multiple MACs on a port

multiple MACs on a port
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Subject Author Date
multiple MACs on a port matthias 04-27-2007
Posted by matthias on April 27, 2007, 7:53 am
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is it allowed by ethernet-standards that a switch has multiple (actually two)
MACs on the same port? the port is part of a VLAN with a L3-Interface i.e. an
IP-Address assigned. I am seeing the following effect:

When an IP-packet is send to and through the switch it answers arp-requests
with its first MAC-address, the answer packet however comes from the second
MAC-address back to the client.

AFAIK it doesn't matter from wchich MAC an IP-packet is returned, as the
IP-layer doesn't care.

(btw. its an Extreme Summit 48si Switch configured for soft rate limiting and
Extreme said its a feature of the chipset)

Pure Networks
Posted by Robert Redelmeier on April 27, 2007, 9:17 am
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> is it allowed by ethernet-standards that a switch has
> multiple (actually two) MACs on the same port?

Certainly should. switches were originally very expensive and
used for network segmentation. Hubs were hung off their ports.

It depends on the sophistication of the switch electronics how
many MACs it can search before resorting to hub behaviour --
pushing the frame out on all ports.

A switch should not alter sending or receiving MACs. A bridge
might and a router will. A proper router has one MAC per port.
Many SoHo routers are really a dual ported router (still more
than a bridge) with an integral switch attached to the LAN side.

-- Robert


Posted by Al Dykes on April 27, 2007, 9:22 am
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>is it allowed by ethernet-standards that a switch has multiple (actually two)
>MACs on the same port? the port is part of a VLAN with a L3-Interface i.e. an
>IP-Address assigned. I am seeing the following effect:
>
>When an IP-packet is send to and through the switch it answers arp-requests
>with its first MAC-address, the answer packet however comes from the second
>MAC-address back to the client.
>
>AFAIK it doesn't matter from wchich MAC an IP-packet is returned, as the
>IP-layer doesn't care.
>
>(btw. its an Extreme Summit 48si Switch configured for soft rate limiting and
>Extreme said its a feature of the chipset)


A network transparent bridge is capable of generating packets that
have whatever MAC address is needed forward each packet. A PC with
two ethernet cards and running Linux can be set up to do this.

--
a d y k e s @ p a n i x . c o m
Don't blame me. I voted for Gore. A Proud signature since 2001

Posted by stephen on April 28, 2007, 7:15 am
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> is it allowed by ethernet-standards that a switch has multiple (actually
two)
> MACs on the same port? the port is part of a VLAN with a L3-Interface i.e.
an
> IP-Address assigned. I am seeing the following effect:

the switch is transparent - so the MAC may be tied to a particular port or
not - if the packet can go between ports internally, then how could you tell
by observing traffic?

lots of bigger switches have a block of addresses to use - 256 on some
Nortel gear i worked on and more on some others.

They tend to get assigned to logical functions, and depending on the switch
there may be more than 1 associated with a port. eg maybe 1 for the "bridge"
MAC allocated to a port, but a different MAC used for L3 on that port. The
Nortel Passport 8600 model is sort of a bridging function and then a logical
router function behind that, and the MACs seem to correspond to those
logical functions.

The standards seem to be much more about how MACs behave and dont really
care.
>
> When an IP-packet is send to and through the switch it answers
arp-requests
> with its first MAC-address, the answer packet however comes from the
second
> MAC-address back to the client.
>
> AFAIK it doesn't matter from wchich MAC an IP-packet is returned, as the
> IP-layer doesn't care.

The switch logic often uses the MAC to decide whether a packet should be
routed or not - if the dest MAC is the logical function for the IP routing
engine on that port then it gets routed, otherwise it is handled
differently.
>
> (btw. its an Extreme Summit 48si Switch configured for soft rate limiting
and
> Extreme said its a feature of the chipset)
--
Regards

stephen_hope@xyzworld.com - replace xyz with ntl



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