"access mode" ports can not accept packets with 802.1q tag?

[teru-lei]

Hi all,

Here are several question I am confused:

1. Can 3550 switch accept packets with 802.1q tags after the interface command "switchport mode access" applied.

2. If the above question answer is "no" there is very strange. Because when I look up the voice vlan config section in 3550 manual, the manual said the feature "should config in access port", But the voice vlan also carry cos bits, and the cos bits are in the 802.1q tags, so it seems that access ports also can accept 802.1q tags. and when I refer to the qos config section in 3550 manual, there is no statement to say that the qos ports must be a trunk port.

3. If the access port can not accept 802.1q tag, so I think if I connect a port to a network card that support 802.1q and I use that feature. I should do this:

switchport mode trunk

switchport trunk allow vlan x

Am I right or wrong? Can anybody give me a hand? Thank You!

Best Regards

Teru Lei

[donewald]

Teru,

Answer to number 1 is "no" 802.1q frames need to come in on trunk ports. COS can be set in 802.1p also. So with an access port you can, by using 802.1p COS, do QOS for your voice.

Here is a link showing Voice VLan configured using access ports and 802.1p frames.

http://www.cisco.com/en/US/partner/products/hw/switches/ps628/products_configuration_guide_chapter09186a00800d84c0.html

Hope this helps you,

Don

[teru-lei]

Hi Don,

Thank you for your reply. But why the manual said (also in the link you gave me):

You should configure voice VLAN on access ports.

Should you please explain it to me for a bit detail?

Thank You!

Best Regards

Teru Lei

[donewald]

Teru,

A voice VLAN allows dot1q tags only for the voice/auxilary VLAN and discards all others. So, I would think it's on access ports for this reason that it will allow a 1q tagged frame only for the VOICE vlan to append your QOS. If this was configured on trunk port all vlans would be treated equally or need to be manually configured... Hope this makes sense.

Hope this helps,

Don

[f.chiesa]

Hi Don,

I have another question. In this situation, what about the data packet behind the phone? They will go on the access vlan or the native vlan? And if I use the normal config with the trunk, IT still works?

Thanks,

Fabio