Teksavvy cable modems

[Lennart Sorensen]

On Fri, Mar 09, 2012 at 05:41:29PM -0500, James Knott wrote:
> Lennart Sorensen wrote:
> >What should work:
> >
> >   +--------------------------------------------+
> >   |                 switch                     |
> >   |     vlan1                 +---vlan2---+    |
> >   |     |   |                 |   |   |   |    |
> >   |    P1   P2                P3  P4  P5  P6   |
> >   +----|----|-----------------|---|---|---|----+
> >        |    |                 |   |   |   |
> >Cable -+    +---Router--------+   |   |   |
> >                                   |   |   |
> >                                    Clients
> >
> >
> That will not work, unless the modem understands VLAN tags and is
> configured to use VLAN 1.

I did say 'ports configured for untagged traffic'.  So the traffic
outside the switch does not carry tags.  The vlan is purely used to
issolate ports from each other inside the switch.

So I am right. :)

This is actually how many home routers work.  It is not unusualy to see:

                              CPU
                        Ethernet port
                               |
                            (trunk)
                               |
                          switch==chip
                          | |  |  |  |
                     WAN -+ |  |  |  |
                            P1 P2 P3 P4

The CPU uses tagged traffic on the trunk to the switch chip with vlan1
and vlan2, and vlan1 is the "WAN" port, while vlan2 is P1 through P4.
All traffic outside the box is untagged.

It means a CPU with only one port can by using a switch chip provide
issolated interfaces for cheap.  Some boxes do have two actual ports on
the CPU and hence don't connect the WAN port through the switch chip.
My favorite DIR-825 for example.

-- 
Len Sorensen
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