[j-nsp] juniper mpls mtu

[Daniel Roesen]

On Tue, Jun 03, 2008 at 09:15:30AM -0400, cp wrote:
> I hoping someone can provide insight into why by default juniper
> calculates mpls mtu at 12 bytes less than the ip mtu?  I've been testing
> using l3vpn and it seems that mpls mtu pads 8 bytes to its mpls mtu
> reducing the ip mtu the inside packet by 8 bytes. Quick example. So if
> mpls mtu is 1508 the ip mtu of the packet inside is 1500. That means the
> ip mtu of the mpls interface is 1520. The only reason I can see
> additional bytes needed on top of the l3vpn two is for fast reroute
> bypass.  I assume it's for the safety factor although I am probably
> missing something. Any information is appreciated.

Exactly that's the case as far as I'm aware. They have chosen a default
"enough for most usages" headroom.

Then again, defaults may change, so it's good engineering practice to
set MTUs for all protocols explicitly.

Best regards,
Daniel

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