VLAN trunk port flapping

[ajimmor]

Hello,

Im trying to connect 2 hosts via a Cisco Catalyst 2960 switch. I want to send traffic between both hosts in 3 different VLANs (10,11 and 12). Both hosts are in the same subnet (172.16.10.0/24). I configured the switch as follows:

-Host 1:

configure terminal

interface Fa0/1

switchport mode trunk

switchport trunk allowed vlan 1,10,11,12

end

- Host 2:

configure terminal

interface Fa0/2

switchport mode trunk

switchport trunk allowed vlan 1,10,11,12

end

When pinging from host 1 (172.16.10.10) to host 2 (172.16.10.20), i get the following log error: 

*Mar 1 00:18:12.314: %SW_MATM-4-MACFLAP_NOTIF: Host 082e.5f2f.70e2 in vlan 11 is flapping between port Fa0/1 and port Fa0/2
*Mar 1 00:18:12.314: %SW_MATM-4-MACFLAP_NOTIF: Host 082e.5f2f.70e2 in vlan 12 is flapping between port Fa0/1 and port Fa0/2
*Mar 1 00:18:12.314: %SW_MATM-4-MACFLAP_NOTIF: Host 082e.5f2f.70e2 in vlan 10 is flapping between port Fa0/1 and port Fa0/2

I am a newbie at networking and i dont really know what is causing the problem.

Thanks in advance.

[Reza Sharifi]

Hi,

The ports that are facing the hosts need to be in one vlan and configured as access and not trunk.

configure terminal

interface Fa0/1

switchport mode access

switchport access vlan x

end

- Host 2:

configure terminal

interface Fa0/2

switchport mode access

switchport access vlan x

HTH

[ajimmor]

Hi, thanks for your reply,
Your configuration would assign just 1 vlan to each port. I want to accept traffic from vlans 10,11 and 12 in each port so i guess i have to configure the 3 vlans in each port.

[Reza Sharifi]

Hi,

A regular host (laptop, desktop, printer, etc..) usually reside in one vlan. On the other hand, devices like servers, VMs, etc.. can support trunking and can be in multiple vlans.

If you want to test communication between vlans, you can simply put one host in one vlan and the other host in a different vlan.

example

host-1 in vlan 10 and ip segment 10.10.10.0/24

and host-2 in vlan 20 and ip address 10.10.20.0/24

your switch needs to support inter-vlan routing or you need a router.

Does this make sense?

HTH

[ajimmor]

Hi,

What im actually doing is controlling traffic via OpenFlow. I've defined 3 vlan interfaces in each host via ip link command. Now im trying to define the 3 vlans in each port of the switch to pass traffic through them. I just wanna choose with my OpenFlow controller the vlan in which traffic should be sent depending on the network parameters.
Thanks.

[Reza Sharifi]

is vlan trunking with dot1q enabled on these hosts?

[ajimmor]

Yes, lsmod | grep 8021q shows:
[root@localhost ~]# lsmod | grep 8021q
8021q 33159 0
garp 14384 1 8021q
mrp 18542 1 8021q

[Reza Sharifi]

Are the same vlan (10,11, and 12) added the hosts?

You need to have the same vlans on both sides.

HTH

[ajimmor]

Yes, i added vlan interfaces eth0.10, eth0.11 and eth0.12 with ip link command in both hosts.