MAC addresses on switch ports

Hello all, i have a quick question thats doing my head in!

For example say i have a simple switched network, Switch 1 has PC A and PC B connected to it. When PC A sends a packet to PC B, included in that packet (after the ARP is sorted) is the source mac address of PC A and the dest MAC address of PC B.

The question i have is, why does each port/interface on the switch have its own MAC address? PC A is sending its packet to PC B's MAC address so why does the switch port need its own?

Any help appreciated!!

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mattau

the switch port doesnt have its own i didnt think ? doesnt this happen ?

when the switch first starts up it sends broadcasts to every one connected to the switch asking them what their mac is?

it learns what devices are connected to it and then adopts that devices mac address to the specific switch port, so when the switch receives a packet it knows hey switch port 1 has pc A connected to it lets forward it out that port ?

I am learning to so dont trust what i say is true

billyr

mattau wrote: »

when the switch first starts up it sends broadcasts to every one connected to the switch asking them what their mac is?

Not quite. The mac table is empty on a switch to start with, it fills it up as it receives MAC addresses from the host devices, it doesn't go out to ask for them. P.Cs tend to be quite chatty though so when you first look in the MAC table and you see a bunch of addresses listed it does appear as if the switch has asked for them.

masterk

mattau wrote: »

the switch port doesnt have its own i didnt think ? doesnt this happen ?

when the switch first starts up it sends broadcasts to every one connected to the switch asking them what their mac is?

it learns what devices are connected to it and then adopts that devices mac address to the specific switch port, so when the switch receives a packet it knows hey switch port 1 has pc A connected to it lets forward it out that port ?

I am learning to so dont trust what i say is true

Thanks for the reply, if you log onto a switch and view the ports you can see each port has its own MAC address which increments upwards from the switches core MAC address, i just wondered why each port needed one!

The question is asked here aswell: Mac Address On Switches Ports/interfaces - ProProfs Forums

But the answer doesnt really answer the question as fully as i would like, as the PC isnt sending its frame to the ports MAC address?

wastedtime

masterk wrote: »

Thanks for the reply, if you log onto a switch and view the ports you can see each port has its own MAC address which increments upwards from the switches core MAC address, i just wondered why each port needed one!

The question is asked here aswell: Mac Address On Switches Ports/interfaces - ProProfs Forums

But the answer doesnt really answer the question as fully as i would like, as the PC isnt sending its frame to the ports MAC address?

Not positive on this and I can't test it at the moment, but I believe that those are used on a routed interfaces. A regular layer 2 switch would probably only have 1 for configuration.

Heero

wastedtime wrote: »

Not positive on this and I can't test it at the moment, but I believe that those are used on a routed interfaces. A regular layer 2 switch would probably only have 1 for configuration.

As far as i'm aware this is how it's done. Layer 3 interfaces need MACs, not layer 2 only interfaces.

chmorin

how do you think the switch knows what port to forward packets out of? it needs a unique identifier of some kind, wouldn't it?

hermeszdata

Heero wrote: »

As far as i'm aware this is how it's done. Layer 3 interfaces need MACs, not layer 2 only interfaces.

This is not correct. Interfaces operate on layers 1 and 2. MAC address are a subset of the layer 2 topology to provide physical addressing of devices (Ports). IP addresses are Layer 3 an provide for logical addressing (path determination) within the internetwork.

Going back to each port on a switch having its own mac address. This is standards based. The standard states that every device/port has its own hard coded mac address. This even includes the core cpu of a device (which as an example, MicroSoft uses when registering their OSs). Routers with several serial modules have individual MACs coded into each module as well as the core CPUs MAC.

These are concepts one must completely understand if they are going to pass the tests necessary to gain certification.

Heero

hermeszdata wrote: »

This is not correct. Interfaces operate on layers 1 and 2. MAC address are a subset of the layer 2 topology to provide physical addressing of devices (Ports). IP addresses are Layer 3 an provide for logical addressing (path determination) within the internetwork.

Going back to each port on a switch having its own mac address. This is standards based. The standard states that every device/port has its own hard coded mac address. This even includes the core cpu of a device (which as an example, MicroSoft uses when registering their OSs). Routers with several serial modules have individual MACs coded into each module as well as the core CPUs MAC.

These are concepts one must completely understand if they are going to pass the tests necessary to gain certification.

Tell me why a layer 2 only switch would need to have a mac for each switchport instead of just 1 for the actual switch? I'm thinking about it from the perspective of "how would it work?" and not "this is what they do." When does the switch use the MAC address of the port?

hermeszdata

Heero wrote: »

Tell me why a layer 2 only switch would need to have a mac for each switchport instead of just 1 for the actual switch? I'm thinking about it from the perspective of "how would it work?" and not "this is what they do." When does the switch use the MAC address of the port?

I am going to answer this question with a question!

How does a switch know which port is which when it makes forwarding decisions?

