Allowed vlans on trunk

Answered Question
Jul 10th, 2009

Hi every body!

Today while discussing trunking with my friend, i came to know a new thing.

According to Cisco press book, a switch has tree reasons to prevent a particular vlan's traffic from crossing a trunk:

1) manually configured to remove vlan

2) vtp pruning

3) A vlan does not exist or is not active, in the switch's vlan database.

The third reason is mind boggling atleast for me.

Let say we have three switches.

sw1 has vlans,1,2

sw2 has vlan 1,

sw3 has vlan 1,2

All switches are connected by trunk. Based on the above reason(reason #3), sw2 will not forward vlan 2 frame to sw3.

Am i correct?

sw1---------sw2-----------------sw3

(v1,v2) (v1) (v1,v2)

=======================================

Let take another example.

sw1---------------sw2(sw2 layer 3 switch)

sw1 has vlans 1,2

Sw2 has vlan 1.

To route traffic between vlan 1 and vlan2, sw2 is used.

A host in vlan 1 connected to sw1 though not shown, wants to send ip packet to host in vlan2.

sw1 forwards this frame over trunk to sw2.

What sw2 will do? Will it drop this frame bacause sw2 has no vlan2 in its database ? or sw2 will route the packet?

Here is my guess:

Since sw 2 is performing routing function, or acting as a router , it will be able to send frame to vlan 2 over trunk though no vlan 2 exists on sw2.

Thanks a lot and have a nice weekend!

I have this problem too.
0 votes
Correct Answer by Edison Ortiz about 7 years 5 months ago

In order to create an interface vlan 2 and have an IP address under it and this interface to be up/up a Layer2 representation of that Vlan must exist on the Vlan DB.

Correct Answer by Edison Ortiz about 7 years 5 months ago

On your first example, yes - SW2 won't forward packets for Vlan 2.

On the second example, how SW2 is going to route between Vlan 1 and Vlan 2 when you said SW2 only has Vlan 1 on the Vlan Database?

___

Edison.

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Correct Answer
Edison Ortiz Fri, 07/10/2009 - 17:51

On your first example, yes - SW2 won't forward packets for Vlan 2.

On the second example, how SW2 is going to route between Vlan 1 and Vlan 2 when you said SW2 only has Vlan 1 on the Vlan Database?

___

Edison.

sarahr202 Fri, 07/10/2009 - 18:15

Thanks Edison .

Should we need to configure vlan 2 on sw2 so that sw2 can route packets to vlan 2?

Is it something usualy done in such situation 1.e create a vlan on multilayer switch so that it can route traffic to that vlan?

thanks and have a nice weekend!

Correct Answer
Edison Ortiz Fri, 07/10/2009 - 18:17

In order to create an interface vlan 2 and have an IP address under it and this interface to be up/up a Layer2 representation of that Vlan must exist on the Vlan DB.

sarahr202 Fri, 07/10/2009 - 19:18

Thanks Edison. I can recall now for a multilayer switch to route traffic between two vlans, say vlan 1 and vlan 2, two interface interface vlan 1 and interfacve vlan 2 are created. We can not create interface vlan 2 without creating the vlan 2 first.

Thanks Edison and you have a good weekend !

abhishek.chanana Sat, 07/11/2009 - 04:27

Hi Edison,

In which VTP mode are these switches running.Is SW2 is in transparent mode ?

regards

Abhishek

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