[c-nsp] GSR and 3750 GE-LX link problem

[Michael Smith]

-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1


> -----Original Message-----
> From: Gert Doering [mailto:gert at greenie.muc.de]
> Sent: Tuesday, January 11, 2005 1:27 PM
> To: Michael Smith
> Cc: Gert Doering; Yu-lin Chang; cisco-nsp at puck.nether.net
> Subject: Re: [c-nsp] GSR and 3750 GE-LX link problem
> 
> Hi,
> 
> On Tue, Jan 11, 2005 at 01:23:58PM -0800, Michael K. Smith wrote:
> > > For local ping, this isn't necessary - the routers won't
> > > forward 
> packets
> > > if "no ip routing" is set, but that's not necessary to ping.
> >
> > Are you sure?  I'm not sure how the 3750 deals with encapsulation
> without
> > the underlying protocols enabled.  Does the ethernet interface
> encapsulate
> > the traffic as a Layer 3 packet or a Layer 2 frame?  Is the
> encapsulation
> > based solely upon the "no ip switchport" command on the 3750?
> 
> "no ip routing" doesn't mean "IP switched off".
> 
> It switches off IP *forwarding*, but the box will still do IP
> pracket processing as a host - no forwarding, no use of routing
> protocols, use of "ip default-gateway" to decide where to
> default-route packets (which is also what the boot monitor does
> when you boot from TFTP).
> 
> (I admit that I'm not sure about the 3750, but the basic principle
> of "no ip routing" vs. "ip routing" is embedded so deeply into IOS
> that it would surprise me very much if the 3750 is different here).
> 
I *think* there is a difference when configuring the router
intelligence on the switching platforms, but I could be wrong.  :-) 
When there is a VLAN interface on the switch and the Gigabit Ethernet
port is set to switchport mode, either trunking or no, the frames are
forwarded across the Gigabit Ethernet port as Ethernet frames and
"terminate" on the VLAN interface.  The VLAN interface then has the
IP address on the same subnet as the ip default-gateway for local
management use.

On the other hand, when the Gigabit Ethernet port is given an IP
address and set to "no switchport" the port forwards the traffic as
encapsulated IP packets, not Ethernet frames.  That's why I wondered
about the "ip routing" process being activated on the device. 
Without it, does the switch still forward the packets?  Or, does it
forward them as frames?  Or, is it an incomplete configuration so it
doesn't forward them at all?

I'd love to hear the answer from Cisco on this one just for my own
edification.

Mike

-----BEGIN PGP SIGNATURE-----
Version: PGP 8.1

iQA/AwUBQeRX2Jzgx7Y34AxGEQLp4wCfW/eJNKoEy6tWgBdqnJjo2Nvcf9gAn06H
4PBjOUVP0Zcb80Y77yvSx5XH
=ldKI
-----END PGP SIGNATURE-----