VLAN Trunking and VLAN Tagging, what’s the difference?

Over the years, I have worked with both Cisco and non-Cisco switches (such as HP, Netgear, Foundry, etc). There is the Cisco way and then there is the non-Cisco way. Both comply with the 802.1q VLAN standard. Lets just forget about the Cisco ISL standard, thats another topic!

So for those who are new, let me try to explain.

In the Cisco method, you basically put ports in “access mode” or “trunk mode”. Trunks are not to be confused with non-Cisco port aggregation trunks (i.e. LACP). Cisco “trunks” are actually ports that have a VLAN “tag” within the ethernet frame that designates the VLAN the packet belongs to. Cisco “access” ports are the ports that isolate specific VLAN packets configured for that port. The “tag” is stripped off and delivered to the port.

In the non-Cisco method, ports are either “tagged” or “untagged”. Tagged ports are ports that contain packets with vlan “tags”, same as the Cisco “trunk” port. Non-Cisco “untagged” ports are ports that vlan tags are stripped off, same as the Cisco “access mode” ports”.

What makes it tricky, is that in the non-Cisco environment, you need to tag & untag ports WITHIN the vlan. In the Cisco method, you set trunk & access vlans WITHIN the ports.
For example, suppose you want to setup port 1 as a trunk port (tagged port), and port 3,4 and 5 as access ports on vlan 100 (tagged ports).

Cisco method:

create the vlan and then assign the vlans –> to the ports

conf t
int fas0/1
switchport mode trunk
int fas0/3
switchport mode access
switchport access vlan 100

int fas0/4
switchport mode access
switchport access vlan 100
int fas0/5
switchport mode access
switchport access vlan 100

Non-Cisco method:

you tag and untag the ports –> to the vlans

conf t
vlan 100
tagged eth 1
untagged eth 3 eth 4 eth 5

Both accomplish the same goal, both are 802.1q, totally different methods! It seems as though Cisco made it more difficult, but for some reason, I get more confused with the vlan tagging and untagging. You start mixing in multiple vlans and things seem easier to follow on the Cisco method. Also keep in mind, it is perfectly fine to mix Cisco and non-Cisco switches and be able to communicate with 802.1q vlans back and forth even though the commands seem completely different. The standard is still 802.1q. Obviously it would be preferred that you keep all the switches the same within your network, but sometimes you have no control. If you have both kinds of switches (Cisco and non-Cisco), I hope that this has helped you understand.