Doubts of the 802.1Q and trunk

[eduangelo]

How to configure trunk mode on both switch ports?

what is difference between 802.1Q with trunk ? 

[luis_cordova · Accepted solution]

Hi @eduangelo,

Check what I found on this link:

https://www.cisco.com/c/en/us/support/docs/lan-switching/8021q/17056-741-4.html

Background Theory

Trunks are used to carry traffic that belongs to multiple VLANs between devices over the same link. A device can determine which VLAN the traffic belongs to by its VLAN identifier. The VLAN identifier is a tag that is encapsulated with the data. ISL and 802.1Q are two types of encapsulation that are used to carry data from multiple VLANs over trunk links

...

802.1Q is the IEEE standard for tagging frames on a trunk and supports up to 4096 VLANs. In 802.1Q, the trunking device inserts a 4-byte tag into the original frame and recomputes the frame check sequence (FCS) before the device sends the frame over the trunk link. At the receiving end, the tag is removed and the frame is forwarded to the assigned VLAN. 802.1Q does not tag frames on the native VLAN. It tags all other frames that are transmitted and received on the trunk. When you configure an 802.1Q trunk, you must make sure that you configure the same native VLAN on both sides of the trunk. IEEE 802.1Q defines a single instance of spanning tree that runs on the native VLAN for all the VLANs in the network. This is called Mono Spanning Tree (MST). This lacks the flexibility and load balancing capability of PVST that is available with ISL. However, PVST+ offers the capability to retain multiple spanning tree topologies with 802.1Q trunking.

Regards

[Dennis Mink · Accepted solution]

simply put, with the use of a trunk, you can stretch the same VLANs across multiple physical switches, using a single physical port on both ends. without a trunk you create an access port that can only carry 1 VLAN across between switches.

[sankarmay · Accepted solution]

Hi,

ISL and 802.1q is a technology used to form trunk port.

[eduangelo]

What is the protocol the default at the trunk ?

[sankarmay]

Hi,

ISL and 802.1 q is a technology used to form trunk port.

[eduangelo]

But i can to create a trunk without this tech 802.1q and ISL ?

[Joseph W. Doherty · Accepted solution]

"But i can to create a trunk without this tech 802.1q and ISL ? "

Don't believe so, although on later Cisco switches, they allow access ports with an additional tagged VLAN, generally used to support a data host and IP phone on the same port, yet keeps their traffic in different VLANs. I believe this "special" trunk port, which again is configured as an access port, uses 802.1Q frame tags for the voice VLAN (and the data VLAN is untagged, like the "native" VLAN on a Cisco trunk port).

To carry multiple VLANs across a link (i.e. the purpose of a trunk), you need some method to tell them apart. Both ISL (old and Cisco proprietary) and 802.1Q (newer and RFC'ed) add/extend some data to the L2 frame to identify to which VLAN the frame belongs. (BTW, the additionally added information provides a bit more data than just the VLAN ID.)

[LukesWANadventure]

Is there a reason you would want to? You need to set up the "encapsulation" command when setting up a trunk and this can be either ISL or 802.1q. This lets a switch know which protocol to use when setting up the trunk.

A trunk can't simpy be "a trunk" as it were, you need a underlying protocol to acheieve this.