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