Associate one Switch port to multiple VLANs on Cisco c4500 Switch

[motiar]

I am newly appointed Network Engineer to a company. I see ports, which are associated with multiple vlans. Here is an example: the port Gi7/20 is associated with the vlan 110 (name VOIP_new), vlan 120 (name Invoice), vlan 500 (name Human_resource) and so on.  The Switchs are WS-C4507R+E and deployed as Core and Distribution switchs, intervlan routing is running to the network.

I have the question: how a port be associated with multiple vlans. I know sofar that one port can be associated with one vlan only.

I would be thankful to your helpful comments.

[Seb Rupik · Accepted solution]

Hi there,

What you are looking at is a trunk port which is most likely configured to use dot1q encasulation. the encapsulation inserts the VLAN ID into the frame allowing frames from multiple VLANs (specified in the switchport trunk allowed vlan <id1...2>  statement) to be transmitted across the link to the receiving switch.

Likewise the receivng switch's swithpoty must be configured to recieve these frames with VLAN tags; so must be a trunk port and have the correct switchport trunk allowed vlan <id1...2> statement.

https://www.cisco.com/c/en/us/support/docs/lan-switching/8021q/17056-741-4.html

cheers,

Seb.

[motiar]

Hi Seb,

Thank you for your quick reply and yes, the ports are really configured as trunk ports.

If I unterstand you correctly: if you configure a port as trunk with the command switchport trunk encaptulation dot1q, then all allowed vlans on this trunk port will be put into those allowed vlans, right? If so, then why don't I see the same result into my lab (packet tracer, switch 3560) with the similar configuration? Attached you are getting a copy of the configuration (from production environment)

Regards

Motiar

[Seb Rupik]

Looking at the output, ports gi1/1, gi1/7, gi1/10 are trunking all VLANs.

What is the switchport configuration for gi1/9, gi1/11-13 ?

Do you have VTP configured and pruning enabled?

[motiar]

Hi Seb,

The switchports gi1/9, gi1/11-13 are also trunking all vlans. The VTP modes are configured either transparent (core) or off (distribution) and pruning is disabled. The attached configs are copied from a production switch- just renamed/renumbered the vlans for security reseans.

Please let me know whether my understanding is correct and if so, then why I dont get the similar output in my lab environment.

Regards

Motiar

[Marvin Rhoads]

(NOTE - I moved this post from the Optical Networking forum to the more appropriate LAN Switching and routing forum.)

In addition to the trunking port mentioned by Seb, there is also the possibility of an access mode port having a voice and a data VLAN association.

Access ports are typically used by endpoints (PCs, VOIP phones, printers etc.) while trunk ports most commonly are used between switches.