[c-nsp] SVI or Subinterfaces?

[Peter Rathlev]

Hi Asad,

On Wed, 2008-07-23 at 15:56 +0500, Asad Ul-Islam wrote:
> We are ISP and have Catalyst 6513. And I want to terminate Trunks on it. Can
> someone tell me what is the better approach to achieve this.
> 
> 1) using Subinterfaces on Trunk links. or
> 2) Using SVIs
> 
> Which will provide more flexibility and scalability and what are the
> limitations of each method?

On LAN cards on the Catalyst 6500 platform there is no local VLAN
significance, which means that with a subinterface definition like this:

interface GigabitEthernet1/1.100
 encapsulation dot1q 100
!

you can't use VLAN 100 anywhere else on the box. This also means that
you cannot do local switching, i.e. using this VLAN on more than one
physical port. You can't do this:

interface GigabitEthernet1/1.100
 description CPE A
 encapsulation dot1q 100
!
interface GigabitEthernet1/2.100
 description CPE B
 encapsulation dot1q 100
!

You'd get a "Command rejected: VLAN 100 not available" when defining the
second port.

On the other hand, with LAN cards facing the core, you cannot use EoMPLS
on SVIs, only on subinterfaces and physical ports.

With the MUX-UNI feature (SXH and newer) you can combine regular
switchport trunks and subinterfaces, using the latter for subinterface
based EoMPLS. You cannot do any L3 termination on them though, they can
only be used for EoMPLS.

We use regular switchport trunks and SVIs everywhere and then of course
make sure to limit VLANs to ports where they are relevant. (No open
trunks.) We only use physical ports where we do EoMPLS.

Hope this helps,
Peter