Kindly correct my understanding of trunking concept

I have a switch, and I have configured 3 VLANs on it. So, that would be 3 subnets, right? To move data between those subnets I would be trunking. And the switch would be a Layer 3 switch which would route the packets between the three VLAN subets on the same switch.

And what happens when VLAN 1 on SW1 wants to communicate with VLAN 2 on SW2. Would this be Layer2 switching - to - Layer 3 routing - to - Layer 2 switching?

Please correct me

An access port does not establish trunk. So that means just one VLAN right? And that mean a single subnet? Could someone help me understand a scenario when you would want to use an Access port and not allow trunking?

Find more posts tagged with

Save $250 on 2025 certification boot camps from Infosec!

Book now with code EOY2025

Button

Comments

sucanushie

In terms of CCNA, it doesn't cover Layer 3 switching.

In order for devices on different VLAN's to communicate you would need to implement a router on a stick.

Magic Johnson

sucanushie wrote: »

In terms of CCNA, it doesn't cover Layer 3 switching.

In order for devices on different VLAN's to communicate you would need to implement a router on a stick.

?

Odom's book certainly covers it!

OP, yes in fact he actually states the 'proper' terminology for 'routing between VLANs'.

So trunk your ports for the link between the switches, make sure both switches have the VLANs configured correctly in the sense they are up, aware and allowed, and boom job done. VTP is not covered in CCENT at least so you have to like for like configure the VLANs on each switch.

p.s of course someone who is actually certified feel free to confirm or correct that haha. But according to what I've learnt so far that is correct (I flaming well hope!)

networker050184

Let's look at this from a different perspective.

When a device wants to communicate with another device in the same subnet how does it find that device? It broadcasts an ARP request. This is defined to a single broadcast domain. A VLAN is essentially a broadcast domain.

When a device wants to communicate with a device off it's own subnet what does it do? It sends the packets to it's default gateway because it is on a different broadcast domain and an ARP request will not reach this device. The gateway is a router that routes the packet on to the destination subnet.

So, same subnet it's all L2 within a broadcast domain and disparate subnet its routed.

sarahanand

Yes, thanks, but I was asking about the frames between the VLANs on the interfaces of a single switch.

networker050184

If it's a single switch the frames are switched if it is a single broadcast domain or routed if it is not a single broadcast domain. How many interfaces or switches that come into the equation do not change this behavior.

EdTheLad

sarahanand wrote: »

An access port does not establish trunk. So that means just one VLAN right? And that mean a single subnet? Could someone help me understand a scenario when you would want to use an Access port and not allow trunking? [/B]

If you connect a PC to a switchport, the switchport should be configured as an access port and have an access vlan assiged.The PC has a single ip address and the subnet to which this ip address belongs now belongs to the access vlan, the switch has no idea about this connection between ip and vlan, this is for you as a network designer to document.If you want multiple devices to communicate using this subnet across your layer 2 network you should assign all access ports to the same vlan.

Lets say you have a server which multiple ip addresses on a NIC, i.e. the server NIC supports dot1q tagging, in this case you would setup the switchport as a trunk, allowing the vlans that have been configured on the server nic. Each ip address on the nic would be in a different subnet and hence the allocated vlan to subnet mapping would be recorded by you.Later if you had a pc that needed to communicate with a specific subnet you would connect it to a layer 2 switch and assign it the appropriate access vlan id.

Samo3411

when you move traffic between diffrent vlans you will be routing (not Trunking) so you need a router or a layer three switch.

when you move traffic in the same vlan but in diffrent switches you will be Trunking (not routing).

a trunk is a port that forward all diffrent vlans traffic between switches.

imagin you have Vlan3 in switch 1 and the same Vlan3 in switch 2 for the traffic of Vlan3 to go between the switches you need to create a TRUNK .

Picker

Think about frames and packets first... Layer 2 = frames & Layer 3 = packet then you should be able to figure out your question. Vlan = subnet = broadcast therefore a layer 3 devise is required to send packets to another Vlan.

Picker

Devise on vlan1 will send a frame to its switch and the switch will send packets to its router (gateway). If its the first time, it will send out an ARP request asking for the MAC address of router. The Router receives this message and replies with the MAC address of its interface. Later, when the devise decide to send packets out of its network (another vlan), it will include the IP address of a devise on another vlan and the MAC address of its gateway in the destination IP address and destination MAC address” fields, respectively.