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Re: Tag-switching mtu doubt

  • From: jesus.ferran@amena.es
  • Date: Fri, 15 Mar 2002 09:32:25 +0100
  • Cc: mpls-ops@mplsrc.com
  • Resent-Date: Fri, 15 Mar 2002 04:30:00 -0500
  • To: Rui Francisco Negrão Valente <rui.valente@vodafone.pt>
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Rui,
i believe you are omitting the transport header in your compute t. I show
you the formula  for estimating the size of packets traveling through the
core Network:

CORE MTU >= Edge MTU + Transport  type header+ MPLS header

where transport type header depends of the transport type you are using (
for example hdlc=4 bytes, Frame relay DLCI=4 bytes...)

Regards






Rui Francisco Negrão Valente   <rui.valente@vodafone.pt> on 14/03/2002
20.09.28

To:    mpls-ops@mplsrc.com
cc:

Subject:    Tag-switching mtu doubt




Hi all,



Imagine the following VPN scenario:



CE----PE---P---P---PE----CE



I have FastEthernetin all the links, and tag-switching mtu1508 at the PEs

andPs interfaces.



Due to the label stacking(2 labels) we have additional 8 bytes in the MTU

size. So, at the CPE the maximum datagram size I can send is 1500 (What I

didwas pings with 1500 and with the DF=1).



My question is:



Why pings fail with datagram size of 1504 from PE to PE?

 From what I understand there will be only one label (4 bytes) pushed in
the

packet, therefore the mtusize will be 1508...







Rui Valente

DRE-SE Vodafone(Portugal)

Tel: +351210914085

Mob: +351919513001

rui.valente@vodafone.pt





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