Vanessa Avalos
Cisco Networking Academy Semester 3
8/22/2001
Lecture Notes

Chapter 2: LAN Switching
Reasons for LAN Congestion:

• More users
• Multi-tasking
• Faster Operating Systems
• Client/Server Applications

Most common reliable LAN technology today:
Ethernet 802.3

• Factors that affect performance
• CSMA/CD
•  Collisions
•  Repeaters
•  Normal latency
•  Multimedia applications that require higher bandwidth

Congestion/Bandwidth
 Faster and more intelligent workstations plus network intensive applications have made a need for more bandwidth
 More congestion on a network = loss of user productivity
 You need more bandwidth to relive congestion

Latency (delay)

• Time it takes the NIC to put the voltage on the wire
• Propagation (travel time on the wire) .556 microseconds / 100m Cat5
• Network Devices
• Duration of host sending bits
• Routers have higher latency because they perform more complex decision making functions

Transmission Time:
bit or slot time
Transmission Time = Number of bits being sent, times the bit time for a given technology

Example:
10BaseT 100 ns bit time
1 byte = 800 ns bit time
10BaseT – smallest frame size is 64 bytes
64 bytes x 800 ns = 51,200 ns  /  1000 = 5.12 microseconds
(in a perfect world)

Switch – a device
Switching – a function

Layer 2 Switching -  Switch
Layer 3 Switching - Router
 

Router	Switch
Path Selection	Switching Frames
Switching	Maintaining and building switching tables (MAC to port)

Segmenting…
     Dividing a network into smaller units can decrease network congestion

…with bridges

•  Can provide more bandwidth because there are less users per segment
•  Bridges learn segments by building address tables
•  Layer 2 device -  uses MAC addresses
•  Considered a store-an-forward device
•   Looks at destination address and calculates CRC before forwarding out all ports
•  A bridge can temporarily store frames until port is available

…with routers

•  Routers – Layer 3
•  creates the highest level of segmentation
•  has the highest level of latency because it makes lots of decisions
•  some protocols require acknowledgements for every packet, this causes a 30% to 40%
• loss of throughput 
•  sliding-windows protocols  have a 20 to 30% loss of throughput

…with LAN switches

• Switching eases bandwidth shortages and bottlenecks
• Can segment into microsegments, creating a collision free domain (see below)
• They are still the same broadcast domain
• Switches can use close to 100% of the bandwidth

Microsegmentation: a hardware concept, makes a virtual circuit that is collision free for the duration of the circuit.  (Done by a switch)
 

Switches learn addresses dynamicly, store them in content-addressable memory (CAM) and time stamps the addresses. Every time an known address comes in, it gets a new time stamp.  Addresses that time out are removed.

Benefits of Switching:

• Users can communicate in parallel
• Segments are collision free
• Cost effective (no rewiring)
• Gives network administrator greater flexibility

Symmetric / Asymmetric Switching
Symmetric – Both sides are the same (All ports have like bandwidth)
Asymmetric – 10 – 100 non like bandwidth (usually used in a client/server environment)

Memory Buffer & 2 methods for forwarding frames:
Port based – Frames get in a line. A single frame can delay all the frames
Shared Memory – All the frames are put into a common memory buffer.

Each frame is dynamically linked to transport port
Allows frames to be received on one port and transmitted out another
Switch keeps a map of ports that the frames need to be transmitted to
Clears map after frame has been transmitted successfully

2 Switching Methods
 Store-and-Forward – has the highest latency
 Cut-Through: (no error checking)

· Fast-Forward – has many errors and collisions
· Fragment-Free – checks for 64 bytes (min size)

VLAN
Virtual LAN:  A logical grouping of users that are not restricted to a physical switch segment

•  Creates a single broadcast domain
•  VLAN standard: IEEE 802.1q
•  Can be grouped by function, department etc…

Spanning-Tree protocol – How VLANS talk to each other:

• Allows duplicate switch paths with out loops
• Bridge protocol data units – BPDUs
• BPDUs are sent and received by all switches at regular intervals
• Each VLAN has its own spanning-tree

STP States: States your ports are in – Default Order

• · Blocking: No frames forwarded, but BPDUs are heard
• · Listening: No frames forwarded, but listening for frames
• · Learning: No frames forwarded, learning address
• · Forwarding: Frames are forwarded and learning address
• · Disabled: No frames forwarded, no BPDUs heard

Initially set by configuration, later modified by STP
(command show spantree)
Ports can be set to immediately enter STP forwarding and not go through the sequence