Traffic in same VLAN goes up trunk

Andy White · ‎02-12-2013

Hello,

I have 2 hosts, 1 plugged in fa 0/21 in VLAN 101 and another in fa 0/22 in VLAN 101 on our L2 Cisco 2960. If I try and transfer files from either host the gig 0/1 trunk port on the 2960 leading tot he 3750 fa 0/1 port hits 100mb (using a real time bandwidth monitor tool), but why? This VLAN is on the same switch, why does it go one way up the trunk to the L3 3750 switch? The L3 3750 is the VTP server and the 2960 is a client. I would of thought the traffic stays local. The 2 hosts don't even have a gateway set.

To sum up the typology the 2960 and 3750 are trunked using a single cable. The 3750 hangs of a ASA firewall using SVIs.

Here is whatthe traffic looks like when copying a file between hosts (2gb file).

3750 L3 Switch (VTP Server)

interface FastEthernet1/0/4

description Trunk to Cisco 2960 Gig 0/1

switchport trunk encapsulation dot1q

switchport mode trunk

switchport nonegotiate

2960 L2 Switch (VTP Client)

interface FastEthernet0/21

description test

switchport access vlan 101

switchport mode access

spanning-tree portfast

spanning-tree bpduguard enable

!

interface FastEthernet0/22

description test

switchport access vlan 101

switchport mode access

spanning-tree portfast

spanning-tree bpduguard enable

interface GigabitEthernet0/1

description Trunk to 3750 1/0/4

switchport mode trunk

!

interface GigabitEthernet0/2

shutdown

!

interface Vlan1

no ip address

!

interface Vlan12

ip address 10.1.2.254 255.255.255.0

no ip route-cache

Port Mode Encapsulation Status Native vlan

Gi0/1 on 802.1q trunking 1

Port Vlans allowed on trunk

Gi0/1 1-4094

Port Vlans allowed and active in management domain

Gi0/1 1-6,9-15,100-101,123,300

Port Vlans in spanning tree forwarding state and not pruned

Gi0/1 1-6,9-15,100,123,300

Drac-Switch#

Any ideas?

Thanks

cadet alain · ‎02-12-2013

Hi,

vlan 101 is not there:

Port Vlans in spanning tree forwarding state and not pruned

Gi0/1 1-6,9-15,100,123,300

so there should be no vlan 101 traffic going through this trunk.

Regards

Alain

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Andy White · ‎02-12-2013

So is it pruned?

A "sh vlan bri" shows it there though, I had to create the VLAN on the VTP server then I coudl see it onthe client and then assigned the ports to VLAN 101:

sh vlan bri

VLAN Name Status Ports

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

1 default active Fa0/13, Fa0/16, Fa0/23, Fa0/24, Gi0/2

2 **** active

3 **** active

4 **** active

5 **** active

6 **** active

9 **** active

10 **** active

11 **** active

12 **** active Fa0/1, Fa0/2, Fa0/7, Fa0/8, Fa0/9, Fa0/10, Fa0/11, Fa0/12, Fa0/14, Fa0/15

13 **** active

14 **** active

15 **** active

100 **** active Fa0/17, Fa0/18, Fa0/19, Fa0/20

101 Test_VLAN active Fa0/3, Fa0/4, Fa0/5, Fa0/6, Fa0/21, Fa0/22

123 **** active

300 **** active

1002 fddi-default act/unsup

1003 trcrf-default act/unsup

1004 fddinet-default act/unsup

1005 trbrf-default act/unsup

Drac-Switch#

cadet alain · ‎02-12-2013

Hi,

it is pruned or the STP is not forwarding this vlan on this interface, so i wonder how you get traffic.

could you do a span session to see which traffic is passing through this trunk when you transfer your files ?

Regards

Alain

Don't forget to rate helpful posts.

Andy White · ‎02-12-2013

Hi,

I found out what it was. The switch had RSPAN running and was sending the VLAN traffic up the trunk to the remote network analyzer - doh. Silly me.

Thanks for your time though.

Giuseppe Larosa · ‎02-12-2013

Hello Andy,

you have been kind to provide feedback on this issue.

You don't need to apologize.

Best Regards

Giuseppe

Andy White · ‎02-12-2013

Thanks, what I love about finding issues like this is a resolution, even if it was something simple or silly as for me it is the best way to learn. Once I found it out what it was it all made sense as to why the traffic was only going up the trunk and never down