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MAC address learning of mulitcast/broadcast

When I got prepared for my CCNA, I read that switches would never learn multicast or broadcast addresses, because they could never be the source MAC address of a frame. Although that seems logical, I don't think that's the whole truth.

Now, my PC's LAN driver won't let me change its MAC address when the new one I try to give it has its I/G bit set.

But if I were to use some rogue utility that actually allowed me to send frames with a multicast address as a source, would the switch actually learn it or does it have some sort of verification mechanism that checks wheter or not the source MAC address is really unicast?

Unfortunately, I couldn't find any documentation on this.

Thanks,

Patrick

P.S. Setting a multicast address statically doesn't seem to bother him, but he won't accept the broadcast address!

C3560-LAB_SW1(config)#mac address-table static 0100.5e00.000a vlan 1 interface FastEthernet 0/1
C3560-LAB_SW1(config)#mac address-table static ffff.ffff.ffff vlan 1 interface FastEthernet 0/1
%Cannot add this address

1 REPLY
Cisco Employee

Hi, As per your output on

Hi,

 

As per your output on earlier switches there was no check to configure static mac entry for multicast mac. I checked on 7600 as well and able to configure static mac-address table entry for multicast mac. 

but i see in Nexus platform there is check

 

R2_7606B(config)#$-table static 0100.5e00.000a vlan 1 interface gig1/1       
R2_7606B(config)#

 

http://www.cisco.com/c/en/us/td/docs/switches/datacenter/nexus1000/sw/4_0_4_s_v_1_2/layer_2/configuration/guide/n1000v_l2/n1000v_l2_2mac.html

"You can configure MAC addresses, which are called static MAC addresses, to statically point to specified interfaces on the device. These static MAC addresses override any dynamically learned MAC addresses on those interfaces. You cannot configure broadcast or multicast addresses as static MAC addresses. The static MAC entries are retained across reboots."

 

 However on switches it does not allow to change mac address of an interface to multicast address

 

R1_7606A(config-if)#mac-address 0100.5e00.000a 
% Malformed hex mac address
R1_7606A(config-if)#mac-address 44d3.ca51.6d50
R1_7606A(config-if)#

 

--Pls dont forget to rate helpful posts--

Regards,

Akash

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