IST 228\Ch2\Layer2 Switching
One Upon A Time Computer Networks
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IST 228\Ch2\Layer2 Switching
The First Switched Network
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IST 228\Ch2\Layer2 Switching
Layer-2 Switching
• Advantages:
– Hardware based (Uses the MAC address from
the host’s NIC card)
– Not software devices
– Fast and efficiency
– Increases the bandwidth for each user
• Limitations:
– Does not break up broadcast domains
– Make sure users spend 80% of time on the
local segment
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IST 228\Ch2\Layer2 Switching
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Address Learning
PC:0200.2222.2222 WWW:0200.3333.3333
E01
DNS:0200.1111.1111
E02
E03
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ARP (DNS)
DMAC:FFFF.FFFF.FFF
SMAC: 0200.222.2222
ARP
DMAC:0200.2222.2222
SMAC: 0200.1111.1111
0200.2222.2222
E01
0200.2222.2222
0200.1111.1111
E01
E03
ARP Request
DMAC: 0200.1111.1111
SMAC: 0200.2222.2222
ARP
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DMAC: 0200.2222.2222
SMAC: 0200.1111.1111
ARP (web)
DMAC: FFFF.FFFF.FFFF
SMAC: 0200.1111.1111
DMAC: 0200.1111.1111
SMAC: 0200.3333.3333
0200.2222.2222
0200.1111.1111
0200.3333.3333
E01
E03
E02
IST 228\Ch2\Layer2 Switching
Forward/Filter Decisions
• When a frame arrives:
– Case 1: DMAC is known and in the database
– Case 2: DMAC is not listed
– Case 3: It is a broadcast
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IST 228\Ch2\Layer2 Switching
Loop Avoidance
• Redundant links are
use between
switches/bridges to
increase survivability
• Disadvantages:
– Broadcast storms
– Trashing the MAC table
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IST 228\Ch2\Layer2 Switching
Spanning Tree Protocol (STP)
• STP is used to avoid networks loops on
layer-2 networks (bridges and switches)
• STP is a bridge protocol that make sure
that transmission takes place over a
spanning tree topology
• Spanning tree terms:
– Root Bridge:
• the bridge with the best bridge ID
• all decision in the network are made on this bridge
– Bridge ID:
• lowest bridge ID becomes the root bridge
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IST 228\Ch2\Layer2 Switching
– Nonroot Bridge
– Bridge Protocol Data Unit (BPDU)
– Root Port
• The link directly connected to the root bridge, or
the shortest path to the root bridge
– Designated/Nondesignated port
– Port cost
– Forwarding port
• send and receives all data
– Blocked port
• does not participate in frame forwarding
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IST 228\Ch2\Layer2 Switching
Spanning-Tree Operations
• Selecting the root bridge
• Selecting the designated port
• Port States
– Blocking
– Listening
– Learning
– Forwarding
– Disabled
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IST 228\Ch2\Layer2 Switching
Example: Find The Root Bridge and Root Ports
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