Ethernet VLAN using LLC/SNAP encapsulation

Ethernet VLAN using LLC/SNAP encapsulation
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Subject Author Date
Ethernet VLAN using LLC/SNAP encapsulation Albert Manfredi 10-22-2007
Posted by Albert Manfredi on October 22, 2007, 8:52 pm
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IEEE 802.1Q seems not to be terribly clear in answering this question:
can a VLAN-tagged frame use LLC/SNAP encapsulation within the tagged
frame?

Clause 9.4 says that a VLAN over Ethernet, i.e. which supports the
Protocol Type field, uses the TPID indicating VLAN tag (which is
0x81-00).

Say this is followed by the expected TCI tag, and that the CFI bit is
0 (indicating canonical format of MAC addresses). In this case, the 16-
bit TCI tag is not followed by any other header extension. Simple
case.

Now comes the Protocol ID encapsulated within the tagged frame.

The question is, is there any reason why I can't encode that 2-byte
field following the TCI tag as a length, i.e. less than 1500, and
follow that with LLC header 0xAA-AA-03 and a SNAP header?

Should be legitimate, but I wanted to be sure.

Thanks.

Bert


Pure Networks
Posted by anoop on October 23, 2007, 3:46 am
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> IEEE 802.1Q seems not to be terribly clear in answering this question:
> can a VLAN-tagged frame use LLC/SNAP encapsulation within the tagged
> frame?
>
> Clause 9.4 says that a VLAN over Ethernet, i.e. which supports the
> Protocol Type field, uses the TPID indicating VLAN tag (which is
> 0x81-00).
>
> Say this is followed by the expected TCI tag, and that the CFI bit is
> 0 (indicating canonical format of MAC addresses). In this case, the 16-
> bit TCI tag is not followed by any other header extension. Simple
> case.
>
> Now comes the Protocol ID encapsulated within the tagged frame.
>
> The question is, is there any reason why I can't encode that 2-byte
> field following the TCI tag as a length, i.e. less than 1500, and
> follow that with LLC header 0xAA-AA-03 and a SNAP header?
>
> Should be legitimate, but I wanted to be sure.

It is legitimate and it should be fairly clear from
the spec. Fig C-9 in Annex C of 802.1Q covers
exactly this. All that adding the VLAN tag does is
displace the original contents of the frame starting
with the type/length by 4 bytes.

Anoop


Posted by Albert Manfredi on October 23, 2007, 10:11 am
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>
> > IEEE 802.1Q seems not to be terribly clear in answering this question:
> > can a VLAN-tagged frame use LLC/SNAP encapsulation within the tagged
> > frame?
>
> > Clause 9.4 says that a VLAN over Ethernet, i.e. which supports the
> > Protocol Type field, uses the TPID indicating VLAN tag (which is
> > 0x81-00).
>
> > Say this is followed by the expected TCI tag, and that the CFI bit is
> > 0 (indicating canonical format of MAC addresses). In this case, the 16-
> > bit TCI tag is not followed by any other header extension. Simple
> > case.
>
> > Now comes the Protocol ID encapsulated within the tagged frame.
>
> > The question is, is there any reason why I can't encode that 2-byte
> > field following the TCI tag as a length, i.e. less than 1500, and
> > follow that with LLC header 0xAA-AA-03 and a SNAP header?
>
> > Should be legitimate, but I wanted to be sure.
>
> It is legitimate and it should be fairly clear from
> the spec. Fig C-9 in Annex C of 802.1Q covers
> exactly this. All that adding the VLAN tag does is
> displace the original contents of the frame starting
> with the type/length by 4 bytes.

Thanks, Anoop. Also Figure C-6 shows this, but never the complete
example, explicitly showing the SNAP header. On the other hand, no
reason why they should get so specific about just one type of LLC
encpapsulation, I suppose.

Thanks again.

Bert


Posted by Michelot on October 24, 2007, 5:13 am
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Bonjour Albert,

> Thanks, Anoop. Also Figure C-6 shows this, but never the complete
> example, explicitly showing the SNAP header. On the other hand, no
> reason why they should get so specific about just one type of LLC
> encpapsulation, I suppose.

Perhaps you can see much more in the G.8012 (08/2004) ITU-T
recommendation, for example the figure 6-15.

You have :
(1) LLC encapsulation : DSAP 0xAA, SSAP 0xAA, Control 0x03
(2) SNAP encapsulation : OUI 0x00-80-C2, Pid 0X00-07 (for MAC without
FCS and no padding), PAD 0x00-00
(3) SNAP payload : DA, SA followed by the MAC SDU.

The MAC SDU could be a tagged MAC.

You can see also RFC 2684 and RFC 2364 to see all the possible client
services.

Take profit of the amendment 05/2006: "the length encapsulation of
client data since that encapsulation is not used for Ethernet over
Transport" only TYPE is recommended for the transport.

Best regards,
Michelot


Posted by Michelot on October 24, 2007, 5:16 am
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> Take profit of the amendment 05/2006: "the length encapsulation of
> client data since that encapsulation is not used for Ethernet over
> Transport" only TYPE is recommended for the transport.

Sorry, the G.8012 amendment.

> Best regards,
> Michelot



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