Catalyst 2950 Questions

jt003649

Hi Folks,

I've some questions regarding the 2950 switches I'm configuring as part of my CCNA lab activites.

1) Why are there two sperate vty statements in the configs (vty 0 4 and also vty 5 15)

2)When I run a "show mac-address-table" it shows me that there are 47 System Self Address Counts. Why does the switch have 47 MAC addresses by default? I'd understand if there were 24 as there would be one for each port.

Cheers.

Danman32

Show mac not only would show you the statistics, but the mapping as well. Do you see 47 entries? If so, what ports do the MACs point to?

david_r

This is what he's talking about (off a 2912 EN)

Switch>en
Switch#show mac-
Switch#show mac-address-table
Dynamic Address Count: 0
Secure Address Count: 0
Static Address (User-defined) Count: 0
System Self Address Count: 35
Total MAC addresses: 35
Maximum MAC addresses: 2048

Danman32

Yes, followed by the table map.

Danman32

Tough to find anything on self addresses. However, show mac static will show CPU MACs. So there seems to be some hidden MACs in the switch for internal use.

Based on http://puck.nether.net/lists/cisco-nsp/0232.html

Try Show Mac Self

Danman32

Reading further, found the following info:

Table 1-3 lists and describes the commands that are not supported in Cisco IOS Release 12.1(6)EA2 or later. These commands are supported only in software releases earlier than Cisco IOS Release 12.1(6)EA2. If you are running Cisco IOS Release 12.1(6)EA2 or later, the switch supports the commands listed in Table 1-3 only if they are in a saved configuration file.

Table 1-3 Commands Not Supported in Cisco IOS Release 12.1(6)EA2 or Later:

...
show mac-address-table self
Displays the addresses added by the switch itself to the MAC address table.
...

jt003649

Thanks for the responses. So what exactly are the CPU Ports on the Mac Address table?

Danman32

Couldn't say. Cisco seems to be mum about even the self assigning macs. May be internal proprietary functions.

lwwarner

jt003649 wrote:

Thanks for the responses. So what exactly are the CPU Ports on the Mac Address table?

SW1#show mac-address-table static 
          Mac Address Table
-------------------------------------------

Vlan    Mac Address       Type        Ports
----    -----------       --------    -----
 All    000d.284a.6680    STATIC      CPU
 All    0100.0ccc.cccc    STATIC      CPU
 All    0100.0ccc.cccd    STATIC      CPU
 All    0100.0cdd.dddd    STATIC      CPU
Total Mac Addresses for this criterion: 4

The first "CPU" address is the base MAC address of the switch. The others are multicast addresses used by various protocols such as CDP, VTP, etc. Which address is used by which protocol(s) is left as an exercise for the reader.

mp3spy

Nice

john_linuxuser

jt003649 wrote:

Hi Folks,

I've some questions regarding the 2950 switches I'm configuring as part of my CCNA lab activites.

1) Why are there two sperate vty statements in the configs (vty 0 4 and also vty 5 15)

2)When I run a "show mac-address-table" it shows me that there are 47 System Self Address Counts. Why does the switch have 47 MAC addresses by default? I'd understand if there were 24 as there would be one for each port.

Cheers.

1) Some routers/switches don't support vty 5 to 15, any of their code can shovel the first range into their facility. The second "line vty" command will bounce back right away and waste all the commands inside. My guess.

2) This is extremely extremely disturbing to me. I am going to look at it on a pair of 3550s tonight. Good question and extremely disturbing. The internal MAC addresses are hard wired to the switch's ports and will not be associated with a port as an outside MAC address. I am so troubled right now. Why do they call it CPU? I looked up their command reference "show mac address-table". Didn't say any thing there.

We all know that sometimes we let the traffic go out of the swith, say via port 14, and let it be routed back into our own switch, say port 13 , mac 3333.3333.3333, with an IP address. Now the IP traffic is destined to 3333.3333.3333 when it arrives at our switch, say port 12. This packet is not going to be sent out of port 14. Because 3333.3333.3333 is not associated with port 14. In fact, traffic destined to 3333.3333.3333 will be terminated at our switch. And because it arrives at port 12, a switching port. The switch process will try to look for the ethernet protocol of the traffic, say BPDU. And because the traffic is IP, the switch process will ignore it!

I can't ping, I can't ping, I can't ping. I can't ping. I can't ping. I can't ping. I can't ping. I can't ping.


John

lwwarner

john_linuxuser wrote:

2) This is extremely extremely disturbing to me.

Why is this disturbing?? As I noted above these are mostly just multicast MAC addresses which the switch uses for various Cisco-specific protocols. (Hint: 0000.0cxx.xxxx == Cisco-specific MAC addr & 0100.0cxx.xxxx == Cisco-specific multicast MAC addr) They are labeled "CPU" simply because they are not tied to any individual port, but are processed internally by the switch for all ports. Why "CPU"? Because someone at Cisco thought that was a good way to designate such things...

john_linuxuser wrote:

I can't ping, I can't ping, I can't ping. I can't ping. I can't ping. I can't ping. I can't ping. I can't ping.

What in the world are you jabbering about?