[c-nsp] MPLS Label stack depth and MTU

[Gert Doering]

Hi,

On Tue, Apr 29, 2008 at 09:34:49AM -0700, Christopher Hunt wrote:
> ah, but alas, with 7205 FA-TX you cannot set the IP MTU>1500, so I have 
> set the mpls mtu 1508.  

This *should* work.  If the path in between is indeed big-MTU capable.

Some ethernet bridging devices we've seen in the past permit just enough 
so that dot1q tagging works, but ISL tagging doesn't - so you really
need to check with all the telcos on the path to make sure that they 
are permitting 1508 bytes (+ L2 encaps) packets.

> The only way to get the extra bits attached is 
> to do an end-to-end ping _within_ the vrf, which cannot ping the middle 
> hops.  

Indeed.  Or get equipment that is fully big-MTU capable - I seem to 
remember that the PA-2FE-TX (which is using a different chipset) can
do that.

> However, someone has suggested that my math is wrong and that in 
> fact the L2 header and CRC is 18 bytes, 

This is so.

> then i need to add an additional 
> 4 bytes for the VLAN tag which does answer my question.  

Well, indeed... - but as far as I understand, the tagging is happening
on the Telco equipment, so it's really *their* business to get that
sorted out.  But yes, I have worked with Telcos, and I know the answer...

> Problem 
> solved!  Now to convince my Service Provider to raise the MTU :-P

Indeed.

gert
-- 
USENET is *not* the non-clickable part of WWW!
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Gert Doering - Munich, Germany                             gert at greenie.muc.de
fax: +49-89-35655025                        gert at net.informatik.tu-muenchen.de
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