One Upon A Time Computer Networks
description
Transcript of One Upon A Time Computer Networks
IST 228\Ch2\Layer2 Switching 1
One Upon A Time Computer Networks
IST 228\Ch2\Layer2 Switching 2
The First Switched Network
IST 228\Ch2\Layer2 Switching 3
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
IST 228\Ch2\Layer2 Switching 4
Address LearningDNS:0200.1111.11
11WWW:0200.3333.3333PC:0200.2222.2222
E01 E02E03
DMAC:FFFF.FFFF.FFFSMAC: 0200.222.2222
1ARP (DNS) 0200.2222.2222 E01
DMAC:0200.2222.2222SMAC: 0200.1111.1111
2ARP 0200.2222.2222 E01
0200.1111.1111 E03
DMAC: 0200.1111.1111SMAC: 0200.2222.2222
3ARP Request
DMAC: 0200.2222.2222SMAC: 0200.1111.1111
4ARP
DMAC: FFFF.FFFF.FFFFSMAC: 0200.1111.1111
5ARP (web)
DMAC: 0200.1111.1111SMAC: 0200.3333.3333
60200.2222.2222 E010200.1111.1111 E030200.3333.3333 E02
IST 228\Ch2\Layer2 Switching 5
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
IST 228\Ch2\Layer2 Switching 6
Loop Avoidance• Redundant links
are use between switches/bridges to increase survivability
• Disadvantages:– Broadcast storms– Trashing the MAC
table
IST 228\Ch2\Layer2 Switching 7
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
IST 228\Ch2\Layer2 Switching 8
– 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
IST 228\Ch2\Layer2 Switching 9
Spanning-Tree Operations• Selecting the root bridge• Selecting the designated port• Port States
– Blocking– Listening – Learning – Forwarding– Disabled
IST 228\Ch2\Layer2 Switching 10
Example: Find The Root Bridge and Root Ports