No Mac Address on Switch port

[nygenxny123]

Is it possible for a server not to advertise its mac address?

The reason I ask is that we have

a tumbleweed server attatched to our

switch..

The gig port shows up up

however no mac-address is being displayed

when issue the command sh-mac-address int gig 0/3

GigabitEthernet0/3 is up, line protocol is up (connected)

Hardware is Gigabit Ethernet, address is 0018.b985.c403 (bia 0018.b985.c403)

Description: "connection to tumbleweed"

MTU 1500 bytes, BW 100000 Kbit, DLY 100 usec,

reliability 255/255, txload 1/255, rxload 1/255

Encapsulation ARPA, loopback not set

Keepalive set (10 sec)

Full-duplex, 100Mb/s, media type is 10/100/1000BaseTX

MC5.1#sh mac-address-table interface gigabitEthernet 0/3

Mac Address Table

-------------------------------------------

Vlan Mac Address Type Ports

---- ----------- -------- -----

MC5.1#

At this point..we can not ping the server..but i can ping the gateway of the server from the switch

[John Blakley]

This may be a stupid question, but do you have an IP address on the server? Can you ping it from the switch and then do your "sh mac add int g0/3?"

HTH,

John

[nygenxny123]

yep..we have the ip configured on the server..had them verify configuration

no ping.

[John Blakley]

You can't ping the server from the switch? Can you ping the server from anything? If not, you won't get an address in your mac table.

HTH,

John

[nygenxny123]

hmm i should be able to get at least a mac-address on the switch..its a layer 2 function..

i am not even getting the mac address

[John Blakley]

I don't believe your mac address is going to show up in the cam table just because you connect the cable. There needs to be some sort of traffic going to that device, and then it will be added after the address is arp'd for. I could be wrong, but I would fix the "not being able to ping" problem, and your other problem should fix itself.

HTH,

John

[milan.kulik]

Hi,

the switch is learning the MAC address from frames received on the port.

And there is 5 minutes age timer.

So if your server was not sending any frame within last 5 minutes, you will not see any MAC address on the port.

I'd try to force the server to send some broadcast (Ping form the server should send an ARP request, e.g.).

If doesn't work, I'd suggest checking duplex/speed on your server NIC, cable (might be a cross) and the NIC finally.

BR,

Milan

[Richard Burts]

Richard

John got it slightly wrong and Milan provides the correction. The switch does not learn the MAC address when something goes to the device. The switch learns the MAC address when something comes from the device.

Milan's advice about trying to get the server to send something is good. If some device in the same VLAN (and same subnet) sends an ARP request (which it should do if you enter the ping command to the server on that device) then the server should respond to the ARP and the switch should learn the MAC address.

If you try the ping and the switch is still not learning the MAC address of the server then I would suggest that you look for why the server is not responding to the ARP request. You should check carefully the configuration of the server: make sure that the IP address is correctly in the subnet and make sure that the mask of the server matches the mask for the subnet.

HTH

Rick

[John Blakley]

This may be a stupid question, but do you have an IP address on the server? Can you ping it from the switch and then do your "sh mac add int g0/3?"

My original post suggested that the device needs to have some sort of L3 connectivity in order to show up in the table. True that it will learn the address when it "comes from the device", but when the address is arp'd for, it floods the switch to see who owns it. When it gets a response, it will add it to the mac table. Therefore, you're sending something TO the device, and the device then sends something in return having the switch learn the mac address. I wouldn't say that I was slightly wrong; it's all how you look at it.

Either way, without pointing fingers, the device needs to be able to communicate with other devices for the switch to add it to the table.

John

[Richard Burts]

John

I do not want to be overly picky - but the switch learns nothing when it sends. It only learns when the device sends.

Your position is basically that if you send an ARP then the device should respond and the switch will learn the MAC. But the switch does not learn anything from sending, it only learns when the device sends. So from my perspective to say that the switch learns when you send is slightly wrong (not entirely wrong but not quite right).

HTH

Rick

[John Blakley]

Rick,

I understand where you are coming from, but it almost sounds like you're saying:

If I connect a new system into the switch, and I ping that new system from another device that is connected into the switch, I'll never see that new system unless I go to the new system and ping from it outbound. That's not the case.

The switch will then arp for the device, and the device will respond to the system that is requesting the response. I'm not seeing where I'm incorrect in this, other than maybe we are explaining this two different ways and coming to the same conclusion.

John

[Richard Burts]

John

Tell me how the switch learns when you send the ARP request. Do not tell me how it learns from the ARP response, but tell me how it learns while it is sending the request.

It is a fundamental concept of layer 2 switching that the switch looks at incoming frames, examines the source MAC address, and places the source MAC address in the MAC address table (or CAM) associated with the incoming port. There is no other way that the switch learns MAC addresses. To say that the switch learns when you send the ARP request is to mis-represent this concept.

HTH

Rick

[John Blakley]

You're correct, BUT you leave out the part that when the switch receives a frame for an unknown destination, it floods out all ports to find the destination. When the destination responds, it adds the learned mac address to the table creating communication between those two hosts.

Again, the server that he's having problems with does NOT have to be the one to send traffic for the mac address to be learned. It will be learned as long as L3 is working.

HTH,

John

[Richard Burts]

John

Flooding to an unknown destination has nothing to do with how the switch learns MAC addresses. It may help with how we get the server to send something. But flooding to unknown destination does not, by itself, help the switch learn the MAC address.

And you paint yourself further into a corner when you say:"the server that he's having problems with does NOT have to be the one to send traffic for the mac address to be learned". In fact the ONLY way the switch will ever learn the MAC address is for the server to send something.

You seem to be starting from the point of how can we get the server to send something (respond to ARP) and then confusing how we ask the server to do something with what happens IF the server does respond.

[edit] I am not saying that the server has to be the one that first initiates traffic. But I am saying that the only way that the switch will learn the MAC address is when the server DOES send something. It does not matter at all what we send to the server. The only way the switch learns the MAC address is when the server sends something to the switch.

HTH

Rick

[John Blakley]

Rick

We're talking about exactly the same thing, but we're not conveying it the same way.

Here's a link that I found (because I was questioning what I was saying):

http://www.cisco.com/en/US/docs/switches/lan/catalyst6500/ios/12.2SX/configuration/guide/layer2.html#wp1020399')">http://www.cisco.com/en/US/docs/switches/lan/catalyst6500/ios/12.2SX/configuration/guide/layer2.html#wp1020399

So yes, the switch learns from the source address, but in one of my posts above I did say that they should be able to ping from another server and, when the server replies, the mac address would be added, therefore fixing the problem with the missing mac address on that port. They still need to get the L3 fixed.

Have a good day,

John