Subject: Re: Why two Ethernet Types for PW + MTU question

From: "Andrew G. Malis" <[email-masked]>
Date: Thu, 14 Nov 2002 06:37:56 -0500

Dawn,

In this case, you should signal Ethernet (0x0005) in both directions, and 
MTU without the tag.  It's up to PE2 to strip the tag on ingress from CE2 
and add the tag on egress to CE2.  This allows CE2 to have PWs to multiple 
other CEs, one per VLAN tag, and the tag only has significance between CE2 
and PE2.  VLAN-capable equipment must be able to handle the extra four 
octets between CE2 and PE2 (see Figure C-6 in 801.1Q).

Cheers,
Andy

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At 11/13/2002 06:44 PM -0500, Dawn Xie wrote:

>There was a discussion on whether two PW types (Ethernet and Ethernet
>VLAN) is needed a couple months ago. Was there any conclusion reached on
>that discussion? From the newest draft
>http://www.ietf.org/internet-drafts/draft-ietf-pwe3-control-protocol-01.txt,
>it does not look like a conclusion is reached.
>
>Without the conclusion, there is difficulty to support the following PW
>case:
>
>    | CE1 |--untagged--| PE1 |----------| PE2 |--VLAN--| CE2 |
>
>The PW at PE1 side accepts untagged Ethernet, and the PW at PE2 side
>accepts 802.1Q tagged frames.  It is a quite useful and common scenario.
>Now there are two issues:
>
>1. What PW-type PE1 and PE2 shall advertise? If PE1 advertise Ethernet
>(0x0005) and PE2 advertise Ethernet VLAN (0x0004), the PW will not be
>established based on the draft, isn't it?
>2. How PE1 and PE2 advertise MTU? If PW agrees on MTU equals to X, and a
>frame of size X reaches PE2 from PE1, PE2 will need to insert 4 bytes
>for the VLAN tag, and the new frame size may become more than the link
>between PE2 and CE2 can handle.
>
>Any recommendation on solving the issues? One way to solve issue 1 is to
>remove the need of two Ethernet types. One way to solve issue 2 is that
>PE2 needs to make sure that the MTU it has negotiated with PE1 is 4
>bytes smaller than it is actually able to transmit between PE2 and CE2..
>
>-- Dawn