When a port is set in access mode, we set it to an untagged VLAN. Let’s say we have port 2 set to access mode untagged VLAN 10. We then connect a PC to port 2 and send from frames to the switch. Let’s also say that port 1 on the switch is in trunk mode tagged VLAN 10 and is connected to another switch. When the frames from the PC hit the switch on port 2, they do not have a tag. Let’s say those frames need to get to some other device on the other switch, so they need to travel over the trunk. However, the trunk only accepts tagged frames. So, does the switch add a tag to the untagged frames from the PC and then send them over the trunk? Is that right?
Sounds you like got it yes.
If you set an interface in access mode then the switch will expect untagged frames from the device that is connected to it. Nowadays, a Cisco Catalyst switch will refuse any tagged frames on an interface in access mode.
Your trunk interface will accept tagged frames.
When the switch receives an untagged frame on port 2 that needs to cross the trunk then your switch will tag this frame before it forwards it on the trunk.
Thank you very much!
I just wanted to open a thread with this subject and look here. I have one more question, can a huge drop-amount in your network (backbone) be an issue where only untagged frames are geting rejected?
Last weeks i came across some interfaces in backbone where our edge devices are connected to and there are lot of rejected packets. After doing some researches i found out that the droped packers are coming from the untagged vlans. Did you had such scenario till now?
What is dropping your traffic? That makes a big difference. If it’s an access-list that’s dropping packets, then it might be an issue since the ACL puts a load on your CPU.
If the switch is dropping frames since it doesn’t have a destination MAC address then it’s no problem since this doesn’t add to your CPU load.