VLAN Trunk Link Issue Between Cisco and BDCom Switches – Troubleshooting Guide

cisco-small-businessvlan

I have a cisco SF 300 switch which is connected to a chinese brand BDCOM switch (2500 series full gigabit port) . When both the ports of switch are configured in access mode, it works. But when they are configured in trunk mode tagging two vlans , it doesn't work. I need to pass two vlans in between these two switches. What might be the issue ?
Here is the configuration of cisco switch:

interface gigabitethernet1 switchport trunk allowed vlan add 17,2029 !

BDCom switch configuration:

interface GigaEthernet0/51
switchport trunk vlan-allowed 17,2029
switchport mode trunk

Adding more: there is only single link in between two switches. If there was multiple physical link between them, we could suspect about STP issue. But it was not the case.

Best Answer

I realized that the problem was with STP. The Bdcom switch interface was in blocking mode. So I manually disable STP in both the connecting interfaces at both switches: BDcom and cisco.

For Bdcom:

interface GigaEthernet0/36
switchport trunk vlan-allowed 17,18
switchport mode trunk
spanning-tree bpdufilter enable

For Cisco:

interface gigabitethernet1
spanning-tree disable
switchport trunk allowed vlan add 17,18

I am not clearly understanding though how STP status is changing in trunk mode. But it worked after disabling STP.

Related Solutions

Cisco – Accidentally Removed Allowed VLANs from Cisco Switch Dot1Q Trunk

You need to use the following command to add your VLAN 30 to an existing Dot1Q trunk on a Cisco Catalyst switch:

switchport trunk allowed vlan add 30

Otherwise IOS just thinks you're trying to overwrite the existing configuration and you are left with an accidentally deleted set of allowed VLANs.

You could similarly use "remove" in place of "add" to remove only one VLAN. See the entire syntax below. (It is actually the same syntax in Cisco Nexus OS or IOS, FYI.)

SW-FOO(config-if)#switchport trunk allowed vlan ?
  WORD    VLAN IDs of the allowed VLANs when this port is in trunking mode
  add     add VLANs to the current list
  all     all VLANs
  except  all VLANs except the following
  none    no VLANs
  remove  remove VLANs from the current list

Another option is to put all of your allowed VLANs into the command, like so:

switchport trunk allowed vlan 10,20,30

This option is more time consuming but also works.

Native VLAN – Configuring Native VLAN Between Switch and Router

Hope I understand what you are asking here (your writing is long and my memory is short :))

The HP untagged VLAN on a tagged interface functions similarly as the native VLAN you configure on Cisco switch trunk interface (actually tag/untag makes more sense than access/trunk technically speaking), the VLAN 8 in your case.

On your router interface facing HP switch, it is running as access/untagged port. So all ingress traffic on that router interface is VLAN 69; meantime router will remove VLAN 69 tag of the egree traffic to switch and when traffic arrives on switch, it will be VLAN 8. However if you have traffic marked with other VLAN, like VLAN 10 and send the traffic to router as the interface facing router is tagged/trunk port, router will not be able to understand it as the edge interface is expecting something without VLAN tag.

There is no native VLAN mismatch just like you connect a laptop to a trunk interface with certain native VLAN configured and switch does not complain about mismatch, right?

Hope this helps.

Best Answer

Related Solutions

Cisco – Accidentally Removed Allowed VLANs from Cisco Switch Dot1Q Trunk

Native VLAN – Configuring Native VLAN Between Switch and Router

Related Topic