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66 Cards in this Set

  • Front
  • Back
What is Microsegmentation?
A switch’s effect of segmenting an Ethernet LAN into one collision domain per interface
Bridges create what?
Two separate collision domains.
Can frames on one end of the bridge collide with frames on another end of the bridge?
If the bridge needs to forward a frame over to a busy segment, the bridge:
buffers the frame in memory until the segment is no longer busy.
What happens when you add a bridge between two hubs?
It creates two separate 10BASE T networks. It also doubles the bandwidth, combining the bandwidth from both networks.
What is a Switches logic based on?
logic based on the source and destination MAC address in each frame’s Ethernet header.
The IEEE defines three general categories of Ethernet MAC Addresses:
Unicast, Broadcast and Multicast
The first half of a multicast address is always
01 00 5e
The second half of a multicast MAC address will fall in the range of
00 00 00 through 7F FF FF
The primary purpose 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.
What are a LAN switches three main jobs.
1. Decide when to forward a frame or when to filter (not forward) a frame, based on the destination MAC.

2. Learn MAC addresses by examining the source MAC address of each frame received by the bridge.

3. Create a (layer 2) loop free environment with other bridges by using STP.
To decide whether to forward a frame, a switch uses:
a dynamically built table that lists MAC addresses and outgoing interfaces
A switch’s MAC address table is also called (three things):
the switching table, or bridging table, or even the Content Addressable Memory (CAM), in reference to the type of physical memory used to store the table.
Switches build the address table by:
listening to incoming frames and examining the source MAC address in the frame. It creates an entry if the source MAC is not in the table already. The MAC address is placed into the table, along with the interface from which the frame arrived.
When the switch receives a frame destined for an address that’s not in the MAC address table, the switch:
forwards the frame out all interfaces (except the incoming interface). These are called unknown unicast frames.
The host that receives the unknown unicast frame will reply to it IF:
It matches its MAC address. The switch will then update the unknown destination with the correct MAC address.
What is Flooding?
The process of sending frames out all other interfaces except for the one that it was received on
In what two scenarios do switches flood frames?
Switches flood for unknown unicast frames as well as broadcast frames
What does Forwarding a frame mean?
Forwarding a frame means the frame is being sent out only one port on the switch.
What is Filtering on a switch?
Filtering is performed when the switch does not have an entry for both the source and destination MAC address. The switch drops this packet.
What is a Switches inactivity timer?
An inactivity timer is a timer for each entry in the MAC address table. A timer is reset to 0 every time a frame is received from a MAC.
What happends when the MAC address table gets full on a switch?
the MAC entry with the longest timer gets removed.
What does STP stand for?
Spanning-Tree Protocol
What would happen without STP in Ethernet networks with physically redundant links?
frames would loop for an indefinite period of time.
How does STP prevent loops?
STP blocks some ports from forwarding frames so that only one active path exists between any pair of LAN segments (collision domain).
STP causes each interface on a switch to settle into either a ______ state or a _____ state.
Blocking, Forwarding
What is a Blocking State?
A blocking state means that the interface cannot forward or receive data
What is a Forwarding State?
A forwarding state means that the interface can send and receive data frames.
What could happen if a PC sends a frame to another PC (turned off) that hasn’t been learned by the switch?
The frame could loop forever without STP and unknown unicast frames would flood out every port
What is the best solution when dealing with looping and the need for redundant links?
The right solution includes switched LANs with physical redundancy, while using STP to dynamically block some interfaces so that only one active path exists between two endpoints at any instant in time.
What is "store and forward processing"?
When the switch must receive the entire frame before forwarding the first bit of the frame.
What processing methods allow the switch to start forwarding the frame before the entire frame has been received, reducing time required to send the frame (the latency, or delay)?
"cut through" and "fragment free" processing
What is "cut through" processing?
When the switch starts sending the frame out the
output port as soon as possible. This might reduce latency, but it causes errors. The FCS is in the Ethernet trailer, so the switch cannot determine if the frame had any errors before starting to forward the frame
What is "fragment free" processing?
works similarly to cut through, but it tries to
reduce the number of errored frames that it forwards. One interesting fact about CSAMA/CD is that collisions should be detected within the first 64 bytes of a frame. Fragment free processing waits for 64 bytes of a frame before forwarding it.
Switches support rate adaptation, which means:
devices that use different Ethernet speeds can communicate through the switch (hubs cannot).
What are two advantages to Fiber Optic cabling?
Fiber supports longer distances and better physical security
What is a collision domain?
A collision domain is the set of LAN interfaces whose frames could collide with each other, but not with frames sent by another device on the network.
Each port on a switch, bridge, and router represents its own ____ _____.
Collision Domain
Broadcast domains relate to where broadcasts can be _______.
What is a Broadcast domain?
a set of devices for which, when one of the devices send a broadcast, all the other devices receive a copy of the broadcast.
How many broadcast domains does a switch create?
Just one
Only routers stop the flow of ________, making each port on a router its own ______.
Broadcasts, broadcast domain
A LAN consists of all devices in the same ____ ______.
Broadcast Domain
How are all the interfaces on Cisco switches in the same broadcast domain by default?
Cisco accomplishes this by putting all interfaces in VLAN 1 by default.
VLANs separate switch interfaces into different _____.
Broadcast domains
No traffic can go between VLANs without passing through a _____, most likely a ______.
layer 3 device, router
Switches create multiple broadcast domains by:
putting some interfaces into one VLAN and other interfaces into other VLANs.
What are five reasons to use VLANs?
1. To create more flexible designs that group users by department or workgroups instead of by physical location.

