Learning Switches (Basic)

Tags: #◇CSNETWK #◇Computer Science #◇Computer Networks #◇Networking Protocols #◇Graph Theory Basics

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◇Overview

A switch is "transparent." It doesn't have a hardcoded map. It learns by listening.

◇Key Concepts

If Known If Unknown Switch 1 (Right) Switch 2 (Left) Inter-Switch Link

◇Details / Theory

◇1. Concept Overview: Self-Learning Switches

Definition

A switch is "transparent." It doesn't have a hardcoded map. It learns by listening.

The Algorithm

1. Listen (Learn): When a frame arrives on Interface $X$ with Source MAC $A$, the switch writes in its table: (MAC A $\rightarrow$ Interface X).
2. Forward (Act): The switch looks at the Destination MAC.
   If Known: Send only to the specific interface found in the table.
   If Unknown: Flood the frame out to all interfaces (except the incoming one).

◇2. The Network Topology

Connections
!◇Pasted image 20251121094303.png
Switch 1 (Right): Connects computers A-F and link to Switch 2. Switch 2 (Left): Connects computers G-L and link to Switch 1. Inter-Switch Link: Switch 1 (Int 7) $\leftrightarrow$ Switch 2 (Int 8).

◇3. Problem Analysis (Step-by-Step)

Time $t=1$: $K \rightarrow L$

Action: Node K sends to Node L. Switch 2 (Left): Receives frame from K on Int 14.
Learn: Records (K, 14).
Forward: Does it know L? No (Table is empty). Floods to everyone, including Switch 1 via Int 8. Switch 1 (Right): Receives flooded frame from Switch 2 (Source K) on Int 7.
Learn: Records (K, 7). (Note: Switch 1 thinks K is "out that way" towards Int 7).

Table Entries Added:
Switch 1: (K, 7) Switch 2: (K, 14)

Time $t=2$: $B \rightarrow I$

Action: Node B sends to Node I. Switch 1 (Right): Receives from B on Int 2.
Learn: Records (B, 2).
Forward: Does it know I? No. Floods. Sends to Switch 2 via Int 7. Switch 2 (Left): Receives from Switch 1 (Source B) on Int 8.
Learn: Records (B, 8).
Forward: Does it know I? No. Floods.

Table Entries Added:
Switch 1: (B, 2) Switch 2: (B, 8)

Time $t=3$: $F \rightarrow E$

Action: Node F sends to Node E. Switch 1 (Right): Receives from F on Int 6.
Learn: Records (F, 6).
Forward: Does it know E? No. Floods. Sends to Switch 2 via Int 7. Switch 2 (Left): Receives from Switch 1 (Source F) on Int 8.
Learn: Records (F, 8).

Table Entries Added:
Switch 1: (F, 6) Switch 2: (F, 8)

Time $t=4$: $G \rightarrow B$

Action: Node G sends to Node B. Switch 2 (Left): Receives from G on Int 10.
Learn: Records (G, 10).
Forward: Does it know B? YES! (Learned at $t=2$). Table says B is on Int 8.
Selective Forward: Sends only to Int 8 (Switch 1). Switch 1 (Right): Receives from Switch 2 (Source G) on Int 7.
Learn: Records (G, 7).
Forward: Does it know B? YES! (Learned at $t=2$). Sends to Int 2.

Table Entries Added:
Switch 1: (G, 7) Switch 2: (G, 10)
(Note: No entry for B is added because it already exists).

◇Summary of Logic (Cheat Sheet)

1. Source Check: Always add the Source MAC and the Incoming Interface to the table.

2. Dest Check: Look for Destination MAC.
   Found? $\rightarrow$ Send to specific port.
   Not Found? $\rightarrow$ Flood (send to all).

3. Inter-Switch Rule: If a packet comes from another switch, the "Interface" recorded is the link connecting the switches.

◇Common Pitfalls

Assuming a switch knows all MAC addresses from the start — switches start with empty tables and learn incrementally. Confusing flooding (sending to all ports except the source) with forwarding (sending to a specific known port). Forgetting that the switch records the source MAC and the incoming interface, not the destination.

◇Summary

TL;DR — Self-learning switches build MAC-to-port tables by recording source addresses of incoming frames. Known destinations are forwarded to specific ports; unknown destinations are flooded to all ports.

◇Links

Related: - ◇Pasted image 20251121094303.png Source / Reference:

◇References & Navigation

• Previous Topic: ◇Addressing and Forwarding
• Next Topic: ◇Learning Switches (Advanced)
• Related Notes: ◇Internetworking