Native VLAN and ISL

Answered Question
Jan 25th, 2009

Hi,

I know that Native VLAN concept in used only in 802.1Q Trunk and not in ISL.

Then how cisco handles the packets without tagging, For instance PC connected to IP Phone.

In which VLAN it puts this PC.?

shall anybody throw light on this concept.

R.B.Kumar

I have this problem too.
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Correct Answer by Giuseppe Larosa about 7 years 10 months ago

Hello R.B. Kumar,

ISL is Cisco proprietary so it cannot be used by a PC.

However, I suppose that untagged ethernet frames shouldn't be accepted on an ISL trunk port it should accept only ISL frames (each of them carry an ethernet frame inside)

The only note is that ISL in its bigger overhead has space for connections via an hub it is necessary that the link has to be point-to-point.

So if you connect a PC to a port configured to be an ISL trunk the PC NIC should be isolated from the network.

Hope to help

Giuseppe

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Giuseppe Larosa Mon, 01/26/2009 - 00:09

Hello R.B. Kumar,

on an ISL trunk there are no untagged frames.

IP phones can use implicit mini trunks with 802.1Q encapsulation:

int fx/y

switchport

switchport access vlan 10

switchport voice vlan 20

frames for the PC are sent out untagged (native vlan) frames for the phone are sent with 802.1Q header with vlan-id = 20 and 802.1p CoS set to the appropriate values ( EF for bearer /vouce packets CS3 for signalling that means CoS 5 and CoS 3 respectively)

As far as I know IP phones don't support ISL encapsulation and this should answer your question.

There are multiple options about how to handle frames for the PC and for the phone:

the usage of a distinct voice vlan

the usage of a single vlan with voice frames tagged with vlan-id=0 (just to be able to carry a CoS field)

untagged: the switch tells in CDP what is the vlan but instructs the phone to transmit ethernet frames.

none: no information is provided to the phone

Only the first two options allow for real QoS on the switch port.

Hope to help

Giuseppe

hclisschennai Mon, 01/26/2009 - 01:58

Hi Giuseppe/Istvan,

Thankyou..

I think my question is not very specific. I quoted IP Phone as an example only. But that is not the thing i need. Let me put the question in this way.

How ISL trunk handle the packet that comes without any tagging. This occurs in two scenario

1. Connecting a Hub in between a ISL trunk and when a PC is connected to the Hub

2. IP phone and a PC as mentioned in the earlier.

Hope this makes the question more precise

Please clarify.

R.B.Kumar

Correct Answer
Giuseppe Larosa Mon, 01/26/2009 - 04:46

Hello R.B. Kumar,

ISL is Cisco proprietary so it cannot be used by a PC.

However, I suppose that untagged ethernet frames shouldn't be accepted on an ISL trunk port it should accept only ISL frames (each of them carry an ethernet frame inside)

The only note is that ISL in its bigger overhead has space for connections via an hub it is necessary that the link has to be point-to-point.

So if you connect a PC to a port configured to be an ISL trunk the PC NIC should be isolated from the network.

Hope to help

Giuseppe

Istvan_Rabai Mon, 01/26/2009 - 00:10

Hi Kumar,

The following interface level commands will do it:

switchport access vlan x

switchport voice vlan y

The result is that the PC traffic (untagged) will be forwarded in vlan x, the IP phone traffic (tagged) will be forwarded in vlan y.

Cheers:

Istvan

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