2. To segment devices into smaller LANs (broadcast domains) to reduce overhead caused by each host in the VLAN.

3. To reduce the workload for STP by limiting a VLAN to a single access switch.

4. To enforce better security by keeping hosts that work with sensitive data on a separate VLAN.

5. To separate traffic sent by an IP phone from traffic sent by PCs connected to the phones.
What is a Campus LAN?
The term campus LAN refers to the LAN created to support larger buildings, or multiple buildings in somewhat close proximity to one another.
Cisco uses three terms to describe the role of each switch in a campus design:
Access, Distribution, and Core switches
The roles of Access, Distribution, and Core switches differ mainly in two main concepts:
1. Whether the switch should connect to end user devices.

2. Whether the switch should forward frames between other switches by connecting to multiple different switches.
Learn the Access, Distribution and Core switch drawings!!!!!!
Learn the Access, Distribution and Core switch drawings!!!!!!
Discuss the role of Access switches:
Access switches connect directly to end users, providing access to the LAN.
Under normal circumstances, access switches normally send traffic to and from the end user devices to which they are connected.

Access switches should not be expected to forward traffic between two other switches.
In larger campus LANs, distribution switches provide
a path through which the access switches can forward traffic to each other.
By design, each access switch connects to at least one
distribution switch
Discuss Core switches:
Core switches provide even more aggregation benefits than do the distribution switches.

Core switches provide extremely high forwarding rates – these days into the hundreds of millions of frames per second.
Succinctly define Access switches:
Access – provides a connection point (access) for end user devices. Does not forward frames between two other access switches under normal circumstances.
Succinctly define Distribution switches:
Distribution – provides an aggregation point for access switches, forwarding frames between switches, but not connecting directly to end user devices.
Succinctly define Core switches:
Core – aggregates distribution switches in very large campus LANs, providing very high forwarding rates.
Describe the Core layer. What are some Dos and Donts?
Core Layer: The core of the network. Its job is to switch traffic as fast as possible with reliability.

Dont: Dont do anything to slow down the network such as access list, packet filtering and routing between VLANS. Don't support workgroup access and don't add devices.

Do: Utilize speed such as fast ethernet, ATM, etc... DO select routing protocols with fast convergence time (EIGRP, OSPF).
Describe the Distribution layer
Distribution Layer: Communication point between the core and Access layer. Handles traffic for remote services (WAN, routing, filtering). Here you would implement routing, access list, security and network policies.
Describe the Access layer
Access Layer (a.k.a Desktop layer): Controls user and Work group access to the network resource. Here you would implement work group connectivity and segmentation.
How to switches forward frames (intelligence)?
Switches use ASIC (application specific integrated circuit) to switch frames. It is also much faster than a bridge.
A switch has three methods on how to process a frame for forwarding:
1) Store and forward- The entire frame will be stored and check to see if it has no errors. If no errors forward the frame, else drop it. (Best error correction but slowest).

2) Cut through- The frame is not checked at all for errors and it will be forward out asap. (Best speed but no error correction).

3) Fragment free- The first 64 bytes of a frame will be checked for errors. If not errors it will be forward, else drop it. (Good speed and good reliability).
Describe FCS in relation to CRC
The switch will check the FCS(Frame check sequence) for errors. The FCS contains the cyclic redundancy check which houses a specific value. If the host doesn't match the CRC it is considered an error.
How does STP work:
STP uses an algorithm to determine a loop free path. All ports that are not on the loop free path are placed into blocking mode.