It's been a long time, and I no longer have a Token Ring Sniffer (sniff
;-), but as I recall, an IP station on Token Ring sends an ARP broadcast as
All-Routes Broadcast. The recipient sends a unicast frame back as a
Specific Route frame. At the SRB layer, the recipient can just reset the
direction bit. Theoretically the original ARPer should have a path to the
destination then. It is in the ARP reply.
Now I do remember seeing some silliness with this, though, where stations
did not learn the path even though the info was in the frame. This causes
the stations to use All Routes Broadcast in subsequent frames. But I think
that silliness was with SNA on Token Ring, not IP on Token Ring.
Priscilla
At 03:07 PM 3/22/01, John Neiberger wrote:
>I'm reading through Lou Rossi's token ring paper and read something that
>is ponderous. Over ethernet, an ARP request is broadcast but the reply
>is unicast. In this paper (p. 4) there is a scenario where two hosts
>are separated by two bridges and a ring. Host A wants to transmit to
>Host B so it ARPs for B's MAC address.
>
>Now, the paper mentions that after the ARP reply, A knows the MAC
>address of B but not the location. My question is this: is an ARP
>reply over token ring unicast or broadcast? If it's unicast then how
>did B send the response to A? Wouldn't it have to send an explorer
>packet first to find the path to A?
>
>It seems to me that this is the process:
>
>A wants to say hi to B
>A sends an ARP request (broadcast) to B
>B receives request and wants to send unicast response to A
>B sends a local explorer for A (no response)
>B sends an all-routes explorer for A (gets a response)
>B sends a unicast ARP reply to A
>
>Is that right? If that's the case, then here's what I gather happens
>next:
>
>A now has B's MAC address
>A sends a local explorer for B (no response)
>A sends an all-routes explorer for B (gets a response)
>A proceeds to transmit data to B
>
>Here's what doesn't make sense to me. If B had to know the path to A
>to send an ARP reply, why doesn't A just take the data in the RIF from
>that reply to figure out the reverse path back to B? Why waste time
>with explorer packets when it was just given the path in the ARP
>reply??
>
>Or, do ARP requests/replies even have RIFs? If not, are they all
>broadcast in token ring?
>
>Can you tell I'm just starting out studying token ring and SRB? <g>
>
>Thanks as usual!
>
>John
>
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Priscilla Oppenheimer
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