VLANs

[jonnybegood]

Can anybody tell me what exactly a 802.1q trunk is ?

 

[DanielBowen]

An 802.1Q trunk is an encapsulation method by which multiple VLANs can be transmitted by one interface/port. By default, a switch port can only tx and rx packets for the VLAN it is configured for, and it can only be configured for one VLAN. With a trunk, multiple VLANs or even all VLANs can be passed by one switch port. 802.1q is an encapsulation method configured on trunk ports. It works by adding a 4-byte tag to frames passing down a trunk link containing the source VLAN or color. This tag allows the recieving station to recognise what VLAN the packet was sourced from, and stops packets from one VLAN being sent to a station in another VLAN (security risk!)

Daniel,

 

[jgercken]

Naw, switchports can be configured for multiple VLANS, called a hybrid port. Also, trunked ports have a native vlan that will contain untagged traffic. 802.1Q (often dot1q) is an IETF standard. The other popular tagging method ISL is Cisco propriatory and adds another layer of encapsulation around the original packet.

I use ISL, any thoughts on which is better?

 

[DanielBowen]

802.1q adds less to the frame, 4 bytes to be exact which is added into the existing frame. ISL on the other hand encapsulates the packet within a 30 byte header and trailer, which sometimes breaks Ethernets maximum packet size which can produce baby giants. I use ISL, due to the fact that all of our kit is Cisco, but then again, 802.1Q does have a place.

Sorry, when you asked the question I assumed you were just talking about a default 802.1q. Hybrids can be configured but I don't see it done very often.

Hope this has helped