Answer: MAC Addresses

Below is the output from one of my 3550 switches:

SwitchVoIP2#sh mac-address-table
          Mac Address Table
-------------------------------------------
Vlan    Mac Address       Type        Ports
----    -----------       --------    -----
 All    0009.e8b5.2300    STATIC      CPU
 All    0009.e8b5.2301    STATIC      CPU
 All    0009.e8b5.2302    STATIC      CPU
 All    0009.e8b5.2303    STATIC      CPU
 All    0009.e8b5.2304    STATIC      CPU
 All    0009.e8b5.2305    STATIC      CPU
 All    0009.e8b5.2306    STATIC      CPU
 All    0009.e8b5.2307    STATIC      CPU
 All    0009.e8b5.2308    STATIC      CPU
 All    0009.e8b5.2309    STATIC      CPU
 All    0009.e8b5.230a    STATIC      CPU
 All    0009.e8b5.230b    STATIC      CPU
 All    0009.e8b5.230c    STATIC      CPU
 All    0009.e8b5.230d    STATIC      CPU
 All    0009.e8b5.230e    STATIC      CPU
 All    0009.e8b5.230f    STATIC      CPU
 All    0009.e8b5.2310    STATIC      CPU
 All    0009.e8b5.2311    STATIC      CPU
 All    0009.e8b5.2312    STATIC      CPU
 All    0009.e8b5.2313    STATIC      CPU
 All    0009.e8b5.2314    STATIC      CPU
 All    0009.e8b5.2315    STATIC      CPU
 All    0009.e8b5.2316    STATIC      CPU
 All    0009.e8b5.2317    STATIC      CPU
 All    0009.e8b5.2318    STATIC      CPU
 All    0009.e8b5.2319    STATIC      CPU
 All    0009.e8b5.231a    STATIC      CPU
 All    0009.e8b5.231b    STATIC      CPU
 All    0009.e8b5.231c    STATIC      CPU
 All    0009.e8b5.231d    STATIC      CPU
 All    0009.e8b5.231e    STATIC      CPU
 All    0009.e8b5.231f    STATIC      CPU
 All    0009.e8b5.2320    STATIC      CPU
 All    0009.e8b5.2321    STATIC      CPU
 All    0009.e8b5.2322    STATIC      CPU
 All    0009.e8b5.2323    STATIC      CPU
 All    0009.e8b5.2324    STATIC      CPU
 All    0009.e8b5.2325    STATIC      CPU
 All    0009.e8b5.2326    STATIC      CPU
 All    0009.e8b5.2327    STATIC      CPU
 All    0009.e8b5.2328    STATIC      CPU
 All    0009.e8b5.2329    STATIC      CPU
 All    0009.e8b5.232a    STATIC      CPU
 All    0009.e8b5.232b    STATIC      CPU
 All    0009.e8b5.232c    STATIC      CPU
 All    0009.e8b5.232d    STATIC      CPU
 All    0009.e8b5.232e    STATIC      CPU
 All    0009.e8b5.232f    STATIC      CPU
 All    0009.e8b5.2330    STATIC      CPU
 All    0009.e8b5.2331    STATIC      CPU
 All    0009.e8b5.2332    STATIC      CPU
 All    0100.0c00.0000    STATIC      CPU
 All    0100.0ccc.cccc    STATIC      CPU
 All    0100.0ccc.cccd    STATIC     CPU
 All    0100.0ccd.cdce    STATIC      CPU
 All    0180.c200.0000    STATIC      CPU
 All    0180.c200.0001    STATIC      CPU
 All    0180.c200.0002    STATIC      CPU
 All    0180.c200.0003    STATIC      CPU
 All    0180.c200.0004    STATIC      CPU
 All    0180.c200.0005    STATIC      CPU
 All    0180.c200.0006    STATIC      CPU
 All    0180.c200.0007    STATIC      CPU
 All    0180.c200.0008    STATIC      CPU
 All    0180.c200.0009    STATIC      CPU
 All    0180.c200.000a    STATIC      CPU
 All    0180.c200.000b    STATIC      CPU
 All    0180.c200.000c    STATIC      CPU
 All    0180.c200.000d    STATIC      CPU
 All    0180.c200.000e    STATIC      CPU
 All    0180.c200.000f    STATIC      CPU
 All    0180.c200.0010    STATIC      CPU
   1    000d.6517.092c    DYNAMIC     Fa0/48
   1    0014.6988.206c    DYNAMIC     Fa0/2
  25    000d.6517.092c    DYNAMIC     Fa0/48
Total Mac Addresses for this criterion: 75

Note that there are only 3 lines that give a port association. These are the hostes connected to the switch. The majority (72) have the CPU association and are hard coded in the switch's ASIC and the ASIC, which takes care of the forwarding decisions uses the appropriate MAC to forward frames through and get to teh proper switch port.

