Re: [RRG] Re: [RAM] Tunneling overheads and fragmentation

[Pekka Savola <pekkas at netcore.fi>]

On Tue, 11 Sep 2007, Dino Farinacci wrote:
> > But what about inter-ISP links? I'm assuming this isn't a big issue for 
> > high capacity private peering, but here in Europe a lot of peering happens 
> > over exchanges, which obviously use equipment that can easily handle larger 
> > packets, but so many people are on a big fat shared subnet that it's a 
> > given at someone will bring down the lowest common denominator to 1500.
>
> All GigE and higher. We are okay.

While it may be technically OK, it's nowhere near operationally 
simple, whether or not you count that as "OK".

Many/most inter-ISP links are through GE-based IXes.  I believe most 
of those use a common VLAN for all the peers.  So in order to upgrade 
from MTU 1500, all the ISPs connecting to the VLAN need to update 
basically at once to MTU 9000 or big packets get blackholed to 
non-updated peers.  Another alternative is having a separate "big-MTU 
VLAN", but moving operators from one VLAN to another introduces IP 
address changes, resulting in BGP flaps and rather big operational 
cost.  I believe these big-MTU VLANs haven't been very popular to date 
due to lack of demand (= you'll need to connect to both VLANs in any 
case, and there's rather little benefit _to date_ of supporting 1500+ 
inter-ISP MTUs).

--
Pekka Savola                 "You each name yourselves king, yet the
Netcore Oy                    kingdom bleeds."
Systems. Networks. Security. -- George R.R. Martin: A Clash of Kings

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