[j-nsp] MX, ARP cache with L2 bridging

Chuck Anderson cra at WPI.EDU
Tue Sep 18 17:35:54 EDT 2012

On Tue, Sep 18, 2012 at 11:24:04PM +0200, Nicolaj Kamensek wrote:
> Am 18.09.2012 20:57, schrieb Chuck Anderson:
> Hi,
> >That is not true in my experience.  L2 MAC Learning takes effect
> >immediately upon seeing traffic enter the new MX port.  The ARP entry
> >will point to the new L2 next-hop immediately.
> interesting because we just had a server being relocated to a
> different switch on a differekt xe port on the mx and after clearing
> the arp cache for the specific irb interface, the host was up
> immediately. We are running 11.4R2.14 and a
> show arp
> actually shows the xe interface instead of the irb interfaces as one
> would expect.

It may be that if the server or client host is "quiet" and not sending
anything that MAC learning will not occur right away.  In that case
the traffic will be sent to the old port until the MAC table entry
ages out.  By clearing the ARP, you cause the router to send an ARP
broadcast to all ports (because something is probably trying to reach
the server still), which triggers the otherwise quiet server/client to
respond, causing the MX to learn the new L2 port where the MAC address
now lives.  This is a generic problem with Ethernet MAC learning, not
something specific to the MX.

If you keep a pinging going FROM the server to the default gateway for
example, it should pick up the L2 move fairly quickly.

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