Layer 2 vs. Layer 3 to TOR

David Coulson david at
Thu Nov 12 15:37:24 CST 2009

Jonathan Lassoff wrote:
> I was recently looking into this (top-of-rack VPLS PE box). Doesn't seem
> to be any obvious options, though the new Juniper MX80 sounds like it
> can do this.  It's 2 RU, and looks like it can take a DPC card or comes
> in a fixed 48-port GigE variety.
The MX-series are pretty nice. That should be able to do VPLS PE, 
however I've never tried it - MX240 did it pretty well last time I 
tried. I've no clue how the cost of that switch compares to a cisco 4900 
or something (not that a 4900 is anything special - L3 is all in software).
> Are there any applications that absolutely *have* to sit on the same
> LAN/broadcast domain and can't be configured to use unicast or multicast
> IP?
The biggest hurdle we hit when trying to do TOR L3 (Cisco 4948s w/ /24s 
routed to each one) was devices that either required multiple physical 
Ethernet connections that we typically use LACP with, or any 
environments that do IP takeover for redundancy. Both are obviously 
easily worked around if you run an IGP on your servers, but that was 
just insanely complex for our environment. It's hard to convince people 
that a HP-UX box needs to work like a router now.

So now we have a datacenter full of 4948s doing pure L2 and spanning 
tree... What a waste :-)

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