicast (a unicast not listed in the MAC table), the switch floods the frame.

If the destination address is a known unicast address (a unicast address found in the MAC table):

If the outgoing interface listed in the MAC address table is different from the interface in which the frame was received, the switch forwards the frame out the outgoing interface.

If the outgoing interface is the same as the interface in which the frame was received, the switch filters the frame, meaning that the switch simply ignores the frame and does not forward it.

Back 7

Switches use the following logic to learn MAC address table entries:

a. For each received frame, examine the source MAC address and note the interface from which the frame was received.

b. If they are not already in the table, add the address and interface, setting the inactivity timer to 0.

c. If it is already in the table, reset the inactivity timer for the entry to 0.

Front 8

Switches use STP to prevent loops by causing some interfaces to block, meaning that they do not send or receive frames.

Back 8

a collision domain is the set of LAN interfaces whose frames could collide with each other, but not with frames sent by any other devices in the network.

Front 9

Ony a hub does not create a collision domain. Routers, switches and bridges ALL create collision domains.

Back 9

broadcast domain relates to where broadcasts can be forwarded. A broadcast domain encompasses a set of devices for which, when one of the devices sends a broadcast, all the other devices receive a copy of the broadcast.

Front 10

A switch creates a single broadcast domain.
VLAN's breakup broadcast domains as do routers.

Back 10

Cisco switches put all interfaces on VLAN 1 by default.

Front 11

LAN Design:
Access switches connect directly to end users, providing access to the LAN.

Back 11

LAN Design:
distribution switches provide a path through which the access switches can forward traffic to each other.

Front 12

LAN Design:
Core: Aggregates distribution switches in very large campus LANs, providing very high forwarding rates.

Back 12

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