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<== Date ==> <== Thread ==>

Subject: RE: result: CA links dont connect
From: johill@lanl.gov (Jeff Hill)
To: "'Matthias Clausen DESY -MKS-2/KRYK-'" <CLAUS@heax03.desy.de>
Cc: "EPICS-tech-talk (E-mail)" <tech-talk@aps.anl.gov>
Date: Thu, 22 Oct 1998 10:44:09 -0600
> 
> you remember our conversation about IOC's which won't talk CA to each other?
> 
> We found out that an entry in the boot params defining the default route
> will disable CA.

As I recall, adding a "gateway" IP causes a aggressive route to be installed which results in
_all_ packets being sent to the router. So when the IOC tries to talk to even its neighbors which 
are on the same subnet it does not use ARP to determine the neighbor's Ethernet address. Instead, 
it uses the Ethernet address of the router. This results in all packets being sent first to the router, 
and then the router sends the packet again to the neighbor host despite the fact that all hosts involved
are on the same Ethernet.

If your router had an option selected (or changed) so that it never generates a Ethernet broadcast
on the local subnet then this would cause all of the CA search broadcasts to be discarded.

> Adding the route in the statup command file is ok.

Yes, in fact we have had to add the gateway IP to the boot parameters so the IOC would boot,
but later deleted this route and added another one in the startup script.

> 
> We think that the reason is the startup configuration of the Ethernet device.
> Defining the default route will (probably) disable broadcasts...

Probably the router is configured to discard network directed IP broadcasts. I suspect that
network system administrators do not always clearly understand issues related to broadcasting 
{ICMP responses, subnet masks, proper host IP address assignment ranges etc} and therefore 
tend to take all options that turn off broadcasting when they set up a router.

The router configuration issue is: if a router receives a packet addressed to IP N.N.N.255,
should it go ahead and generate an Ethernet broadcast on the interface that is attached to
network N.N.N.X with netmask 0xffffff00, or should it just discard the packet.

Jeff

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