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Are vlans passed on a trunk when not configured

earl.hinkle
Level 1
Level 1
5 Replies 5

vasanth77
Level 1
Level 1

Better you should allow VLAN which are present and traverse over network ,so that other VLAN will be blocked and It's the best practice to secure your network.

So you have to configure cisco's Gi 0/18 ,0/19 ports to allow vlan 12,20,30 traffic.

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johnnylingo
Level 5
Level 5

By default, a trunk port on a Cisco switch will pass all vlans.

To successfully pass traffic from end to end, the intermediate switch must know that each vlan exists. 

Hello Earl.

Echoing Johnnylingo: By default, a trunk passes all traffic from all VLANs, also, you must check which VLANs are you going to filter in the trunks :)

Majid Jalinousi
Level 1
Level 1

hi Earl,

Only Vlans there are in the "show vlan brief" can traverse the trunk link, even though in "show interfaces trunk" you can see all the Vlans are enabled in the trunk port by default.

Then at first step you need to create your desired vlans locally or by VTP protocol in switch.

BR,

Majid

LJ Gabrillo
Level 5
Level 5

TO put it simply, with default configuration and no filtering involved, trunks passes ALL CONFIGURED VLANs on its port 

To stress out, the switch must have a configured VLAN for it to recognized and be able to passthrough, see scenario below:


DIAGRAM:
[SW1]----[SW2]---[SW3]

SCENARIO:
SW1: VLAN10,20
SW2: VLAN10
SW3: VLAN10,20

-Based on the vlan configuration above, even though SW3 has VLAN20, he wont be able to receive VLAN20 traffic since the middle switch(SW2) does not have VLAN20.

In order for SW3 to get VLAN20 traffic, SW2 should have VLAN20



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