Initial mac address table of L2 switches

[ragulkarthick]

According to the textbooks, I learned that the MAC address table of a switch is initially empty. However, when I deployed a topology in Cisco Packet Tracer, I noticed that there were already entries in the MAC address table even before I sent any ping requests.

It appeared as if the switch already knew the MAC addresses of directly connected interfaces — not the global MAC addresses, but the MAC addresses of the adjacent interfaces.

For example:

H1: fa0/1 ↔ SW1: fa0/1

SW1: fa0/2 ↔ SW2: fa0/2

In the MAC address table of Switch1, there were already entries for the MAC address of H1 (fa0/1) as well as the MAC address of SW2 (fa0/2).

I thought i would have empty mac address in each switch can anyone please help me with this. am i missing any other theory concepts in it

[M02@rt37 · Accepted solution]

Hello @ragulkarthick 

First, switches are passive learners.

Even before you initiate traffic like a ping, a switch can learn MAC adresses from various background or control-plane traffic, such as cdp/lldp, stp, gARP and dhcp discover/request. 

These types of frames include valid source MAC addresses, so the switch learns those MAC as soon as it receive the frame...even before you manualy send traffic like ping or arp.