CCNA Chapter 2: Network Access

Switching Concepts

Switches are intelligent Layer 2 devices that forward Ethernet frames using MAC addresses. They reduce collision domains and improve network efficiency.

• Store-and-forward: Entire frame is checked for errors before forwarding.
• Cut-through: Faster, but forwards possibly corrupted frames.

Command Example:

Switch(config)# mac address-table static aaaa.bbbb.cccc vlan 10 interface fa0/1

VLANs (Virtual LANs)

VLANs isolate broadcast traffic by logically segmenting the network. Each VLAN creates a separate Layer 2 broadcast domain.

• Native VLAN: VLAN that carries untagged frames (typically VLAN 1)
• Voice VLAN: Used to separate voice traffic from data

Switch(config)# vlan 10
Switch(config-vlan)# name SALES

Trunking and 802.1Q

Trunk ports allow multiple VLANs across a single physical link using tags. 802.1Q inserts a VLAN tag inside Ethernet frames.

Switch(config-if)# switchport trunk encapsulation dot1q
Switch(config-if)# switchport mode trunk

STP (Spanning Tree Protocol)

STP prevents broadcast loops in Layer 2 networks. It elects a root bridge and places redundant links in a blocking state to break loops.

• BPDU: Bridge Protocol Data Units used for STP communication
• Port States: Listening, Learning, Forwarding, Blocking

Switch(config)# spanning-tree vlan 1 priority 4096

EtherChannel

EtherChannel provides fault-tolerant, aggregated links using PAgP or LACP. It avoids STP blocking and increases bandwidth.

Switch(config)# interface range fa0/1 - 2
Switch(config-if-range)# channel-group 1 mode active

Summary Table