Heero

Check out this thread.

https://learningnetwork.cisco.com/thread/10473

The following is what i see on my 2960s AND my 3750s with ip unicast-routing disabled.

Switch#show mac-address-table
          Mac Address Table
-------------------------------------------
 
Vlan    Mac Address       Type        Ports
----    -----------       --------    -----
All    0100.0ccc.cccc    STATIC      CPU
All    0100.0ccc.cccd    STATIC      CPU
All    0180.c200.0000    STATIC      CPU
All    0180.c200.0001    STATIC      CPU
All    0180.c200.0002    STATIC      CPU
All    0180.c200.0003    STATIC      CPU
All    0180.c200.0004    STATIC      CPU
All    0180.c200.0005    STATIC      CPU
All    0180.c200.0006    STATIC      CPU
All    0180.c200.0007    STATIC      CPU
All    0180.c200.0008    STATIC      CPU
All    0180.c200.0009    STATIC      CPU
All    0180.c200.000a    STATIC      CPU
All    0180.c200.000b    STATIC      CPU
All    0180.c200.000c    STATIC      CPU
All    0180.c200.000d    STATIC      CPU
All    0180.c200.000e    STATIC      CPU
All    0180.c200.000f    STATIC      CPU
All    0180.c200.0010    STATIC      CPU
All    ffff.ffff.ffff    STATIC      CPU
Total Mac Addresses for this criterion: 20
Switch#

Apparently, they are all for different protocols where the switch has to respond to traffic.

CDP UDLD/DTP/VTP/Pagp= 0100.0ccc.cccc
CGMP = 0100.0ccc.cccd
system-cpp-bpdu-range = 0180.C200.0000 - 0180.C200.000F
GARP = 0180.C200.0020 - 0180.C200.002F

None of the MACs from the interfaces are in the mac-address-table.

I understand that from a programming standpoint, the switch needs some way to identify switchports, but I can't think of a reason it couldn't be an identifier other than a MAC address.

networker050184

The port MAC is used as the source in STP, DTP, CDP etc. I'm not quite sure if they are used in forwarding decisions to identify interfaces though. Never heard of that and never seen any of the interface MACs in the MAC table, but they could be hidden entries or something.

Heero

networker050184 wrote: »

The port MAC is used as the source in STP, DTP, CDP etc. I'm not quite sure if they are used in forwarding decisions to identify interfaces though. Never heard of that and never seen any of the interface MACs in the MAC table, but they could be hidden entries or something.

So then how about a dumb layer 2 switch that doesnt do STP, DTP, CDP, or anything like that. Lets say auto-negotiation (speed/duplex), auto mdi/mdix, and ethernet switching only (like what you would get out of a cheapo unmanaged switch.

networker050184

I have no idea. I could look around on google a bit, but I'm sure you have access to that as well.

Forsaken_GA

I think that this might be being overthunk.

Essentially, any device which has the possibility of originating traffic needs to have a MAC address. Switches can originate traffic. Fairly simple concept.

Now I could see an argument as to whether or not one MAC for the entire switch would do, but usually it's the MAC of the interface it's going out that's used as the source (for example, dual homed server... traffic which originates on eth1 is going to have eth1's mac, not eth0's), so the switch would be following common practice with that as well.

It should be even more obvious in the case of layer 3 capable switches. Since *every* port on the switch has the potential to be turned into a routed link, that would also require each interface to have a MAC

masterk

Forsaken_GA wrote: »

I think that this might be being overthunk.

Essentially, any device which has the possibility of originating traffic needs to have a MAC address. Switches can originate traffic. Fairly simple concept.

Now I could see an argument as to whether or not one MAC for the entire switch would do, but usually it's the MAC of the interface it's going out that's used as the source (for example, dual homed server... traffic which originates on eth1 is going to have eth1's mac, not eth0's), so the switch would be following common practice with that as well.

It should be even more obvious in the case of layer 3 capable switches. Since *every* port on the switch has the potential to be turned into a routed link, that would also require each interface to have a MAC

Ok, so your saying when i send a packet through my switch, the device i am sending to on the same switch see's the source MAC address on that packet as the interface on the switch port that the sender is attached to? Not the sender themselves?

Forsaken_GA

masterk wrote: »

Ok, so your saying when i send a packet through my switch, the device i am sending to on the same switch see's the source MAC address on that packet as the interface on the switch port that the sender is attached to? Not the sender themselves?

No, not if they're on the same network segment. If the switch is in layer 3 mode and the hosts are on different segments, then yes, it would do that, because the switch is functioning as a router at that point, not a switch.

But if the two devices that are communicating with each other are in the same layer 2 domain, then the source and destination macs will be that of the two nodes that are communicating.

What I am saying is that if the switch itself originates traffic, whether it be for STP, CDP, IGMP, or whatever, it'll be using it's own MAC address.