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Cisco LAN Switching Basics

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Cisco LAN Switching Basics

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Cisco CCNA Study Guide

By Cbrzana

Table Of Contents

Cisco LAN Switching Basics


Bridges create separate collision domains, add bandwidth to the network.

Transparent Bridging simply means that devices connected to bridge don't need to know that the bridge exists (no additional configuration necessary).


Transparent Bridges Perform

1. Learn MAC addresses (examine source address) and add to table

2. Determine when to forward or filter a frame

3. Use Spanning Tree Protocol (STP) to prevent looping


The Forward versus Filter Decision: Examines bridge table, if source and destination port are the same, filter.

1. Frame is received by bridge.

2. If destination is broadcast or multicast, forward to all ports except source port.

3. If the destination is unicast and address not in address table, forward on all ports except source port.

4. If destination is unicast, address is on different port than source, forward.

5. Otherwise, filter.

Hub = Half-duplex

Switch = Full or half-duplex


Internal Processing on Cisco Switches

Store and Forward: Wait for entire frame, analyze destination, and forward

Cut-through: Reduces latency, propagates errors. Sends packet as soon as destination is known, doesn't check FCS for errors.

Fragment-free: Waits for first 64-bytes, can detect errors by then.

*Note: Most today use Store and Forward, as speeds as very fast.

Autonegotiation: NIC and switch determine speed automatically, both most support full and half duplex and 10/100 Mbps.

- > Many say static assignment is still the best option


Comparing Bridges to Switches

Feature/Fact Transparent Bridge Switch
Unicast forwarding Based on bridge table/MAC Same
Broadcast forwarding All broadcast forwarded Same
Learning the bridge table Examining source MAC Same
Loop Avoidance STP STP
Popular in market today? No Yes
Support dozens/hundred of ports? No Yes
Allows full duplex No Yes
Uses specialized hardware for faster processing? No Yes (ASICs)
Cut-through/store and forward processing? No Yes


LAN Segmentation: Breaking the LAN into different parts = collision domains

- > Collision domain = Set of LAN interfaces whose frames could collide with each other

Broadcast Domain: Set of LAN interfaces where, when broadcast sent, all devices receive it.

- > Only routers stop the flow of broadcasts


Summary of Collision/Broadcast Domains

Feature Bridging Switching Routing
Greater cabling distances allowed Yes Yes Yes
Decrease in collisions Yes Yes Yes
Block Broadcasts? No No Yes
Block multicasts? No No Yes
Increase in bandwidth? Yes Yes Yes


Spanning Tree Protocol

Without this, frames would loop forever if a physically redundant link existed in the network. Block some ports, forwards others, so that only one link between devices exist at one time.

1. STP sends Hello Messages (or Bridge PDU/BPDU)

2. Each bridge/switch claims to be the root, but the one with lowest bridge-ID is root.

a. Bridge ID = 8 bytes (2 for priority, 6 for MAC)

3. All the Root bridges ports are set to forwarding state

4. Root bridge sends out BPDUs to all other devices

5. Those devices set root port according to what the lowest cost port is.

6. The switch w/ the lowest cost on the BPDU is the bridge for that segment

7. All other ports are blocked.


Other States Include:

Listening: Listens to BPDUs, doesn't learn MAC or forward traffic

Learning: Listens to BPDUs, learns MAC, doesn't forward

Disabled: Administratively down


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