Trunk links Vs Access links

Unanswered Question
May 26th, 2007

Hi Experts ,

I want to know the differences between Access and Trunk Links in switched networks with images if it's possible.


Ibrahim Alwahidi

I have this problem too.
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ankbhasi Sat, 05/26/2007 - 07:55

Hi Ibrahim,

Trunk links and Access links are available on all images on switches as it is not a feature but a standard.

Trunk links are capable of carrying multiple vlans and traffic carried on trunk links are tagged (dot1q) /encapsulated (isl). Dot1q also has a concept on native vlan and data for native vlan is not sent tagged.

Access link are mapped to one particular vlan and only pass data for that vlan to which it is associated/mapped. Data on access link are not sent tagged and does not carry any vlan information when data moves out of access port.



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alwahidi22 Sat, 05/26/2007 - 08:26

Hi Ankur ,

But i meant that i need images that demonstrate Trunk and Access Links.


Ibrahim Alwahidi

mohammedmahmoud Sat, 05/26/2007 - 11:45

Hi Ibrahim,

The links provided by Ankur are awesome, just to brief:

VLAN - Broadcast Domain (using VLANs increases the number of broadcast domains but reduces the size of each broadcast domain) creating independent logical networks within a physical network - we often configure a VLAN to map directly to an IP sub-network, or subnet.

It is obvious that VLANs operate at Layer 2 (the data link layer) of the OSI model.

Each switch access port can be assigned to a certain VLAN, and thus you can configure several switch ports to a certain VLAN and thus having your separate broadcast domain / subnet.

Switch trunk ports can have multiple VLANs traversing it, for cases where you need a VLAN to span multiple switches (multiple hosts on multiple switches you need to be member of a single VLAN), all VLANs traffic can traverse the trunk port to go from hosts on the first switch to ports on the second switch sharing the same VLAN.

There are 2 famous trunking protocols, ISL (Cisco Proprietary - ISL is no longer supported by Cisco) and 802.1Q (standard).

Please do check the links Ankur has recommended and if you have any further questions please do come back.

HTH, please do rate all helpful replies,

Mohammed Mahmoud.

alwahidi22 Sun, 05/27/2007 - 05:44

Thanx Mohammad,

I want to ask another question that if i have 3 routers and 3 switches , each switch attach to one router...

switch no.1 have 2 VLANs and the switch no.2 have 3 VLANs but they aren't connect to each other directly ( links between switches ) but by routers....

switch no.3 has no VLAN so 1)should i have trunk links between switch no.1 and switch no.2 and their routers?

2)should i have trunk link between switch no.3 and the router which it connects ( as i said it hasn't any VLAN on it only the default one VLAN1 ) ?


Ibrahim Alwahidi

ankbhasi Sun, 05/27/2007 - 06:21

Hi Ibrahim,

It all depends what exactly you want? If all of your devices you mentioned are in same lan and location then why are they seperated with routers? All switches can be connected together via trunk and finally one switch can be connected to router which can perform routing for all vlans.

Just remember router interface are costly as compared to switch interfaces so it should not be wasted when things can be managed with one router interface.

Also on the other hand if this is your requirement then you can have subinterfaces for respective vlans on router's physical interface and can route between vlans. Suppose router fa0/1 is connected to switch 1 as you said switch 1 has 2 vlans then you can have 2 subinterface on fa0/1 and trunk on your switch 1 and route between those 2 vlans on router and same you can follow on other interfaces of router and finally when you will terminate all vlans on your router respective subinterface they all can talk to each other.

So it all depends on what is your requirement and what exactly you want to achieve then particular solution can be thaught and discussed.



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alwahidi22 Sun, 05/27/2007 - 07:22

Thanks Ankur ,

I understood your nice explanations , but i want form you to explain that Should i set trunk link for switch no.3 which hasn't any VLAN set..?


Ibrahim Alwahidi

Amit Singh Sun, 05/27/2007 - 08:15


You really dont need to set the link from switch number 3 to the router connected to it as a trunk link. You need trunk links only when you want to carry the layer 2 vlans from one switch to another. You need to configure the trunk link to the router only when you want to do inter-vlan routing on the router. In this case as the switch has only be default vlan do you dont have to configure the link as trunk link.

Let us know if you have more queries.

HTH,Please rate if it does.

-amit singh

alwahidi22 Sun, 05/27/2007 - 08:58

Thanks Amit ,

But i have another question that Switch no.3 can see VLANS on both Switch no.1 and Switch no.2 if it isn't use trunk link.

And i want to know that 1)where should i configure VTP ?

s)can i use VTP with switches that are connected to each other directly or by routes ?

Amit Singh Sun, 05/27/2007 - 09:17

Switch3 will see the vlans from switch1 and switch2 only if you have configured a trunk links between all the switches.

In this topology you have all the 3 switches not connected to each other. Each switch has its local router connected.

By the way this is not s suggested topology/design if you have to connect all the 3 switches together. Also, if you have to configure VTP, I will not recommend you this design.

Change the toplogy like this:

Router---Trunk Link--- Switch1----Trunk link Switch2-----Trunk Link----Switch3.

Configure the Switch1 as VTP server and configure Switch2 and Switch3 as VTP clients.

You really dont need 3 routers in this topology.You can configure the intervlan routing using a single external router.

HTH,Please rate if it does.

-amit singh


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