Re: L2VPN and MTU issue

From what I know.

if you do not use the tag-switching mtu command, and a packet arrives with
larger MTU's then the 1500 size the labels pushed to the end of the stack
will be dropped and an icmp packet will be sent back to the source. If
this is a layer 2 switch providing the segment, the packet itself could be
dropped and will report a giant.

Look at RFC 1191, PMTUD
Also, www.ietf.org/ids.by.wg/tcpimpl.html
(draft-ietf-tcpimplpmtud)

Cisco has a work around (soft-of) by increaseing the MTU of the ethernet
port to 1526

(draft-ietf-mpls-label-encaps)

If your switches do not support Giant frames Cisco TAC has a work around,
but you will have to get that from them.

HTH: JS

On Mon, Oct 22, 2012 at 9:03 AM, Routing Freak <routingfreak_at_gmail.com>wrote:

> Hi all
>
> I know the break up of the MTU when doing a L2VPN setup. This was my break
> up
>
> Here is my breakdown of the MPLS MTU:
>
>
>
> Ethernet Payload - 1500
>
> 802.1q header - 18
>
> AToM Header - 4 (Required for ATM and FR only )
>
> AToM Label - 4
>
> LDP Label - 4
>
> TE Label - 4
>
> MPLS Fast Reroute - 4
>
> Total = 1538
>
>
> So If i have a MTU of 1500 on the core, whether the L2VPN packet will be
> fragmented r else it will be dropped , I mean the L2VPN data as well as the
> control pLane traffic.
>
>
> WHen targeted LDP exchange control plane messages to bring up the T-LDP
> session, they negotiate the Attachment circuit MTUs and not the Core side
> MTUs .
>
>
> But when the MTU is lesser on the core side, what problems will happen.
> Will it fragment the packet or else it will attach any Header to find the
> correct sequence of packets
>
>
> Or else Control Word is used when fragmentation happens on the core.
>
>
> Anyone came across this issue ?
>
>
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Received on Mon Oct 22 2012 - 13:28:52 ART