Voice VLAN - 802.1q Special Trunk to IP Phone

Unanswered Question
Jul 18th, 2007

Hi All,

Just spent a while going through previous posts and was very helpful to me in regards to how the Voice VLAN works.

Now one question

If the voice and data VLANs are setup and it uses a "special case" 802.1q trunk, can I assume that when packets are sent on either data or voice VLANs, they will have a full 802.1q header inserted between the Ethernet and IP headers and not some special or Cisco amended 802.1q header :

Ethernet II, Src: Cisco_ef:04:00 (00:03:e4:ef:04:00), Dst: Cisco_fe:d8:c0 (00:0a:f3:xx:xx:xx)

Destination: Cisco_xx:xx:xx(00:0a:f3:xx:xx:xx)

Source: Cisco_xx:xx:xx(00:03:e4:xx:xx:xx)

Type: 802.1Q Virtual LAN (0x8100)

802.1Q Virtual LAN

000. .... .... .... = Priority: 0

...0 .... .... .... = CFI: 0

.... 0000 1100 1010 = ID: 202

Type: IP (0x0800)

Trailer: 000000

Internet Protocol, Src: 202.202.202.202 (202.202.202.202), Dst: 202.202.202.201 (202.202.202.201)

Version: 4

Header length: 20 bytes

Many thx indeed all :)

Kind regards,

Ken

Pls refer to the post from Narayan which is excellent below:-

" When you enable voice vlan, a special-case 802.1Q trunk automatically is enabled through a CDP information exchange between the switch and the IP Phone. The trunk contains only two VLANs a voice VLAN (tagged VVID) and the data VLAN. The switch port's access VLAN is used as the data VLAN that carries packets to and from a PC that is connected to the phone's PC port. "

I have this problem too.
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AJAZ NAWAZ Tue, 07/24/2007 - 14:15

Hi there Ken,

Pay special attention to the following detail in your capture:

Type: 802.1Q Virtual LAN (0x8100) <-- this is where the answer to your question lies.

2.3.1 VLAN Tagging

In 1998, the IEEE approved the 802.3ac standard that defines frame format extensions to support Virtual Local Area Network (VLAN) Tagging on Ethernet networks. The VLAN protocol permits insertion of an identifier, or "tag", into the Ethernet frame format to identify the VLAN to which the frame belongs. It allows frames from stations to be assigned to logical groups. This provides various benefits such as easing network administration, allowing formation of work groups, enhancing network security, and providing a means of limiting broadcast domains, Refer to IEEE standard 802.1Q for definition of the VLAN protocol. The 802.3ac standard defines only the implementation details of the VLAN protocol that are specific to Ethernet.

If present, the 4-byte VLAN tag is inserted into the Ethernet frame between the Source MAC Address field and the Length/Type field. The first 2-bytes of the VLAN tag consist of the "802.1Q Tag Type" and are always set to a value of 0x8100. The 0x8100 value is actually a reserved Length/Type field assignment that indicates the presence of the VLAN tag, and signals that the traditional Length/Type field can be found at an offset of 4-bytes further into the frame. The last 2-bytes of the VLAN tag contain the following information

The first 3-bits are a User Priority Field that may be used to assign a priority level to the Ethernet frame.

The next 1-bit is a Canonical Format Indicator (CFI) used in Ethernet frames to indicate the presence of a Routing Information Field (RIF).

The last 12-bits are the VLAN Identifier (VID) which uniquely identifies the VLAN to which the Ethernet frame belongs.

With the addition VLAN tagging, the 802.3ac standard permitted the maximum length of an Ethernet frame to be extended from 1518-bytes to 1522-bytes. The following illustrates the format of an Ethernet frame that has been "tagged" with a VLAN identifier per the IEEE 802.3ac standard (view frame table section 2.3.1):

<http://www.techfest.com/networking/lan/ethernet2.htm>

hth

Ajaz

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