Cisco 3560, VLANs

Unanswered Question
Jul 16th, 2008

Good morning,

I will first preface this by saying that my Cisco skills are a bit rusty. While I have a CCNA, that was earned about three years ago and I've only had to use it sparingly since then; a testament to the quality of the product I suppose.

Anyway, here's my issue. At my new job we recently installed a Cisco 3560 to replace some Netgears that were on the network (don't ask why those were there, I really don't know). On the Netgears there were two VLANs setup. One was for the subnet with all the company's core servers (172.20.x.x/16) while the other went to a subnet dedicated to an EMC Celerra SAN system (172.21.10.x/24).

The previous Netgears connected into two different Extreme 24 port switches via 1 gigabit copper ports. The Cisco 3560 we put in their place connect back to one of the Extreme switches. It connects via two copper gigabit connections, one which is on the VLAN for the servers, the other for the VLAN of the EMC device.

I'll include some of the config here.

int g0/1

swithcport access vlan 51

switchport mode dot1q-tunnel

This gigabit port connects back to the Extreme switch which then connects to other switches


switchport access vlan 20

switchport access dot1q-tunnel

This one also connects back to the same Extreme switch as g0/1

Interface VLAN1

No ip address

Interface VLAN20

IP address

Interface VLAN51

ip address

While the switching is working between all of the devices (hooray!) there are some other issues that have cropped up that I'm uncertain how to handle.

1. I can not seem to telnet into the device. I feel pretty confident that the IP address I gave the two VLANs were not in use but pinging and telnetting to it does not work.

2. On the EMC Celerra device there are actually 4 NICs going into the switch. on the ports those NICS are connected to, I received notifications that a MAC address on g0/3 and g0/4 is "flapping" while the same is being shown on g0/13 and g0/14. Performance doesn't seem to be hurting, but this notification keeps popping up on the switch when I'm plugged in via the console port.

Any help or suggestions would be greatly appreciated. Thanks!


I have this problem too.
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a.alekseev Wed, 07/16/2008 - 05:04

conf t

vlan 20

vlan 51

int g0/1

no swithcport access vlan 51

no switchport mode dot1q-tunnel

sw tr enc dot1q

sw mo tr

int g0/24

no swithcport access vlan 51

no switchport mode dot1q-tunnel

sw tr enc dot1q

sw mo tr

Interface VLAN20

no sh

Interface VLAN51

no sh


jlf000175 Wed, 07/16/2008 - 05:08

So, if I understand this correctly:

1. Enter config mode

2. Create the VLANs again?

3. On the g0/1 interface turn off the settings I currently have and setup trunking instead.

4. Do the same on the g0/24

5. Do a no shutdown on both VLANs.

a.alekseev Wed, 07/16/2008 - 05:16

You understand correctly.

2. if you already have vlan20 and vlan51 in the vlan database, you may skip the step.


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