Re: route field
- From: roberson@xxxxxxxxxxxx (Walter Roberson)
- Date: Sun, 29 Jun 2008 19:42:44 GMT
In article <fe9d0880-d249-44f8-af4f-75ff7b1a5b47@xxxxxxxxxxxxxxxxxxxxxxxxxxx>,
vicky <vikrant.pandey@xxxxxxxxx> wrote:
is there is a necessity of route field in layer 2 switch ...
please tell me ...
No, there is no necessity for that.
For example, it is possible to build a layer 2 switch in which every
port has a dedicated "send" path to each other port (e.g., the wiring
to send from port B to port D is completely different than the wiring
used to send from port C to port D); and when a packet came in to a
port, the port could send the packet literally simultaneously to
every other port, and then each output port could discard the
packets that are not destined for that output port. No routing
information of any kind would have to be kept on such a layer 2
switch, as each output port would independantly decide whether
the output packet was to be transmitted or not.
I am not saying that this would be a *good* way to build a
layer 2 switch, but your question was about whether there is
a *necessity* of having some kind of "route fiele", and my example
above demonstrates that it is not *necessary*.
Now, it could potentially be much more *efficient* to have some kind
of "route field" in real layer 2 switches -- but it is not *necessary>
Note, by the way, that in networking, the word "route" usually
refers to layer 3 information. Unmanaged layer 2 switches do not need
to hold any layer 3 information for basic layer 2 operations, so
if you are referring to "route" in the layer 3 sense, then the
answer is a flat NO. My answer about the hypothetical layer 2
switch was for the case where you might have meant "route field"
more generally, such as some kind of internal table that listed
MAC addresses and their associated port: my example shows that
even that kind of internal table is not *necessary* in a layer 2 switch.
You have been asking a lot of questions about VLANs lately, and
one of your questions that I answered recently but which you did not
understand (or believe?) my answer gave a scenario in which you
were attempting to show that layer 2 switch must have
an intra-vlan router in order to provide VLAN connectivity in single-
instance STP cases. I suspect that your current question is just
a rephrasing of that earlier question, and I will reinforce my
answer of that time: if you use a layer 2 switch with VLANs that
are restricted to particular ports, and you use a single-instance STP
protocol, then your VLAN connectivity *will* break, and the breaking
would be the fault of the person that configured the situation,
not the fault of the layer 2 switch. Furthermore, there is no
requirement that layer 2 switches support VLANs *at all*.
Several times, I have noticed you get confused about the functionality
that *must* be supported in -all- layer 2 switches, mixing it up
with functionality that needs only to be supported in layer 2 switches
that claim to have standard conformance to particular -optional-
features.
.
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