I agreed to read through the 802.1d spec regarding
the learning
process, and how it would apply to RPR rings using destination
stripping (where the bridge might not see the packet before
it
is stripped).
Overall, I agree with the sentiment that 802.1d is written to
not
place requirements on the underlying MACs. I also agree that
the
bridge will still function even if the destination strips the
packets.
We will get more flooding of packets because the Learning process
is
less effective. However, we can argue that a moderate increase
in
flooding is better than having to make the ring source stripped.
I also realized something important: 802.6 (DQDB) has the same
issue.
DQDB uses a pair of unidirectional links. All nodes must know
the
topology, and send the packet left or right depending on its
destination address. The packet flies by its destination and
"falls
off" the end. Thus, there is no single point on a DQDB net which
will see all packets. 802.1d defines how to bridge 802.6, and
says
nothing about the Learning process or anything special with
DQDB.
Therefore, I think we've got a strong endorsement that destination
stripping is ok.
Nonetheless, for completeness I did a strict reading of the 802.1d
spec. There are four places which discuss the learning process.
The page numbers listed below are relative to 802.1d/d17-May
25, 1998.
On #3 below, the wording is a bit troubling with respect to
destination
stripping. I don't think a strict interpretation is intended
here,
I'm just pointing it out.
1. pg 42, line 7 "Automatic learning of dynamic
filtering
information for unicast
destination addresses through
observation of source addresses
of Bridged Local Area Network
traffic."
This is from the overview,
and doesn't say much.
2. pg 45, line 7: "The learning process, which
by observing the
source addresses of frames
received on each Port, updates the
Filtering Database, conditionally
on the Port state."
This wording implies that
the bridge must only learn those packets
which happen to arrive at
its port.
3. pg 48, line 3: "The individual MAC Entity
associated with each
Bridge Port examines all
frames transmitted on the LAN to
which it is attached."
The wording of this one is
troubling, in that it states the
Port must examine _all_
frames on the LAN segment. Taken literally
and absolutely, this would
be a problem for destination stripping.
4. pg 54, line 23: "The Learning Process observes
the source
addresses of frames received
on each Port and updates the
Filtering Database conditionally
on the state of the receiving
Port."
This wording implies that
the bridge must only learn those packets
which happen to arrive at
its port.