trunk links

[autominus]

Hi,

What are the parameters and types of trunk links?

Thank you in advance.

[Jan Hrnko]

Hi Matthew,

please, can you be more specific in your question?

Because I don't really know if you want to ask about DTP (dynamic trunking protocol) - protocol which can negotiate the trunk dynamically and its different modes. Or you want to ask about different types of trunk like 802.1q and ISL trunk.

Thanks and have a nice day!

Best regards,

Jan

[Steve Fuller]

Hi Matthew,

In Cisco speak a trunk is the term used to describe a link that is configured in such a way as to allow it to carry multiple VLANs. This may be using the Cisco proprietary Inter Switch Link (ISL) protocol, or more normally these days the IEEE 802.1q protocol.

You can find details about both of them in the Inter-Switch Link and IEEE 802.1Q Frame Format TechNote, but if you can be a little more specific what you're after we can probably help further here.

It should be noted that other network vendors use the term trunk to  describe Link Aggregation i.e., the mechanism used to combine two or  more physical links into a single logical link for the purpose of  increasing bandwidth and availability.

Regards

[JohnTylerPearce]

Matthew,

A trunk, is a link between two switches, that can carry more than one vlan. For instance, an access link, can only have one vlan go across is, whereas a trunk, can have multiple.

For multiple vlans to go across a link (i.e. trunk) it must be encapsulated. You have ISL and dot1q. ISL encapsulated the whole frame, is Cisco Proprietary, and isn't really used anymore. Most newer switches don't even have the ISL option.

dot1q, tag the frame, with a vlan identifer, which includes the specific vlan that is traversing the link.

DTP or Dynamic Trunking Protocol, can dynamically negotiate a trunk link.

You can dynamically setup trunks, by either specifying 'dynamic desirable or dynamic auto'

int x/x

switchport mode trunk

switchport trunk encapsulation [dot1q | isl]

Most switches will only accept dot1q, so you don't even need to run the "switchport trunk encapsulation" command.

Some switches, by default, have interfaces in either dynamic desirable and dynamic auto'

Depending on how they are matched, they can automatically setup a trunk.

Dynamic Desirable + Dynamic Desirable = Trunk

Dynamic Desirable + Dynamic Auto = Trunk

Dynamic Auto + Dynamic Auto + No Trunk

Trunk (switchport mode trunk + (anything besides access mode) = trunk

I hope that helps Matthew.