forgotten vlan...

alliasneo

~Sigh~ I've forgotten this.... A pc doesn't understand the concept of vlan's right? So when you assign a switchport to a vlan a packet from a PC comes in how does it know to tag it with that vlan?

-I think I've just answered this myself through that question. Because the port is assigned to that vlan the switch will add the vlan number in to the packet as it comes in?

thanks

hiddenknight821

You had the right idea all along.

By default, all switchports are in VLAN 1 and they are in native VLAN. Native VLAN exists primarily for trunking to switches that lack VLANs.

drkat

OP,

When a frame comes ingress to a switchport configured as an access port - the frame comes in and is associated with the access vlan but there isnt any official tagging done. The switch keeps track of the the frame and floods it - if it goes across a trunk then the tag is inserted and along it goes.

When a switch floods the frame for the specific vlan, there is no vlan information on the egress toward the PC

zrockstar

Not as it comes in, as it goes out, and how it does it depends on the protocol. ISL encapsulates the whole packet with a header and trailer and dot1q modifies the Ethernet header to include a 4 byte VLAN ID. If it is traffic for the Native VLAN, dot1q doesn't tag it at all, ISL will still encapsulate because it does not recognize the concept of Native VLAN.

drkat

zrockstar, we're talking about access ports, not trunks

so when a frame from PC1 comes into the switchport the switch associates the frame with the access vlan of the port, it does not tag the frame (insert your vlan header information). Once the destination port is determined to be a trunk, when the frame arrives on the trunk port the switch inserts the vlan header and sends the frame up the trunk to the next switch, which performs the same process all over again.

Frames sent egress toward a PC/Server or some other vlan unaware device there is no vlan info in the frame, it's been stripped or otherwise not inserted to begin with

In all honesty in the real world all we need to know is that access ports put the frame in the vlan, it keeps its vlan info over a trunk and loses it headed toward a PC.. that's pretty much all she wrote...

alliasneo

thanks for all the help guys, much appreciated