How many MACs per L3 switch?

[Bojan Zivancevic]

If a L3 switch has, say, two routing ports and the rest of the ports are divided into two vlans, will this switch have four MAC addresses, as appeared to the rest of the world? My guess is four, because I think that is the only way it could properly communicate on the L2 with the other L2 guys on the network.

Am I right?

[Kevin Dorrell]

If a station A is talking to station B through a L2 switch, then he addresses the frame to the so-called "MAC address" of station B, not the MAC address of the switch. If he is talking L2, he doesn't know anything about the MAC address of the switch. He only needs a MAC address for the switch if he is using it as a L3 router.

Kevin Dorrell

Luxembourg

[Bojan Zivancevic]

Yes, you are right. I wasn't correctly explaining my thoughts. But every switch, even a plain old L2 switch, has a MAC address, right? So, I guess every SVI should have it's own MAC.

For example, if you want to remotely administer your switch, you need its MAC address. In the example I mentioned you don't need to know MACs of the SVI interfaces because you have routing ports, of course.

[frenzeus]

Hi,

Each port on the switch has it's own MAC address. If u configured ur L3 switch for say another additional 2 SVIs, then those 2 will also have MAC addresses assigned to it, 1 for each SVI.

by placing your ports on the switch into different vlans, this does not have any direct effect on MAC addresses assigned to it. Like i said earlier, by default, all ports on ur switch, be it L3/L2, they will have their very own unique MAC address.

HTH!

Cheers,

kenny

[Bojan Zivancevic]

How does the switch assign these new MACs? Are they in any way similar to port MACs?

[frenzeus]

Hi,

my mistake. HSRP addresses have MAC addresses assigned to it, generated according to the vlan number. As for SVIs, all of it uses the same MAC address, which is the burnt-in-address on the PFC/MSFC. Meaning, interface vlan 10 will have the same MAC address as interface vlan 20.

[Bojan Zivancevic]

Hmm. Tell me then, if the SVI MACs are all the same, how can one gain access to the switch from various vlans? For example, I have two SVIs with two IP addresses. I need two MACs, right? Or maybe the same MAC will be used in both vlans?

Come to think of it, vlans are separate networks, why shouldn't there be a same MAC for both SVIs?

[frenzeus]

when any host on a network needs to cross vlans, they will contact their default gateway. So, when the host has the right MAC address of the gateway (from arp or host cache), the mac of the gateway will be appended to the packets' destination address. Switches will then check on their cam table for an entry for this gateway mac address and will switch on until it reaches the msfc card with the svi gateway. so, since it uses mac address, doesnt really matter if they're from different vlans, they still point back to the very same place, which is the msfc card!

[Bojan Zivancevic]

Of course. Why would anybody give a dam - the boxes only want to make sure that the frame arrives on the right place. :)

Thanks a lot. You've been patient and helpful!

[frenzeus]

no prob, would appreciate u give appropriate rating if u feel that i have answered ur questions! thanks.