MTU handling in 6RD deployments

Anfinsen, Ragnar Ragnar.Anfinsen at
Thu Jan 9 20:36:05 CET 2014

On 09.01.14 17:36, "Templin, Fred L" <Fred.L.Templin at> wrote:

>But, in some environments we might not want the 6rd BRs to suffer from
>sustained fragmentation and reassembly so a responsible network operator
>would fix their IPv4 link MTUs to 1520+. If they can't do that and the
>load on the 6rd BR appears too great, then MSS clamping and a degenerate
>IPv6 MTU reported to the IPv6 hosts is the only option.

The problem with your statement is that many L3 access networks do not
support MTU greater than 1500 on the access port. And if the RA MTU is set
to 1480, you would not see any problems at all. However, there are some
retail routers which do not set the MTU to 1480 when using a 6rd tunnel.
In these cases adjusting the MSS if a good and efficient way of correcting
that problem. Our experience so far is that MSS-clamping does not have any
additional CPU load compared to not do it.


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