[j-nsp] MPLS transport / 1GE-LH

[Richard A Steenbergen]

On Thu, Oct 21, 2004 at 11:04:55PM +0200, sthaug at nethelp.no wrote:
> > This brings up an issue that I have been playing with. IF you are
> > running mpls lsps over an Ethernet network. What should the media MTU be
> > set to in order to make things as transparent as possible. The config
> > below has 1568, and I'm curious how you came to that number.
> 
> That number was decided for other reasons, we simply had to make it
> "large enough" that ethernet-ccc would work. It could definitely have
> been made smaller than 1568.
> 
> Note that we prefer to have some headroom, and setting for instance IP
> MTU explicitly, instead of trying to make the physical interface MTU
> fit everything. YMMV.

http://www.juniper.net/techpubs/software/junos/junos64/swconfig64-interfaces/html/interfaces-physical-config4.html

Look at these table to determine the highest supported value (or consult 
other vendors if the opposite side isn't also a Juniper), and the amount 
of encapsulation required, and draw your own conclusions.

If you're just doing basic Draft-Martini ethernet handoffs via VLAN-CCC, 
your PE needs:

  1500  (Base frame)
+   14  (Local Ethernet Header)
+    4  (802.1q tag)
+   14  (Encapulated Ethernet header)
  ----
  1532

The requirements for P routers are probably going to be the same, but swap 
out the 4 octets of 802.1q for 4 octets of MPLS shim. The smallest MTU 
supported by a Juniper interface is 1532 on the high-density FastE 
interfaces, so you're generally going to be safe there. You may have 
problems if you're doing Martini plus MPLS plus 802.1q on P routers across 
these FE links, but then again you probably shouldn't be doing this to 
begin with.

-- 
Richard A Steenbergen <ras at e-gerbil.net>       http://www.e-gerbil.net/ras
GPG Key ID: 0xF8B12CBC (7535 7F59 8204 ED1F CC1C 53AF 4C41 5ECA F8B1 2CBC)