From: Jamie Caesar (jamie.caesar@gmail.com)
Date: Wed Aug 17 2005 - 13:14:04 GMT-3
There may be something useful in this article. It contains a few
options on how to handle MTU issues with GRE tunnel interfaces:
http://www.cisco.com/en/US/tech/tk827/tk369/technologies_tech_note09186a0080093f1f.shtml
Jamie
On 8/16/05, Jongsoo <bstrt2004@gmail.com> wrote:
> I thought about this again and here is my thought ( maybe a wrong thought!!!).
>
> The problem is how to deal IP packets w/ MTU1500 and DF set over MPLS
> and over GRE.
>
> Since the GRE-tunnel outgoing physical interface has MTU 1500, no
> matter what packet has to be fragmented.
> And I have two interfaces, GRE tunnel and physical interfaces that can
> fragment IP packet. Obviously, the problematic incoming IP packet to
> GRE tunnel has MTU1500 and DF set so that tunnel interface can not
> fragment due to DF set.
> I can fake the size MTU of GRE tunnel to meet the biggest szie of IP
> packet, which will make GRE IP packets to be generated without
> fragmentation. And when this large GRE IP packet goes out to the real
> physical interface, it will be fragmented because it is larger than
> MTU of physical interface and GRE IP OH doesn't have DF set.
>
> Anyway I think perhaps the correct MTU size for tunnel interface is
> 1532 = 24 ( IP OH + GRE OH) + 8 ( 2 MPLS tags since it is MPLS VPN) +
> 1500 ( ip packet)
>
> Basically, this MTU size of 1532 under tunnel interface will make IP
> packet( 1500 DF set) to be fragmented not in tunnel but in outgoing
> physical interface.
>
> Any thought on this?
>
>
> Jongsoo
> CCIE 14539
>
>
> On 8/16/05, jon kim <bstrt2004@gmail.com> wrote:
> > Group
> >
> > I transmit MPLS VPN traffic over GRE tunnel over public internet and
> > am having some issue, which looks like MTU issue.
> >
> > I made IP MTU 1508 ( 1500 + 2 MPLS tags) but no luck.
> >
> > I am sure soemone did this before and will appreciate any tip about
> > VPN MPLS over GRE .
> >
> >
> >
> > Thanks
> >
> > Jongsoo Kim
>
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