1. One Upon A Time Computer Networks
IST 228\Ch2\Layer2 Switching
One Upon A Time Computer Networks

2. The First Switched Network
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The First Switched Network

3. Layer-2 Switching Advantages: Limitations:
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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

4. Address Learning WWW:0200.3333.3333 PC:0200.2222.2222
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Address Learning
WWW:
PC:
DNS:
E01
E02
E03
DMAC:FFFF.FFFF.FFF
SMAC: 1
ARP (DNS)
E01
DMAC:
SMAC: 2
ARP
E01
E03
DMAC:
SMAC: 3
ARP Request
The following list provides some additional insights relating to the steps shown in Figure 4-8:
The PC is preconfigured with the IP address of the DNS. The PC notices that the DNS IP address is in the same subnet as its own IP address; therefore, the PC sends an ARP broadcast hoping to learn the DNS's MAC address.
The DNS replies to the ARP request with its MAC address,
The PC requests name resolution for the Web server by sending a packet with the destination IP address of the DNS.
The DNS returns the IP address of the Web server to the PC in the DNS reply.
The PC does not know the Web server's MAC address, so it sends an ARP broadcast to learn the MAC address. Because it is a MAC broadcast, the switch forwards the frame on all ports.
The Web server replies to the ARP, stating that its MAC address is
The PC can now connect to the Web server.
The two ARP broadcasts (Steps 1 and 5) are sent out all switch ports because switches and bridges do not perform the broadcast firewall function that a router performs. After the switching table (often called the address table) is built, the switch forwards unicasts only out of the appropriate ports. In other words, frames sent from the client to the Web server, and vice versa (which are unicasts), are never sent out port E2.
The switch network has created three separate Ethernet segments, as compared to the transparent bridge network in Figure 4-7, which creates two LAN segments. Each segment is called a collision domain because frames sent by any device on that
DMAC:
SMAC: 4
ARP
DMAC: FFFF.FFFF.FFFF
SMAC: 5
ARP (web)
DMAC:
SMAC: 6
E01
E03
E02

5. Forward/Filter Decisions
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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

6. IST 228\Ch2\Layer2 Switching
Loop Avoidance
Redundant links are use between switches/bridges to increase survivability
Disadvantages:
Broadcast storms
Trashing the MAC table

7. Spanning Tree Protocol (STP)
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

8. Bridge Protocol Data Unit (BPDU) Root Port
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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

9. Spanning-Tree Operations
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Spanning-Tree Operations
Selecting the root bridge
Selecting the designated port
Port States
Blocking
Listening
Learning
Forwarding
Disabled

10. Example: Find The Root Bridge and Root Ports
IST 228\Ch2\Layer2 Switching
Example: Find The Root Bridge and Root Ports