[c-nsp] Why it won't route vlan 1 ?

[Brett Frankenberger]

On Tue, May 15, 2007 at 07:51:29PM +0200, Jerome Covini wrote:
> >
> > 	There is only one 'vlan1'
> >
> > 	if you have vlan1 on more than one interface (eg: gig1/1 and gig1/2)
> > they are actually the same vlan.  This device is a switch, not an
> > independent router.
> > 	You should be able to 'no shut' the vlan1 interface and use that
> > instead and leave the port as a trunk.  vlan1 has been generally 
> > 'pseudo-reserved' on cisco for as long as I can remember.  I suspect
> > that it was working some other way was some odd artifact of the code that
> > they've since closed to prevent unexpected operation.
> >
> For info, the platform onto which it was working was a totally different 
> one i.e. Cisco 8540CSR with 2port GE modules.
> This was accepting to have multiple vlan 1 routed subinterfaces, as well 
> as multiple vlan x routed subinterfaces . Probably the odd code artefact 
> you are referring to !

It's not a code artifact; that's a completely different platform. 

The 8540CSR was more like a router -- there were no global VLAN
identifiers.  You could use the same VLAN ID on different interfaces
pretty much at will; you could put the same LVAN (bridge-group, in
8540-speak) on multiple interfaces with a different tag on each one,
and so on.  The backplane architecture was ATM, not VLAN based; tags
were applied at interface level as needed.

The 6500 architecture, on the other hand, is built around VLANs.  A
Layer 3 interface is architecturally a VLAN with a SVI and a single
member port; when it's done on a physical interface, the switch
allocates an internal VLAN number for it; when it's done on a
subinterface, the switch more or less has to use the tag you specify as
the VLAN number.  (There's limited hardware support for VLAN tag
translation, but it's not the any VLAN with any tag on any port that
something like an 8540 could do, and Cisco has elected to not try to
magically configure translation to accomodate vlan tag reuse (and
complain when you specify something that the hardware can't support),
and to instead just not allow vlan tag reuse.)

     -- Brett