[j-nsp] EVPN all-active toward large layer 2?

[Eldon Koyle]

On Fri, Apr 19, 2019 at 5:06 AM <adamv0025 at netconsultings.com> wrote:
>
> > Tarko Tikan
> > Sent: Thursday, April 18, 2019 10:14 AM
> >
> > hey,
> >
> > > You have effectively created L2 loop over EVPN, so to cut it you need
> > > a link between bridged network and EVPN to be a single link. There is
> > > no STP in EVPN.
> >
> > To be fair it's not a full loop but only BUM traffic will loop back to
> other PE.
> >
> Yes but there should be an MPLS label associated with that traffic that says
> to the other PE -do not send this traffic back to LAN -cause it's the same
> site.

The problem is actually in the other side: the LAN would send BUM
traffic sourced from the router back to the other router port, and BUM
traffic sourced from the LAN to both router ports.  Two sites
configured like this in an evpn would cause such traffic to loop
infinitely, since Ethernet has no TTL.  Three sites would get you to
the point of exponential packet duplication where a single broadcast
packet could fill your pipes and keep them full until you intervene
(or something dies).

Allowing a MAC to appear on multiple ports would add a _lot_ of
complexity to ethernet (current hardware doesn't support it), and
could often result in traffic taking a suboptimal path (since switches
only know they saw this source MAC on that port -- not how far away it
is).  You would need a routing protocol running at layer 2 to solve
these issues.

Remember that ethernet was initially designed using shared media, and
the MAC address was used to allow your NIC to ignore traffic that was
being sent to other hosts (to save CPU).  The fact that they managed
to shoehorn switching in there without re-writing the protocol is
magical, but we are still living with some inherent limitations.

-- 
Eldon