Networks

Ask something
Discussion Locked

Two Systems with same MAC address

0 helpful
13 replies

by kr_kirru

Closed to new repliesThis discussion is closed. It stays readable, but it takes no new replies.

Hi,
When two systems A & B are having different ip address and same MAC address(suppose i have changed that on linux machine) then how the third system C can ping the exact system(assume it as A)?

How the switch deside the exact system and sends the icmp packet to that perticular port?

13 replies

by sgt_shultz

you can't change the hardware mac address even with mighty linux. so computer is working normally is my guess. besides, you have unique ip address, yes?

by cg it

it is my understanding that NIC cards CAN NOT have the same MAC address, period. Now some routers allow you to change their MAC address to the computer that is connecting to it MAC address but no 2 NICs can have the same one. One would have to reprogram the NIC which isn't an easy task.

Therefore unless you know something that your not telling us your question moot.

by pgm554

Switches work at layer 2 (MAC address).

IP works at layer 3.

When you ping, you are working at the layer 3 level and IP routing doesn't care about the MAC address.

Will having the same MAC address affect the switches speed? Maybe.

Built into every NIC card, according to RFC spec, is a burnt in MAC address, that can't be changed physically.

However,you can change them logically, depending on the OS.

by kr_kirru

We can changethe MAC address if it comprises of EPROM. So rule out the issue that we cant change the MAC address. Try to figure out the exact answer. How does the switch takes decision when it encounters such a situation?

by pgm554

Most switches work at layer 2 of the OSI model.

Some of the newer ones work at layer 3(vlan).
Store and forward switches build a MAC table to figure out how to forward traffic based on MAC address.

As I pointed out before, when you ping using ICMP packets, this is an OSI layer 3 function of which the switch know nothing about and hence, ignores.

When you have the same 2 MAC addresses on a switch, you use lose the advantage of the software forwarding (layer 2) that is built into the switch.

So what the switch becomes is a hub, which means that it is now broadcasting to all nodes that are connected to that collision domain that have the same MAC address.

You now have the same issues as if it were a dumb hub, which are collisions.

by pgm554

Here is an example:
EX 1) You have a switch with 8 nodes attached and all unique MAC addresses. No collisions.

EX 2) You have a switch with 8 nodes attached and if you change MAC addresses one at a time to be the same, you will encounter collisions and slow down the network to the point where you now have little or no speed advantage over a hub.

What you doing by using duplicate MAC addresses, is to defeat the store and forward software built into the switch. It is now broadcasting dumbly, instead of unicasting smartly.

This discussion is closed to new replies.