MAC Address and Logical Router Interface

MAC Address and Logical Router Interface
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MAC Address and Logical Router Interface Graham Drabble 11-05-2008
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Posted by Graham Drabble on November 5, 2008, 5:59 pm
Hi,

Do all the logical interfaces on a physical Ether interface have the
same MAC address? If not how is the MAC address for eahc logical
interface defined?

Thanks,
--
Graham Drabble
http://www.drabble.me.uk/

Posted by bod43 on November 5, 2008, 9:21 pm
> Hi,
> Do all the logical interfaces on a physical Ether interface have the
> same MAC address? If not how is the MAC address for eahc logical
> interface defined?

Well, why don't you configure it and have a look?
I suppose you are asking a theoretical question?

Usually it will not matter of they are the same. The different
VLANs do not usually end up being connected so no
conflict occurs.

It is of course possible that some requirement
might result in the vlans being connected by a
cable somewhere in the network. This could
result in communication failures.

One related thing is that by default Sun workstations
use the same MAC address for all of the interfaces
in the device. This usually works OK since the
different interfaces are not usually in the same
broadcast domain.

Posted by Graham Drabble on November 6, 2008, 12:26 am
3517-4f71-b24c-a86290484222@a3g2000prm.googlegroups.com:

>> Hi,
>> Do all the logical interfaces on a physical Ether interface have
>> the same MAC address? If not how is the MAC address for eahc
>> logical interface defined?
>
> Well, why don't you configure it and have a look?

Because I don't have any CISCO kit available at the moment. I take it
from the rest of your answer the short answer is Yes.

> I suppose you are asking a theoretical question?

Yes. I have a piece of equipment that has 2 physical ports with
addresses in different subnets. They share a MAC but any broadcast
traffic (STP, ARP etc) are always sent out of both ports. I'm trying
to find out how other equipment handles having 2 IP with 1 MAC handle
this, only thing I could think of was logical interfaces.

> One related thing is that by default Sun workstations
> use the same MAC address for all of the interfaces
> in the device. This usually works OK since the
> different interfaces are not usually in the same
> broadcast domain.

Thanks.

--
Graham Drabble
http://www.drabble.me.uk/

Posted by Stephen on November 7, 2008, 4:07 pm
On Wed, 05 Nov 2008 22:59:35 GMT, Graham Drabble

>Hi,
>Do all the logical interfaces on a physical Ether interface have the
>same MAC address? If not how is the MAC address for eahc logical
>interface defined?

it depends :)

the hardware can support different MAC addresses on the same interface
(at least since the 25xx routers).

this used to be common - DECnet forced the MAC address to include the
L3 address, XNS forced all ports to the same MAC and so on.

Different MACs are used with HSRP / VRRP by the current "owner" of a
shared gateway address, for different groups and for the native
address.

i think if you have multiple IP addresses on a port, it will use the
same MAC - but it is so long since i tried i dont remember.

on cisco switches each vlan interfaces gets a separate MAC, so you
will different MACs per IP interface in each VLAN on a trunked
interface.
>Thanks,
--
Regards

stephen_hope@xyzworld.com - replace xyz with ntl

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