From: Jongsoo (bstrt2004@gmail.com)
Date: Tue Aug 16 2005 - 22:43:10 GMT-3
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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