Putting a Trunk port in a vlan?

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Aug 6th, 2007
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I need ot be educated. Why would someone place a trunk port in a vlan. I always thought of a port as either being one that supports a trunk or one that supports an end-user (access port).

Check out this config:

interface GigabitEthernet0/48

description Connection to TLS

switchport access vlan 21

switchport trunk encapsulation dot1q

switchport trunk native vlan 21

switchport mode trunk

load-interval 60

speed 100

duplex full

This trunk port is not pruning any vlans, right?

Given this config, by default, it should be allowing traffic from all vlans, right?

What is the significance of placing it in vlan 21 and configuring the native vlan as vlan 21 also?


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Pari Thiagasundaram Mon, 08/06/2007 - 12:15
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access vlan 21 is the old config. This port was a member of VLAN21 before someone configured as a TRUNK port.

Pari Thiagasundaram Mon, 08/06/2007 - 12:16
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also the command, "switchport mode trunk | access" decided which mode the port is operating as.

Hope that helps

Edison Ortiz Mon, 08/06/2007 - 12:24
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You will see this kind of configuration very often on switches where the edge device is an IP Phone.

During the initial connection, the IP Phone will negotiate its values with the TFTP and DHCP server and form a trunk.

The IP Phone config will indicate its native Vlan and it will forward the native Vlan information from the switch onto the device connected to the phone which is usually a workstation.

Therefore, the workstation will use Vlan21 as the access vlan and the phone will use another Vlan per its configuration. This way you have separate Vlans for voice and data.

If you didn't have a phone connected to this port, the workstation will only understand the native vlan information, on this case Vlan21. If you have a trunk port without a native Vlan information, the workstation will assume that is connected to Vlan1 which is the default native Vlan on a trunk port.


lamav Mon, 08/06/2007 - 16:52
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Edison, thank you very much.

Ill tell you what this switchport is being used for.

It supports a connection to a MAN circuit -- a TLS circuit -- to a hub site. In other words, there are 2 "spokes," each with their dedicated connections into the TLS multipoint-to-multipoint cloud. Coming out of the cloud and into the "hub" is one leg with aggregate traffic from both remote spokes.

furthermore, the "hub" is a 6509 switch port that is configured as a trunk and is taking the traffic and trunking it once again to another switch (6513), which is the true destination of the traffic. In other words, instead of just connecting the TLS circuit directly into the 6513 (the true destination), they didnt want to 'waste' a dedicated fiber going up a riser, so they trunked up the traffic with other vlans on an already-existing fiber riser between the 6509 and the 6513.

The only problem is that the traffic coming from one of the sites is VoIP traffic and its getting killed on that shared riser. Now they want to go nuts and implement QoS over the fiber riser/trunk.

Am I making sense?

Check out the 2 attachments.

Now, given what I just told you, why the hell is that port at the DR site (3560) configured the way it is??? LOL

Edison Ortiz Tue, 08/07/2007 - 08:22
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Vlan 21 represents your Metro-Vlan given by the provider.

Your edge switches are connected to the provider's switches which are configured for dot1q-tunneling and they only accept packets from you if they are tagged with Vlan 21 (that's the metro-tag). This same switch is servicing other customers and each customer is assigned a different metro-tag. Yours happen to be 21.


Pavel Bykov Tue, 08/07/2007 - 09:03
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In your first post, "switchport access vlan 21" command is ignored, since trunk is established.

When trunk is established, all packets are being tagged with 802.1Q.

If for some reason you don't want packets from a certain VLAN to be tagged with 802.1Q, you would use "switchport trunk native vlan". In your case Command "switchport trunk native vlan 21" sends all the packets from VLAN 21 as-is.

Trunks are made to differentiate traffic in different VLANs, so only one VLAN can be untagged at the time.

In your particular case 3560 will be able to communicate with WAN router, but not with 6509 or 6513.

To make it work, you would have to configure subinterface with encapsulation dot1Q on your wan router, and remove native VLAN configuration on your 3560.

Or you would have to type "switchport trunk native vlan 21" on your 6509 port.

But because it's CATOS the command will look different.

Edison Ortiz Tue, 08/07/2007 - 10:29
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The 3560 is connected to a service provider's switch. He mentioned his connection is via TLS (Transparent LAN Services) which is also known as metro-ethernet.

The native Vlan (on this case 21) was assigned by the provider. I'm sure he has this Vlan 21 as native in other spoke and at the hub.

This 3560 should be able to communicate to the 6509 via Layer2 and to the 6513 via Layer3.

There isn't any WAN router on this config. It's strictly a switched MAN.


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