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Webcast-Catalyst9k
New Member

Distinguishing Vlans without Trunking ?

Hi,

How does a switch distinguish one Vlan from another if trunking is not involved ?

For Example: Switch A is connected to Switch B via a X-over cable. I place two ports at either end of the X-over cable into VLAN 2. I place one PC on switch A in VLAN 2 and one PC in switch B in VLAN 2. The result is that they can communicate. If I place one of the PC's into VLAN 1 then they cannot communicate. Is there a tagging mechanism involved here even though trunking isn't being used ?

Has anyone got an example of the frame format for Ethernet for this scenario ?

Cheers,

Phil.

4 REPLIES
New Member

Re: Distinguishing Vlans without Trunking ?

Hi,

I think I can answer my own question.

Does it just work as a transparent bridge and consult the CAM table ?

i.e the MAC address is stored along with the source port and the VLAN and the Ethernet frame remains unmodified.

Cheers,

Phil.

Bronze

Re: Distinguishing Vlans without Trunking ?

Green

Re: Distinguishing Vlans without Trunking ?

Devices connected to an access port (versus a trunk port) do not receive VLAN information.

FOr example, if you connected port 1 from switch A (which happens to be in VLAN 100) via crossover to another switch port 1 (which happens to be in VLAN 200), neither switch would know anything about the VLANs on the other switch.

Tags are added on the ingress to an access port, and stripped at the egress from an access port.

Traffic going from one port to another in the same VLAN (on the same switch) is not tagged (I believe).

If you connected an "dumb" (unmanaged) switch into a port that is associated with a VLAN on the host switch, then all of the extended ports will also appear in the VLAN of the host switch's outbound trunk.

The short version is that VLANs do not come into play for things plugged into an access port.

Good Luck

Scott

Bronze

Re: Distinguishing Vlans without Trunking ?

You're right; devices connected to access port are unaware of VLAN association. Therefore Ethernet frames on access ports do not carry VLAN tags.

VLAN tags aren’t associated with access ports but the switch is responsible for maintaining the MAC, interface, and VLAN association for access ports.

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