[Info-vax] SOLVED: Re: tcpip gateway question

[Rick Jones]

In comp.protocols.tcp-ip glen herrmannsfeldt <gah at ugcs.caltech.edu> wrote:
> In comp.protocols.tcp-ip jbriggs444 <jbriggs444 at gmail.com> wrote:
> < On Oct 2, 6:00?pm, glen herrmannsfeldt <g... at ugcs.caltech.edu> wrote:
> <> Richard B. Gilbert <rgilber... at comcast.net> wrote:

> <> < I'm not certain but I suspect that your problem is due to the fact that
> <> < DECnet sets the Ethernet address to AA-00-04-00-FB-04. ?If you start
> <> < TCP/IP first, it will get the original hardware address for the
> <> < interface. ?When DECnet starts it will set the address to the DECnet
> <> < node address and leave TCP/IP swinging in the breeze!

> <> That should only happen until the ARP cache is updated. ?Any host
> <> that tries to connect after the MAC address change will get the
> <> new address from ARP. ?Most will update on the broadcast ARP reply,
> <> but maybe not all.
>  
> < Broadcast ARP reply?  What broadcast ARP reply?

> OK, that is why I posted here.
>  
> < ARP requests are broadcasts.
> < ARP replies are conventionally sent as unicasts.

> Other hosts should update their cache based on the broadcast
> request, though, right?  

Should?  Perhaps.  Certainly they can.  Most probably do.

Some even went so far as to populate their ARP caches based on the
broadcast requests they saw - even before they tried to talk to that
system. I don't think there are any systems that do that foolish thing
any longer :)

> Many years ago I found a bug related to ARP.  An HP/UX machine

HP-UX :)

> running a third party appletalk (ethertalk) package would,
> it seems, sometimes ARP in SNAP encapsulated mode.  The Cisco
> router would then switch to SNAP encapsulation, but the host
> IP stack would not recognize them.  If I ping another host from
> the HP/UX machine it would ARP for that host, and the router
> would update the cache.

> I wouldn't be surprised if some broadcast replies.  How many
> update the cache on broadcast requests not destined for them?

Unless a gratuitous ARP happens to qualify as a reply (even though it
isn't) I don't think that ARP replies are ever sent as anything other
than unicast frames.

rick jones
-- 
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