Genzis - Blog

Home

Blog

Protocols

RFC 826 - part 2: How switches manage their MAC address table

RFC 826 - part 2: How switches manage their MAC address table

post-details

In a previous article, we explored the ARP protocol and how it worked on endpoints. In this article, we will be analyzing ARP design and behavior on L2 switches. 

 

Facts about switches

 

Before we dive into switch ARP, we should note the following:

  • Each switch interface has its own MAC address
  • Unlike computers and other L3 devices, L2 switches do not store MAC addresses in an ARP table, but rather in a MAC Address Table. (simply because L2 devices cannot store L3 information)

 

On Packet Tracer, we defined the following network. 

 

 

We want to find out how the switch discovers and stores the MAC addresses of each host. And what MAC addresses are stored on the host. 

 

On host a, we send an ICMP echo request to host B. the switch receives the requests and makes a broadcast to all the network devices. In fact, the switch has no clue at this point who is connected to its interfaces. 

 

At this point, the switch stores the Host A Mac address in its MAC Address Table. 

 

Note: Switch MAC address Table can be checked with the following command: 

switch# show mac-address-table  

 

All the hosts receive the frame and check the requested IP. Host C and Host D drop the frame while Host B sends an ARP reply frame to Host A.

 

At this point, the switch stores the Host A Mac address in its MAC Address Table.

 

Host B and Host A updated their ARP tables with each other's MAC Addresses and can now communicate without any more broadcasting. 

 

This is what the switch ARP table looks like now. 

 

Vlan Mac Address Type Ports
---- ----------- -------- -----
1 0001.966a.e016 DYNAMIC Fa0/1
1 0001.c9b8.b8c2 DYNAMIC Fa0/2
Switch#



Note: on the switch, you will notice the Spanning Tree Protocol in action. Spanning Tree Protocol (STP) is a Layer 2 network protocol used to prevent looping within a network topology. 

 

On the packet tracer simulation mode, SCP frames are broadcasted every 10-15s. If the service is disabled, the Switch will not be able to maintain its MAC Address table.  





 


Don’t Forget to Share the Article

All Rights Reserved.

arrow

Back to the top