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in a single switch 2 vlans are created ,if we want to communicate between 2 vlans in the same switch then we required trunk port or not?

pl help to find the answers

16 REPLIES 16
Advisor

Re: in a single switch 2 vlans are created ,if we want to commun

No you do not need to trunk that being said unless the switch is a layer 3 switch then the 2 vlans cannot talk to each other .  What are you asking needs a trunk port?  If you have an external L3 routing device then yes it would have to be a trunk to that L3 device.

Beginner

I have multiple VLANs on

I have multiple VLANs on Switch 1 (3850) and multiple VLANs on Switch 2 (3850).

Switch 1 VLANs talking to each other and same in case of Switch 2.

But VLAN from Switch 1 is not communicating to VALN on switch 2 and vice versa. I make gi1/0/48 as TRUNK port on both switches. Still not able to communicate. I enable IP Routing as well. No router in environment. Anybody can help on this.

Thanks

Hall of Fame Master

It is good to know that you

It is good to know that you have configured a trunk port to connect the switches. But that does not tell us enough about your situation for us to be able to give you good advice. Can you tell us whether there is some vlan and some subnet that exists on both switches? That would be important for being able to route between the switches. Can you tell us if you have any dynamic routing protocol running between the switches? Or can you tell us if you have any static routes on switch 1 for the subnets on switch 2, and any static routes on switch 2 for the subnets on switch 1?

HTH

Rick

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Beginner

OK. VLANs are communicating

OK. VLANs are communicating with each other now. I did stack the switches and made as on entity and configured VLANs and enable "IP ROUTING". Up to here all the communication is good. Addition to this I want establish something more.

I have 2 stacked Cisco 3850 Switches and 1 Standalone 3750 - X Series

Scenario:

Stacked Switch (3850)

Switch have 15 VLANs spreading from gi1/0/1 (Excluding gi1/0/48) to gi2/0/47. All the VLANs are talking to each other. Available ports on Stacked Switch are: gi1/0/48 and gi2/0/48.

Few VLANs:

116 - 10.1.16.254 (on gi1/0/x port)

131 - 10.1.31.254 (on gi1/0/x port)

530 - 10.5.30.254 (on gi2/0/x port)

830 - 10.8..30.254 (on gi2/0/x port)

I want these VLANs to communicate to Cisco 3750-X Series switch VLANs which will be of same subnet of stacked switch.

Assumed VLANs on 3750

116 - 10.1.16.253

131 - 10.1.31.253

530 - 10.5.30.253

830 - 10.8.30.253

For communication I assuming that I can LAG two ports from stack switch and connect to LAG to Cisco 3750 switch and assign IP on each switch. Create the VLANs as written above. Create static routes via LAG IPs between switches. Is this sound doable ?

Thanks

Beginner

Re: in a single switch 2 vlans are created ,if we want to commun

Pradeep,

    If the switch is a special layer 3 switch (3560, 3750, 4500, 6500) then the switch effectively has a router built into it, so it can route between vlans on its own.

    If the switch is a layer 2 switch (2950, 2960) the switch cannot route between vlans.  You can, however, connect a layer 2 switch to a router with a trunk port, and use the router to communicate between the vlans.  This setup, with the router connected to a switch is sometimes called a 'Router on a Stick'.  There are many examples of this configuration, below is a link to one example:

http://www.firewall.cx/cisco-technical-knowledgebase/cisco-routers/336-cisco-router-8021q-router-stick.html

I hope this helps answer your question,

   - Ken

Beginner

in a single switch 2 vlans are created ,if we want to communicat

Hi Pradeep,

you need a layer 3 device to communicate between VLAN's. If your Swicth is layer 3 switch then good and if not then introduce a router or any layer 3 switch.

Contributor

Re: in a single switch 2 vlans are created ,if we want to commun

You just require ip routing command if the switch is a inter-vlan routing capable(l3 switch).
A

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Hall of Fame Master

Re: in a single switch 2 vlans are created ,if we want to commun

As others have pointed out for two vlans to communicate requires a layer three device which provides routing between the broadcast domains of each VLAN. As others have pointed out one good way to do this is to have a layer three switch. Another good possibility is to have a trunk port which will connect both vlans to a router. I would point out a possible solution that does not need a layer three switch or a trunk port. You can have the vlans communicate by having one VLAN have an access port that connects to a router interface in one subnet and have the other VLAN have an access port that connects to another router interface in the other subnet.

HTH

Rick

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Beginner

Re: in a single switch 2 vlans are created ,if we want to commun

Hi Mr. Richard,
I have a question,
I have one layer 2 switch and multiple vlans configured on it, can we communicate within vlans without 'ANY' L3 device such as router or Switch(L3), without using trunk and router on stick. ????
Regards : Haseeb Butt
Beginner

Re:in a single switch 2 vlans are created ,if we want to communi

If you use a l2 switch we need a l3 device ro make communication between 2 vlans .

If you have a l3 switch just enable ip-routing command foe inter vlan communication

Angus

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in a single switch 2 vlans are created ,if we want to communicat

For inter-vlan routing, we need L3 device or IP routing command.

If these VLANs are passing over trunk (dot.1q or ISL) VLAN tags will be added, hence other end switch will be able to distinguish the VLANs .

My question.

Let assume I have two VLANs in same L2 switch.

Vlan 10  (Two Host connected )

Vlan 20 (Two Host connected)

How the hosts in same vlan will communicate?

How the frame structures look like?

How the L2 switch distinguishes the frame is from Vlan 10 and Vlan 20?

(Frame does not have Vlan tag added and I guess tagging will be added only in trunk port.)

Advisor

in a single switch 2 vlans are created ,if we want to communicat

Hi,

by putting a switch port in access mode and configuring the vlan with switchport  access vlan 10, the switch will internally tag the frames( not 802.1q or ISL tag) received on this port as part of vlan 10 and will  use the CAM table for vlan 10 to forward the frame.

Regards.

Alain

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Don't forget to rate helpful posts.
Hall of Fame Master

in a single switch 2 vlans are created ,if we want to communicat

We have a new formulation of the qeustion "Let assume I have two VLANs in same L2 switch." This clears up some of the variables we have been discussing. Based on this version of the question, 2 vlans in a single layer 2 switch with no other devices, then the answer is that the vlans can not  communicate with each other.

HTH

Rick

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Beginner

Re: in a single switch 2 vlans are created ,if we want to commun

On an access port, there is a layer 2 tag pushed on ingress into the switch port and then the tag is popped on egress.

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