traffic across mismatched vlans

Unanswered Question
Aug 15th, 2007

I have a 3560 running 12.1(19)EA1d on which I configured a 2nd vlan with 2 ports in order to force traffic through a compression device. That is, WAN to router, router to switch port vlan 3, crossover cable from 2nd switch port vlan 3 to switch port vlan 4. All other switch port are in vlan 4 also. CDP expectedly reports vlan mismatch between ports on each end of the crossover cable. Transit traffic flows OK through the switch, but there is an issue with traffic sourced from the switch. If I put the IP address on vlan 3 the switch can ping, Telnet, etc to devices localy on vlan 3 but not the gateway router or anything beyond. If I put the IP address on vlan 2 the switch can ping etc with the gateway and everything beyond but not with anything localy. With the IP on vlan 3 I can get to the switch by telneting to another device on vlan 3 and then telneting back to the switch. What is stopping traffic sourced from the switch from crossing the ports on different vlans connected by the crossover cable?

I have this problem too.
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kschleppenbach Wed, 08/15/2007 - 11:27

When I tested this configuration on a 3750 it worked OK. Also fails on a 3550 running 12.1(13)EA1a and a 3650 running 12.2(25)SEB2.

royalblues Wed, 08/15/2007 - 14:13

Do you mean to say you have no trunks configured between your switches?


kschleppenbach Wed, 08/15/2007 - 14:32

It's just one switch. Ports 1 and 2 are in vlan 2, all others in vlan 3. Compression device in port 1, crossover cable between port 2 and 3 tying the vlans together.

szilagyiferenc Thu, 08/16/2007 - 07:34


if U use different subnets on the two vlans, routing will be needed between the vlans. the crossover will not work... If the subnet the same, You can put all ports to the same vlan.



MUHAMMAD SHAHEEN Thu, 08/16/2007 - 07:46


I think it is bit difficulty to understand your topology. It looks like you have a crossover cable connected on port 2 to 3 on the same switch. Does your LAN consists on 1 or more switches ? Please give us more details !! You have also mentioned you have tested this on 3750, 3550 and 3560 switches. May be a diagram can help us to identify the problem. If you can attach switch configs too then we will be able to undestand more clearly of what you trying to do.....

it is hard to suggest anything at the moment...



kschleppenbach Thu, 08/16/2007 - 14:49

Here is what I am trying to accomplish. It works OK except for the fact that traffic sourced by the switch, telnet for instance, can talk to vlan 3 devices but not vlan 2.

MUHAMMAD SHAHEEN Fri, 08/17/2007 - 01:17


Can you check? and change followings:

1.All devices in VLAN3 including the interface configuration on your compression device which connects to Vlan3 has correct IP, subnet mask and default gateway, within VLAN3 IP range, the gateway address will be SVI address from your 3550 switch.

2.All devices which are in Vlan2 have correct IP with VLAN2 IP range and gateway address should be SVI address of VLAN2 from your switch.

3. you should have IP routing in place on your 3550 switch, show IP route should give you list....




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