Traffic billing - L2 encap to include or not?

Richard A Steenbergen ras at
Sat Jun 13 17:53:32 UTC 2009

On Fri, Jun 12, 2009 at 03:32:42PM +0200, Weber, Markus wrote:
> So, is L2 encapsulation (e.g. Ethernet) considered as framing characters
> or not?
> Cisco does count them (looks like they also count the FCS), while 
> Juniper does not (at least not on their routers) with above MIBs.
> So who's right?

Juniper doesn't do this on purpose, it is just an unfortunate
consequence of a hardware architecture which has distributed framing
ASICs at the PIC level. For most PICs, the L2 information is already
stripped away before the packet reaches the hardware components that are
capable of counting or otherwise processing the packets further. This is
why you need a far more expensive IQ pic with special processing
capabilities on the PIC itself to do things like MAC accounting.

Cisco and "everyone else" include the L2 overhead AFAIK. It makes for 
plenty of confusion come billing time.

> PS1: And what about padding? IP packets could be smaller then the min
>      ethernet frame size ... (think of some kind of DOS attacks) ...
> PS2: Oh, maybe someone could check on J switches - would be nice to
>      know ...

The MX uses essentially the same type of hardware as a T-series, but
with an extra EZChip ASIC added to do some basic Ethernet framing. You
actually pose a good question, it might be possible for them to count
the L2 overhead in SNMP, but I'm not sure if they actually do it or not. 
EX is an entirely different architecture from traditional Juniper
routers so I wouldn't even begin to guess.

At any rate, this is a discussion better suited for juniper-nsp mailing

Richard A Steenbergen <ras at>
GPG Key ID: 0xF8B12CBC (7535 7F59 8204 ED1F CC1C 53AF 4C41 5ECA F8B1 2CBC)

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