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

John Edwards jaedwards at gmail.com
Fri Sep 28 00:09:38 EST 2012

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.

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.


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