[AusNOG] Why-do-internet-exchanges-need-layer-2?

[Matthew Moyle-Croft]

On 27/09/2012, at 7:09 AM, John Edwards <jaedwards at gmail.com> wrote:

> 
> On 27/09/2012, at 10:53 PM, Christopher Pollock wrote:
> 
>> tl;dr Layer 2 was cheap and is easy.
> 
> Recall that the first peering exchanges were point-to-point PVCs on ATM. You got expensive backhaul to expensive ports, and then still had to negotiate agreements with other participants.
> 
> Ethernet was more than just cheap - it forced MLP on participants, and everyone has benefited from the agglomeration and scale.

MLP is more recent and less common than ethernet.

Layer2 is all most universal (I don't know specifically of any non-Layer2 exchanges left, but I'm sure there's the odd obscure one) because it's cheap, it scales and it doesn't prevent bilateral peering.  But the reasons are a little more subtle - flat broadcast domains are easy to understand.  The reality is most people have little clue how IXes work in practise - even those connected to them, so any unnecessary complexity is a burden.  I've seen people propose all kinds of wild and crazy schemes but at the end of the day, it needs to be easy and cheap not clever.

Only issue as Mark pointed out is that the broadcast issue creates some issues - especially at scale - so you need to have a careful thought about the features you need when you choose the equipment.   I'll point out that the equipment used by the larger exchanges globally tends to be both Layer2/3 capable (eg. LINX went Juniper MX for one LAN recently) purely because no one actually makes dumb massive switches anymore!

MLP is, at one level, a good thing for emerging markets, but the thing I've discovered is that it makes people very lazy about peering.  It tends to be that people treat peering ports much the same way as transit - as just a paid service, rather than something to actively manage.  So it can be quite frustrating as a peering person to try and get bilateral sessions setup or maintained when necessary.

MMC



> 
> Had our industry been brought up with L3 peering exchanges, we probably still wouldn't be doing IPv6 peering except on private links. 
> 
> Having an L3 exchange binds a peer to accept the most specific routes available. If I advertise a /24 and you advertise a /25 to an exchange that accepts both, then anyone else putting a packet on the exchange is forced to accept the /25. L2 lets us keep some filtering options open.
> 
> John
> 
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